Unix shell program training

Winter Training, December 2011

Unix and Shell Programming

Department of COE and SE,

Delhi Technological University

Instructor: Divyashikha Sethia

Contents

UNIT 1: INTRODUCTION TO UNIX ..........................................................................3

UNIT 2: SHELL SCRIPTING ..................................................................................... 63

UNIT 3: ADVANCED SHELL SCRIPTING, SED, AND AWK .................. 143

UNIT 1: INTRODUCTION TO UNIX

1. THE UNIX OPERATING SYSTEM – AN OVERVIEW.................................7

2. UNIX COMMANDS ................................................................................................... 21

3. UNIX FILE SYSTEM ................................................................................................ 33

4. THE VI TEXT EDITOR ............................................................................................ 45

COE Unit 1, Lesson 1

LESSON 1 THE UNIX OPERATING SYSTEM – AN

OVERVIEW

1. THE UNIX OPERATING SYSTEM – AN OVERVIEW .................................................7

1.0 OBJECTIVES ...............................................................................................................7

1.1 INTRODUCTION ...........................................................................................................7

1.2 INTRODUCTION TO THE COMPUTERS .........................................................................7

1.2.1 Typical hardware components of a computer.................................................8

1.3 OPERATING SYSTEM ..................................................................................................8

1.3.1 Virtual Memory.....................................................................................................9

1.4 UNIX OPERATING SYSTEM .................................................................................... 10

1.4.1 History of UNIX ................................................................................................. 10

1.4.2 Importance of UNIX ......................................................................................... 11

1.5 UNIX OPERATING SYSTEM – ATTRIBUTES AND COMPONENTS ............................ 12

1.6 STARTING WITH UNIX............................................................................................. 14

1.7 CHANGING YOUR PASSWORD ................................................................................ 15

1.8 ENTERING COMMANDS IN THE UNIX SYSTEM ....................................................... 16

1.8.1 Command Options and Arguments ............................................................... 17

1.9 SUMMING UP ........................................................................................................... 17

1.10 ANSWERS TO THE SELF CHECK QUESTIONS ........................................................... 17

1.11 TERMINAL QUESTIONS............................................................................................. 18

1.12 REFERENCES .......................................................................................................... 18


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1. The UNIX Operating System – An Overview

Use and influence of computers has been steadily increasing in the last few decades. Today, computers play a pivotal role in all walks o f life. An operating

system (OS) is a core component of the computer system. An operating system lets a computer function as multi-user, multitasking and multithreading environment, thus

augmenting the power of the computer. UNIX is an operating system that offers its users all these capabilities along with numerous other features. In this lesson we will look upon the features and components of the UNIX system that make it very useful

and popular. In the subsequent lessons we will explore the features and components of UNIX in more details.

1.0 Objectives

After going through this lesson, you will be able to

Understand the concepts of the Operating System

Understand what is the UNIX Operating Systems

Understand the importance and popularity of UNIX Operating System

Understand how to start working on a UNIX machines

1.1 Introduction In the modern age, we have seen the computer doing wonders, from children

playing games to the scientists launching satellites; we can clearly see that the computers are playing a important role. It is the operating system that has made the computing in the modern world possible and efficient.

1.2 Introduction to the computers

Unlike calculator, a computer carries out user specified tasks. An inherent power provided by a computer is that it can be programmed to do variety of tasks. Computers are mostly general purpose computers in the sense that a

computer can be used to play a game and the same computer can be used to perform a circuit simulation.

A computer consists of hardware and software. A computer can be defined as a programmable machine which responds and executes a list of instructions.

These lists of instructions are called programs. The hardware components are the physical components and software is data or instruction.


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1.2.1 Typical hardware components of a computer

Hardware components in computer are what you can see and touch.

Memory: Enables the computer to store the temporary data and instructions. This is used in the computer during the execution of various instruction sets.

Mass storage devices: These are used for the bulk storage of data, such as, disk drives and tape drives.

Input devices: Interface to take the instructions from the user to the computer. Commonly used input devices are keyboard, mouse, web camera, etc.

Output Devices: Display the results of the instruction processing done by the computer. Commonly used are display monitors and the printers.

Central Processing Unit (CPU): The brain of the computer in which all the processing is done. It reads the data from memory or input and executes the instructions. CPU consists of ALU (Arithmetic Logic Unit) and CU (Control

Unit). ALU is responsible for all calculations and CU is responsible for getting instructions and data for execution.

Working with the hardware components alone is very difficult because their controls are very cryptic. Instead, software components are used to drive the

hardware components. The operating system is also one such software.

1.3 Operating System

An Operating System (OS) is an important program that runs on the computer. An operating system performs the very basic tasks, such as

recognizing inputs from the user, sending outputs to the display, keeping track of file and directories on the disk, and controlling the peripheral devices such as the disk drivers and printers.

While evaluating the following expression, the intermediate results are stored in memory Sum = 2 + 1 + 3 * 4


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The OS also works as a traffic cop - it makes sure that different program and

users running at the same time do not interfere with each other. The operating system is also responsible for security and blocking unauthorized users.

Operating systems can be classified as follows:

Multi-user: Allows multiple users to use computers at the same time.

Multiprocessing: Supports running parts of a program in parallel.

Multitasking: Allows multiple programs to run concurrently on a single CPU.

Multithreading: Allows different parts of a single program to run concurrently.

Operating systems provide a platform on which other programs, called

application programs, can run. The application programs must be written to run on a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs,

the popular operating systems are DOS, OS/2, Windows and Linux.

1.3.1 Virtual Memory

Programs that run on a computer may need more memory than what is

available physically on that computer. Many operating systems provide an illusion to the user of much larger memory. This is done by loading only partial

program and data in physical memory. Only the parts that are needed for current execution are brought into physical memory. So, bigger programs can be run even if physical memory is small.


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Self-Check Questions

1. A ____________ is a prerecorded set of instructions, which is executed b y the computer to perform some task.

2. A computer is a specific purpose machine that can not be tweaked to perform some other tasks. (True/False)

3. The operating systems keep the temperature inside the computer down, so that the functioning is proper. (True/False)

4. A ___________ system allows running parts of a program in parallel, on more

than one CPU. 5. In a _______________ system, a large number of users can use the system

concurrently. 6. The ____________ memory is an imaginary memory which is used by the

Operating System to get a larger address space.

1.4 UNIX Operating System

1.4.1 History of UNIX

The UNIX operating system found its beginnings in MULTICS, which stands for Multiplexed Operating and Computing System. The MULTICS project

began in the mid 1960s as a joint effort by General Electric, Massachusetts Institute for Technology and Bell Laboratories. In 1969 Bell Laboratories

pulled out of the project. One of Bell Laboratories people involved in the project was Ken Thompson.

He liked the potential MULTICS had, but felt it was too complex and that the same thing could be done in simpler way. In 1969 he wrote the first version of

UNIX, called UNICS. UNICS stood for Uniplexed Operating and Computing System. Although the operating system has changed, the name stuck and was eventually shortened to UNIX.

Ken Thompson teamed up with Dennis Ritchie, who wrote the first C compiler.

In 1973 they rewrote the UNIX core (called kernel) in C. The following year a version of UNIX known as the Fifth Edition was first licensed to universities. The Seventh Edition, released in 1978, served as a dividing point for two

divergent lines of UNIX development. These two branches are known as SVR4 (Release 4) and BSD.

Ken Thompson spent a year's sabbatical with the University of California at Berkeley. While there are two graduate students, Bill Joy and Chuck Haley,

wrote the first Berkeley version of UNIX, which was distributed to students. This resulted in the source code being worked on and developed by many

different people. The Berkeley version of UNIX is known as BSD, Berkeley


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Software Distribution. From BSD came the VI editor, C shell, virtual memory, Send mail, and support for TCP/IP.

1.4.2 Importance of UNIX

During past 25 years the UNIX OS has evolved into powerful, flexible, and versatile and robust operating system. It serves as the operating system for

variety of computers , for single user personal computers , engineering workstation , multi-user microcomputers, minicomputers, mainframes, super

computers and as well as special application devices . There are approximately 20 million machines now running UNIX and more than 100 million users, and this popularity and rapid growth is estimated to be

increased further. The success of UNIX is due to many factors including its portability to a wide range of machines, its adaptability and simplicity, the wide

range of tasks it can perform, its multi-user and multitasking nature, and its suitability for networking. What follows is a description of the features that have made UNIX system so popular.

Multi-user and Multitasking abilities

The UNIX OS allows the use of a single computer by many users. It is also a multitasking system that is it allows more than one application to be run on the same computer at the same time.

Powerful command set

The UNIX OS provides a consistent and powerful set of commands that has made it very useful particularly for the technical people .

Combining commands

The UNIX provides constructs like pipes and redirection of commands which

enables the user to create his own powerful utilities from UNIX commands.

Excellent environment for Networking

UNIX offers program and utilities that provide the services needed to build networked applications - the basis for distributed, networked computing. With

networked computing, information and processing is shared amongst different computers in a network. It is useful in client server computing where the

machines on the network can be client and servers at the same time. UNIX system is used as the base system for the development of the internet services and the growth of internet.

Portability

The UNIX system is far easy to be ported to new machines than other operating systems. The fact that, it is portable to almost any computer, results

from its being almost entirely written in C programming language.


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1.5 UNIX Operating System – Attributes and Components

The UNIX operating system is made up of several major components. Some

of these components are the commands, the file system, the shell, the kernel and the commands.


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The Commands and User Programs

UNIX provides a number of built-in commands and in addition user programs

can also run.

The File System

The basic unit that stores information in the UNIX system is called a file. The

UNIX file system provides a logical method of organizing files. Files are organized in a hierarchical file system where the fi les are grouped together in

a directory.

An important simplifying feature of the UNIX system is the way it treats the files. For example, physical devices are treated as fi les, this permits the same

command to work for an ordinary file or a device i.e. same command can be used to write to a file and printer.

The Shell and shell scripts

The shell is the command interpreter in the UNIX operating system. It reads the user specified commands and interprets them as requests to execute a program or a set of programs, which it then arrange to carry them out. Shell

also provides a programming language. Shell scripts are covered in subsequent chapters of this unit.

The kernel

The kernel is the core of the OS. The kernel interacts directly with the hardware through a set of programs called the device drivers that are built into

the kernel. It provides the set of services that can be used by the other programs; also it safeguards these programs from hardware layers. The major functions of the kernel are to maintain the file system, manage memory,

access control to the computer, and handle the interrupts (these are the signals to terminate the processes, ctrl + C is a common example)., error

handling, I/O handling which enables the computer interaction with the peripheral devices such as printers, monitors, storage devices, etc.).

Programs use kernel through the system calls. For example, if the user wants some file to be opened then the program generates a system call to open the

directories and then the files.

The figure below shows the relationship amongst various components of the

UNIX file system.

Example: Hierarchical File Structure /dtu/COE_Course/COE_101/schedule

Here ―dtu‖ is the parent directory which is in ‗/‘ root and other directories are in it


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Components of UNIX operating system (shown in gray).

Self-check Questions

7. UNIX is a multi-user OS and also possesses multitasking abilities. (True/False)

8. The first version of the UNIX Operating System was known as _____________. 9. The file system in a UNIX Operating System is a hierarchical structure.

(True/False) 10. The ____________ in a UNIX Operating System is used to interact with the

hardware and executes the user commands and program.

11. The command interpreter in the UNIX system is called ___________. 12. The programs in the UNIX systems interact using the __________ calls with the

kernel to perform the tasks.

1.6 Starting with UNIX

This section is dedicated to the learning of how to log into a UNIX system and how to change password on a UNIX system. We will touch the details of the

different types of system configurations and how we can log on to systems having these configurations.

Selecting a login

Every UNIX user on a multi-user system is recognized by a login name which is the only identity he has on the system. This is to be set before you use a multi-user or a single user UNIX system, to log onto the system.

UNIX provides excellent built-in security. Therefore no users are permitted

unless they are identified. For this identification, each user has a login ID.

The User Commands

The Shell

The Kernel

Hardware


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The login ID is typically allocated by an authority (known as the system

administrator). The system administrator is also responsible to add new users to the system and provide them a login name and an initial work enviro nment

and password on the computer. UNIX shows a login prompt initially. User needs to type-in his login ID. Then

the password prompt comes. After you correctly type in the password, you get logged into the system. The example below shows this process.

Connecting to the UNIX System

In a multi-user system you have to contact the system administrator as to how

you can connect to the system using your PC or terminal. Your PC can be directly wired to a computer or it can be connected via LAN.

Direct Connect - This is a method of connecting to UNIX machines when there is a single machine.

Dial-in Access - You can dial in to the UNIX network using a modem, use terminal emulators to get the UNIX prompt.

Local Area Network (LAN) - LAN is a client server model. Connect to the

server using the client workstation and use the UNIX capabilities. IP Networks

Using IP networks like internet one can connect to some remote machines using telnet capability of UNIX.

1.7 Changing Your Password Your password is very important information that you must not share with

anyone. You must change it regularly (say once in 2 months) and also should remember it (you must not write it on paper). Your password should contain 6 to 8 letters and should not simply be your name, your date of birth, etc. Your

password should also contain at least one non alphabet (maybe a number).

To change the password of your login you can use the passwd command.

bash> password

password: Changing password for sushobhit Old password:

New Password: Re-enter new password: bash>

login: akash

password: ―akash‖ is the user login name.

Note to keep password secure, it is not displayed when

you type it.


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There is a simple scheme to create complex passwords and still remember them! All you do is to take the first letters of a line of your favorite poem or

song and add a number or symbol to make a complex password. Here is an example: Say you pick the like ―Twinkle twinkle little star‖. Take the first

letters to makes a string Ttls. And suppose your favorite symbol is = (equal sign) and favorite number is 2 so you append these to the string to make your complex password as Ttls2=. You can see that for anyone else it will too hard

to find out while it is very easy for you to remember. NOTE: If you forget your password it cannot be retrieved even by the system

administrator. The only remedy in such cases is that the system administrator can reset the password.


13. ________________ is the program which is used to connect to the UNIX system

from a remote system. 14. ___________________ in a multi-user system is the person who is responsible

for maintaining the system.

15. Get the odd one out To connect to a UNIX system one of the following measures can be used

a. Dial-in access b. IP Networks c. LAN

d. System Calls 16. If you forget your password system administrator can give you permissions.

(True/False)

1.8 Entering Commands in the UNIX System

UNIX provides numerous commands. When the user types some command on UNIX prompt then the shell invokes the program for the command, the

command program can invoke many system calls, these calls then interacts with the hardware.


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1.8.1 Command Options and Arguments

UNIX system has a standardized command syntax that is applicable to almost

all the UNIX commands. Every command has some base functionality and additional functionality that are provided by the command line arguments.

For Example, the ls command can be used to list the contents of a directory.

Now let‘s use ls command with some option

This example shows the usage of – l argument of ls command, which outputs the long format of ls command.

Another command that is frequently used is ‗man‘ command. This is used to

displays the manuals of different commands.

1.9 Summing Up

An operating system is the most important software in any computer as it fills the communication gap between a user and the underlying hardware. UNIX operating system with its unique qualities and ease to adapt is a popular and

powerful operating system now days. In the chapters to follow we will explore the powers of UNIX in some details.

1.10 Answers to the self check questions

1. program

2. False 3. False 4. multitasking

5. multi-user 6. virtual memory

7. True 8. MULTICS 9. True

10. Shell 11. Shell

12. System calls

bash> ls –l

-rw-r--r-- 1 anmol friends 10777 Mar 30 16:26 README

-rw-r--r-- 1 achint friends 21483 Feb 28 17:39 2134.tar.gz drwxr-xr-x 2 amit friends 4096 Dec 12 16:41

game_scores drwx------ 3 arat friends 4096 May 10 2006

game_schedule

bash> ls

README 2134.tar.gz game_scores game_schedule


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13. telnet 14. system administrator

15. h 16. False

1.11 Terminal questions

1. List and expand briefly the components of the UNIX operating system.

2. What are the features of UNIX operating system that are the cause of its popularity amongst the users?

3. Explain briefly the possible modes to log onto a UNIX system

1.12 References

1. http://www.uwsg.iu.edu/usail/concepts/unixhx.html


LESSON 2 UNIX COMMAND

2. UNIX COMMANDS ......................................................................................................... 21

2.0 OBJECTIVES ............................................................................................................ 21

2.1 INTRODUCTION ........................................................................................................ 21

2.2 THE COMMANDS CLASS .......................................................................................... 21

2.3 CONNECTING TO UNIX ........................................................................................... 22

2.3.1 telnet command ................................................................................................ 22

2.3.2 rlogin command ................................................................................................ 22

2.4 FILE MANAGEMENT ................................................................................................. 22

2.4.1 mv command..................................................................................................... 23

2.4.2 cp command...................................................................................................... 23

2.4.3 rm command ..................................................................................................... 23

2.5 A COMMUNICATION RELATED COMMAND - FTP ....................................................... 23

2.6 INFORMATION .......................................................................................................... 24

2.6.1 man command .................................................................................................. 24

2.6.2 du – Disk usage ................................................................................................ 25

2.6.3 df – Disk free ..................................................................................................... 25

2.6.4 quota................................................................................................................... 25

2.6.5 who – Finding out who is logged on .............................................................. 25

2.7 PRINTING ................................................................................................................. 26

2.7.1 lpr – Printing ...................................................................................................... 26

2.7.2 lprm – Removing a printing job ...................................................................... 26

2.7.3 lpq – Checking the printing queue ................................................................. 26

2.8 PROCESS CONTROL................................................................................................ 26

2.8.1 ps – Finding the process ................................................................................. 26

2.8.2 & - Running process in background .............................................................. 27

2.8.3 Cntrl-z – Suspending a processes................................................................. 27

2.8.4 Jobs – Finding the process in background................................................... 27

2.8.5 Kill – Killing a process...................................................................................... 27

2.8.6 nice – reducing the priority of process .......................................................... 27

2.9 MISCELLANEOUS COMMANDS ................................................................................. 28


2.9.1 alias / unalias command.................................................................................. 28

2.9.2 cal (calendar) command.................................................................................. 28

2.9.3 clear command ................................................................................................. 28

2.9.4 crontab command............................................................................................. 28

2.9.5 csh command.................................................................................................... 28

2.9.6 history command .............................................................................................. 29

2.9.7 date command .................................................................................................. 29

2.9.8 echo command ................................................................................................. 29

2.9.9 grep command .................................................................................................. 29

2.9.10 unset command ................................................................................................ 29

2.9.11 tar command .................................................................................................... 29

2.9.12 tee command .................................................................................................... 29

2.9.13 touch command ................................................................................................ 29

2.10 SUMMING UP ........................................................................................................... 30

2.11 ANSWERS TO THE SELF-CHECK QUESTIONS ........................................................... 30

2.12 TERMINAL QUESTIONS ............................................................................................ 30


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2. Unix Commands

UNIX as any other operating system provides a set of commands to its users, using which, the users can perform the tasks they want. There is a huge variety of

commands that UNIX provides its user. In the present lesson we will discover and read about the usage of many of the commands in UNIX.

2.0 Objectives After going through this lesson, you will be able to

Use the UNIX commands to perform tasks

Understand how to send and receive mails on UNIX

Understand the file management basic command

Understand the information and communication system using the UNIX

2.1 Introduction UNIX provides a number of commands. For the ease of understanding we can

divide these commands into various categories.

2.2 The Commands class

UNIX commands can be grouped amongst few broader classes:

Starting and Ending These are the commands which are basically used to logon to the UNIX system, or to initiate working on to the UNIX system.

File Management

File is the basic data holding entity in the UNIX systems. There is a set of commands that can be used to maintain the file system so as to keep the data stored in the files, secured, updated and maintained.

Communication

UNIX provides communications at many levels, including mails, writing messages, exchanging files, etc.


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Information UNIX provides a number of commands to get information about the system

like who are logged in, how much disk space is available, etc.

Printing In UNIX user can give the print command and also can monitor the status of

the job or can remove the job if required from the queue.

Job and Process control

As there are lots of processes which are going on in a UNIX system, it is sometimes required to get the information related to the user jobs running on

the system. For this purpose UNIX provides a set of commands to monitor, kill, prioritize and resuming the jobs.

In the present chapter we will look at some of these commands in detail and the other commands will be discussed in the chapters to follow.

2.3 Connecting to UNIX Before we learn anything in details the very first thing we will look at is the

process that a user has to adopt to start with the UNIX system.

2.3.1 telnet command

The telnet command is used for logging into a remote system. The telnet

command presents the same login and password prompts as done on a local system.

2.3.2 rlogin command

The rlogin command is used to connect to a remote computer. It is comparatively easier to use then telnet. Here is the syntax of rlogin command:

rlogin [-l username] hostname

In this the username is taken by default the username of the current user. Hostname is the name of the UNIX machine that is to be logged on.

2.4 File Management A file is a basic data storage entity in a UNIX system. There is a set of

commands that can be used to maintain this system. We will be having an introductory flavor of these commands in this chapter with the complete discussion being taken up in the chapter on file system. Readers are advised

to have a look at the man pages of each of these commands and try to understand what exactly these commands are used for.


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2.4.1 mv command

The mv command moves a fi le. The command can also be used to rename a file. Here is a simple example of mv command.

2.4.2 cp command

The cp command copies a file. Here is a simple example of the cp command.

2.4.3 rm command

The rm command removes a file. Here is an example of the rm command.

2.5 A communication related command - ftp

The ftp (file transfer protocol) command is used for copying fi les from a remote computer to another computer. While mv and cp works on the same

system at a time you might need to get files from across systems at the same time ftp can be used for that.

In the example below we can see how ftp can be used to connect to a remote machine. In this example user ‗achint‘ gets file from machine mitserv.

bash> ls tempPresentation.txt bash> mv tempPresentation.txt

finalPresentation.txt bash> ls

finalPresentation.txt

bash> ls tempPresentation.txt bash> cp tempPresentation.txt

finalPresentation.txt bash> ls

tempPresentation.txt finalPresentation.txt

bash> ls tempPresentation.txt finalPresentation.txt bash> rm tempPresentation.txt

bash> ls finalPresentation.txt


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The ftp prompt provides few limited commands as listed below:

bin – Changes the file transfer type to support the binary image transfer.

get – Used to ‗get‘ the files from remote machine

mget- multiple get commands

ls – Used to list the contents of a directory on a remote machine

cd – Used to change directories on the remote machine

pwd – Used to get the present working directory on remote host

lpwd – Gives the current working directory in local host.

2.6 Information The information UNIX commands, regarding other users, disk quota and other things can be retrieved using some of the UNIX commands. In this section we

will be discussing about some of these commands.

2.6.1 man command

UNIX traditionally provides the manual pages (called ‗man‘ pages) for all the

built-in commands and for system calls.

You can learn a lot by referring to the manual pages for commands. The general syntax of the command is

man [-] [-k keywords] topic/command

The example below shows a part of the manual page of ‗du‘ command.

bash> ftp mitserv Connected to mitserv

Name: achint # User types his login id

31 Please specify the password. Password: # password will not be visible

230 Login successful. Remote system type is UNIX.

ftp> get myPresentation.txt # Now you are in ftp. See the prompt 250KB data transfer successful

ftp> quit bash> # You are out of ftp

now.

bash> man du


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2.6.2 du – Disk usage

This command is used to find out how much disk space is been occupied at present by the files and directories of the user.

2.6.3 df – Disk free

The df command tells how much disk space is left which can be used.

2.6.4 quota

This command is used for knowing as to how much disk space the files are

occupying on the file system. 2.6.5 who – Finding out who is logged on

The who command displays the information like the usernames, terminal IDs

and process IDs of other users and processes running on the computer.

General syntax of the command is: who [-q] [am i]

Following example shows the output of who command.


1. The commands below are used to connect to the remote computers: i. telnet

ii. rlogin iii. rm

2. It is not possible to logon to another machine with another username by any means. (True/False)

3. If some files are needed to be transferred from a remote location to the current

location, we can use the ________________ command for this purpose. 4. If a user needs to know the usage of the write command, he can use the

____________ command to know how the command works. 5. There is a restriction on the usage of the disk space by a user or a group on the

UNIX system and this disk space restriction can be found by using the command

_____________. 6. To know as to how much total disk space your fi les and directories have taken,

issue __________ command.

bash> who

singhs :0 May 28 14:05 achint pts/0 May 28 14:06 (lx-ptiwari:0.0) anmol pts/1 May 28 14:12 (lx-ptiwari:0.0)


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7. On a multi-user system, there are more than one people logged onto a machine and this sometimes chokes that machine off. To get in information as to who all

are logged onto the machine we can use ______________ command.

2.7 Printing

UNIX provides commands that for printing documents. Additionally, it is possible to control the printer queue and also to kill the processes if required to cancel the printing job.

2.7.1 lpr – Printing

This command can be used to print some text in a file. This is used to specify a printer otherwise it issues a print job to the default printer set by the user.

2.7.2 lprm – Removing a printing job

The lprm command can be used to cancel the print jobs that have been queued or printing. It can be used to cancel printing jobs on the specified

printer or to cancel the job on the default printer.

2.7.3 lpq – Checking the printing queue

This command shows the printer queue status on the named printer. Jobs

queued on the default destination will be shown if no printer or class is specified on the command-line.

2.8 Process Control When you run a program in UNIX, the program‘s copy starts to run. This

running program copy is called a process. The concept of process is fundamental to UNIX OS. So, you should find out and understand details about processes. If you run the same commands twice, each time a new

process is started.

Every process is identified by a unique process ID and this ID can be used to refer to this process or to perform any further operations on the process, like killing the process. We will have a look at the commands which can be used

to control the processes.

2.8.1 ps – Finding the process

This command is used to list all the processes being run on the machine.

bash> ps –ef PID PPID User Process …

233 230 achint ls –l 345 342 anmol ps –ef


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2.8.2 & - Running process in background

By put ‗&‘ at the end of any command, that command runs in the background. Time consuming commands can be put into background so that you can continue working on the same terminal.

2.8.3 Cntrl-z – Suspending a processes

If some command is by mistake issued and you want to suspend this command and do something else first. Then you can use Cntrl-z to suspend

this process and get the CPU free for some other more important work.

2.8.4 Jobs – Finding the process in background

To find the processes running in the background you can use the jobs

command. This is different from the ps command.

2.8.5 Kill – Killing a process

If some process is running for long time or is producing some unwanted

results you can use the ‗kill‘ command to kill the process.

The syntax of command is Kill [-signal] [process id]

Sometimes a process may still not get killed and you still want to kill it, you can send the -9 signal to kill it.

2.8.6 nice – reducing the priority of process

This command can be used to reduce the priority of a command and let other commands run earlier than the command.

The syntax of command is nice command [command option]


8. If a print job is fired it is not possible to abort the printing. (True/False)

9. To know as to what all are the print processes that are at the printer in queue, we can use ____________ command.

10. To print some text in a file, use ______________ command.

11. To change the priority of a job we can use the _________ command. 12. If some process is fired which is not required at the moment and we need to fire

another process, then we suspend the process using _______________ command and continue with the process later on.


28

13. If it is required to know the processes running on to the system then we will issue ______________ command.

2.9 Miscellaneous commands Besides the other commands that we have discussed in this lesson by now,

there are numerous other commands in UNIX with lots of options which can be used to perform some amazing tasks. We will be discussing some of these commands with useful and common options that are used. For other options

readers can refer the man pages of these commands.

2.9.1 alias / unalias command

To create or remove an alias for some command these commands are used.

The example shows the use

2.9.2 cal (calendar) command

This command displays the calendar.

2.9.3 clear command

This command clears the screen

2.9.4 crontab command

It is sometimes required to run some commands at a specific date and time. For this purpose ‗crontab‘ command can be used. See man crontab for see

details. The cron (see man cron) maintains a file which is managed using the crontab command. This file contains the information about the command and

the time and date of the execution of the command. Here is an example:

2.9.5 csh command

This command is used to run the C shell or to execute a C shell script.

The syntax for this command is csh [filename]

bash> alias rm ―rm –i‖

bash> unalias rm Creates an alias rm which calls rm –i

Now rm will call rm command

bash> crontab – l

0 0 * * 5 echo ―This is a cron‖ | mail john

Contents of crontab file.


29

2.9.6 history command

This command is used to list the commands that you have typed so far.

2.9.7 date command

This command prints the system date and time. The date command has many formatting arguments. See man date for details.

2.9.8 echo command

This command echoes back string given to it.

2.9.9 grep command

This command is used to search a pattern in a file. We will see more details

on grep command in subsequent chapters. Here is a simple example.

2.9.10 unset command

The unset commands removes a shell variable.

2.9.11 tar command

This command is used to create an archive of files or to extract files from an

existing archive. See man tar for details.

2.9.12 tee command

This command copies text from a pipe into a file. See man tee for details.

2.9.13 touch command

This command changes the date and time of a file without changing the files content. The touch command creates a file if not exiting.

bash> echo ―My name is achint‖

My name is achint

bash> grep goto file.c

/*You should not use goto in c programming */

bash> date Friday 25 Jan 2008


30


14. An ____________ is a short command or word that points at some path, or absolute command name.

15. To change the date and time stamp on a file without reading the file __________ command can be used.

16. To get the text from a pipe into a file ______ command can be used.

2.10 Summing Up UNIX provides a rich set of commands for file management, printing, process

control, etc.

2.11 Answers to the self-check questions

1. telnet, rlogin. 2. False.

3. ftp 4. man. 5. quota

6. du. 7. Who

8. False 9. lpq 10. enscript

11. nice 12. cntrl-Z

13. ps 14. alias 15. touch

16. tee

2.12 Terminal Questions

1. Define and explain the various command classes 2. How is communication handled in UNIX? What is FTP?

3. Describe how File Management is implemented in UNIX 4. List the commands and their usage for various commands used in process

control

5. Explain the various print commands in UNIX


LESSON 3 UNIX FILE SYSTEMS

3. UNIX FILE SYSTEM ....................................................................................................... 33

3.0 OBJECTIVES ............................................................................................................ 33

3.1 INTRODUCTION ........................................................................................................ 33

3.2 FILES ....................................................................................................................... 33

3.2.1 Filenames .......................................................................................................... 33

3.2.2 Filename Extensions ....................................................................................... 34

3.3 DIRECTORIES .......................................................................................................... 34

3.4 FILE TYPE ................................................................................................................ 34

3.4.1 Links ................................................................................................................... 35

3.4.2 Special Files ...................................................................................................... 35

3.5 PATH TO A FILE ........................................................................................................ 36

3.5.1 The root directory ............................................................................................. 36

3.5.2 Absolute Path.................................................................................................... 36

3.5.3 Relative Path ..................................................................................................... 36

3.6 MANIPULATING FILES .............................................................................................. 36

3.6.1 Moving and Renaming Files and Directories ............................................... 36

3.6.2 Copying files and directories .......................................................................... 36

3.6.3 Removing Files and Directories ..................................................................... 37

3.6.4 Creating a directory.......................................................................................... 37

3.6.5 Listing the files .................................................................................................. 37

3.7 FILE PERMISSIONS .................................................................................................. 38

3.7.1 File Permissions ............................................................................................... 38

3.7.2 Permissions for directories ............................................................................. 39

3.7.3 Changing the permissions on the file ............................................................ 39

3.8 CHANGING FILE OWNER AND GROUP .................................................................... 40

3.9 FILE SEARCH ........................................................................................................... 40

3.10 VIEWING BEGINNING AND END OF A FILE................................................................ 40

3.11 ANSWERS TO THE SELF CHECK QUESTIONS ........................................................... 41

3.12 TERMINAL QUESTIONS............................................................................................. 42

3.13 SUGGESTED READING MATERIAL ........................................................................... 42


33

3. UNIX File System

In the UNIX operating system the basic storage block is known as a file. This lesson focuses at understanding the concepts of file manipulation and handling.

3.0 Objectives


Understand the basic concepts of fi les and directories

Understand the paths and pathnames in UNIX systems

Understand the UNIX file types

Understand the basic UNIX commands related to the fi le system

Understand the file manipulation and file security

3.1 Introduction In a UNIX operating system the basic structure that stores data is known as a file. You can store data of any format in a file. Multiple files can be put

together in a directory. Apart from containing files, a directory can contain other directories as well. A directory that is inside another directory is called a

subdirectory. A file is analogous to a notebook. A directory is analogous to a bag that contains files.

3.2 Files A file contains a sequence of bytes stored on a storage device, such as a

disk. On the disk the file is not necessarily stored on a single sector but can be scattered on the disk The OS, keeps track of the information that belongs

to a specific sequence of data.

3.2.1 Filenames

Each file has a name. Any name can be given to a file. The name of a file can

be changed anytime. Unlike windows, UNIX fi le names do not contain spaces.

An important thing to remember here is UNIX is case sensitive. Which means

‗A‘ is different than ‗a‘, so one should be very careful while using the cases for separating the file names. So, myfile.txt and myFile.txt are different files.


34

3.2.2 Filename Extensions

UNIX does not enforce any specific extensions on file names. This is unlike

Windows where extensions are used to invoke applications directly. In UNIX you can choose any extension for your files. Even multiple extensions

are permitted (e.g.,data,tar.gz). Also files need not always have extensions (e.g., myFileOf24Dec2007).

Since it is possible to not give extensions, one can create files where extensions are misleading. For example, myProg.db may be a C program

while myData.cpp may be containing simple text data. Obviously this is not desirable and one must be careful in putting proper extensions.

Though UNIX itself does not enforce any extensions, there are many important uti lities/programs that expect a specific file extension. For example,

the C compiler expects files with .c or .h extensions.

3.3 Directories

Files are kept in directories. Directories are the groups of files in some logical structure totally dependent on the application and the user requirements. A

directory can contain files and other subdirectories. The figure below shows how the directory myData contains subdirectories which in turn contains the files.

Each directory in UNIX contains two special subdirectories: ./ (The dot directory) This indicates the current directory itself. ../ (The dot dot directory) indicates the parent directory of current directory.

3.4

myData/

Investmen

ts/

Official

/

RBI

Bonds

ICI

CI

Reports

Sal

es pla

n

custo

mers

bash> pwd

Investments bash>cd .. bash>pwd

myData

My name is achint

Shows current directory as Investments /

Current directory after cd .. is myData/ (the parent)


35

File Type

Regardless of the data contained in a file, UNIX associates a file type for each file. There are 4 file types - ordinary files, directories, links and special files.

Ordinary file is any file that you commonly use. These include text files, executable programs, shell scripts, etc. Also, we have already see what are

directories. Lets now see links and special files.

3.4.1 Links

A link is not a file but it is a second name to a file. Sometimes linking files is a

good option over copying because once copied, the copies can be changed differently. On the other hand if you create a link then there is actually only

one copy of the file. A link is created using the ln command of UNIX. There two types of links, soft link and hard link. See man ln for more details.

3.4.2 Special Files

UNIX represents even devices with files. These files are special files. For example, the audio output is typically /dev/audio file. What can you do with such a special file? Well, you can write into it or read from a special file and

UNIX hides the details on how it is actually working with the device. For example, you can simply cat a music file to /dev/audio and it will be played!


1. IT is possible to have multiple filename extensions in a file in UNIX. (True/False) 2. It is required to have a filename extension in a file in UNIX, which signifies the

properties of that file. (True/False) 3. Filename work and Work points to the same file in a UNIX file system.

(True/False) 4. Directories acts as a categorization structure of the data in a UNIX file system.

(True/False)

5. __________________ is a directory under the parent directory, which can be used for the categorization of data further down the hierarchical file structure.

6. Which is not a UNIX file type? a) Links b) Symbolic Links

c) Program files d) Directories

7. A ______________ (soft/hard) is only a text file that points to some other file somewhere in the file system and does not contains the data.


36

3.5 Path to a file

3.5.1 The root directory

UNIX OS treats the directory / as the root directory. The root directory is the

ultimate parent of all other directories on a UNIX system.

3.5.2 Absolute Path

Every file on a system has a path that starts from the root.

For example,

.

The pwd command always lists the absolute path.

3.5.3 Relative Path

When in a directory, if you know the relative position of a file, you need not

access that file using absolute path. You can simply use the relative path to the desired file as well. This is shown in an example below:

You can also access files using relative paths. For example,

3.6 Manipulating Files

The file manipulation operations are – file deletion, file renaming and moving files from one location to another.

3.6.1 Moving and Renaming Files and Directories

The mv command of UNIX moves files and directories to specified locations.

3.6.2 Copying files and directories

bash> pwd /dtu/It_Courses/IT_999

bash> ls ../IT-102/schedule.txt

This is the relative path of

―schedules.txt‖ with respect to ―/dtu/It_Courses/IT_999‖

bash> mv –i data data.old bash> mv –i data new bash> mv –i oldDir newDir

Moves data to data.old

Moves data into new/ directory

Moves oldDir to newDir

bash> pwd /dtu/IT_Courses/IT_101/schedules.txt

This is the absolute path to the ―schedules‖ file


37

The cp command of UNIX copies files and directories..

3.6.3 Removing Files and Directories

Often you want to fi les or some directory (including its contents). For example you may be cleaning your system. The rm command deletes files and

directories.

Be careful with rm command. A fi le or directory once deleted cannot e undeleted in UNIX. There is no such thing as trash can in UNIX. It is advisable

to use the –i option of rm command all the time. See man rm for details.

If a directory is empty, then it can be deleted using rmdir command. See man rmdir for details.

3.6.4 Creating a directory

The mkdir command creates a new directory.

3.6.5 Listing the files

The ls command of UNIX lists files and directories in the current directory. lt has a large number of other options (see man ls).

bash> rm file.txt my.txt

bash> rm –f file.txt bash> rm –r directory1

Removes specified

files. -f option indicates that rm will not give error even if file given to be deleted does not exist.

bash> mkdir project bash> mkdir /home/anmol/data

bash> mkdir ../../myDir

bash> cp old new bash> cp –R /home/joe/bread /home/jam/food

Copies file old to new. Overwrites new if exists.

Copies all files and subdirectories to the target

directory

Will create directory project/

Absolute path can be given to create a dir

Relative path can be given

-r option indicates delete all subdirectories as well.


38


8. The __________________ is the parent directory of all types of directories in the UNIX file system.

9. The name of file starting from the root directory is called the _____________

pathname of the file. 10. The relative pathname of a file is the name of the file with respect to the parent

directory. (True/False) 11. Pick the odd one out

Following operations can be performed on the file system

a) Building b) Listing

c) Renaming filenames d) Copying

12. On using the ‗mv‘ command from one file to an existing file it ___________

(appends/overwrites) the contents of the moved file onto existing file. 13. To copy one directory to the other it is mandatory to use the option _______ with

the command ‗cp‘. 14. Command ‗rmdir‘ can be used to delete the complete hierarchical directory

structure. (True/False)

3.7 File Permissions UNIX enforces permissions for fi les and directories. If you are the owner of a

file, you can put permissions whether the file should be readable by others or not, and so on. Lets see more details about file permissions.

3.7.1 File Permissions

The user of the UNIX file system can belong to three classes:

The owner of the file

The group which the file belongs to

Other users

bash> ls -l drwxr--r-- 1 achint editors 4096 drafts

-rw-r--r-- 1 achint editors 30405 edition-32 -r-xr-xr-x 1 achint editors 8460 final_draft

This field explains file permissions and file type the fields are explained in table below

achint is the file owner. editors is the group. Size is

8460 bytes


39

3.7.2 Permissions for directories

For the directories read permissions enables the user to list the contents of

the directory; Write permissions allows the users to create a fi le or a directory inside that directory and execute permissions allows to change the present

working directory to that directory.

3.7.3 Changing the permissions on the file

The chmod command changes the permissions for a file and directory. See

man chmod for details. There are several ways to change the permissions of a file. Here are few examples:

There is another form in which the permissions can be directly set for the files

by using an octal code. With three-digit octal notation, each numeral represents a different component of the permission set: user class, group class, and "others" class respectively.

For example, the number 764 in octal can be represented as following in

binary 111110100.

bash>chmod ug+rw sample bash> ls -ld sample

drw-rw---- 2 achint editor 96 Dec 8 12:53 sample

bash> chmod a-rwx sample bash> ls -l sample

---------- 2 amol editor 96 Dec 8 12:53 sample

Permits user and group to read and write

in file

Removes permissions for all

bash> ls -l drwxr--r-- 1 achint editors 4096 drafts

-rw-r--r-- 1 achint editors 30405 edition-32 -rwxr-xr-- 1 achint editors 8460 final_draft

-rwxr-xr--

First letter:

- means ordinary file d means

directory l means its a

link

These 3 letters indicates file readable, writable

and can be executed by the owner.

These 3 indicate

group people can read/execute but

cannot write into this file

These 3 indicates others can only read this file.


40

The first octal digit when converted to binary represents the permissions for owner (7 in octal is 111 in binary which implies rwx for owner).

The next octal digit when converted to binary represents the permissions for the group (6 in octal is 110 in binary which implies rw- for group).

The last octal digit when converted to binary represents the permissions for the others (4 in octal is 100 in binary which implies r-- for other).

3.8 Changing File Owner and Group The chown command changes the owner of a file. See man chown for details.

The chgrp command changes the group of a file. See man chgrp for details.

3.9 File Search

The find command helps in locating files and directories. This is a powerful command and has lots of options. See man find for details. Here is the syntax

of the find command.

The find command searches through the contents of one or more directories including all of their subdirectories.

Another example in which same file name is searched in two directories:

.

3.10 Viewing Beginning and End of a file

UNIX provides commands using which it is possible to display the contents of the start or end of the file. These are head and tail commands.

head – Start of the file tail – end of the file

bash> find / -name schedule -print /dtu/IT_courses/IT_101/schedule

/dtu/IT_courses/IT_102/schedule

bash> find . –type d –name abc -print

Finds all the files in ‗/‘ named

schedule

Finds ‗directory‘ abc and not file in the present directory

find search_directory –name file_name [-print]


41

Example usage


15. Pick the odd one out The users in a UNIX file system can be categorized as:

a) Owners b) Group c) Friends

d) Other users 16. To change the fi le permissions from one set to another, the command

___________ can be used. 17. __________________ command is used to change the owner and the group of

the file.

18. The _______ command lets you search for files and directories. 19. The _______ command will be useful to show the last few lines of a file.


1. True 2. False

3. False 4. True

5. Subdirectory 6. Program files. 7. Soft link

8. Root. 9. Absolute path..

10. True 11. Building 12. overwrites.

13. –r 14. False

15. Friends 16. Chmod 17. Chown, chgrp

18. Find 19. tail

bash> head –n 10 file

Shows the 10 starting lines of ‗file‘


42

3.12 Terminal questions

1. Write a detailed note about the hierarchical file structure. 2. Explain briefly the manipulating operations possible on the file structure

3. Write a brief note on the permissions on the files and directories in UNIX. Also, explain how we can change permissions of the files in UNIX using the chmod command. Use some relevant examples to explain the concepts.

4. Explain the UNIX system fi le types, also explain the salient features of each file type

3.13 Suggested Reading Material

1. Unix Programming Environment, by Kernighan and Pike.

2. Design of Unix Operating System, by Maurice J. Bach


LESSON 4 THE VI TEXT EDITOR

4. THE VI TEXT EDITOR.................................................................................................... 45

4.0 OBJECTIVES ............................................................................................................ 45

4.1 INTRODUCTION ........................................................................................................ 45

4.2 FILES CONTAIN STREAM OF CHARACTERS .............................................................. 45

4.3 HOW VI HANDLES THE FILES ................................................................................. 46

4.4 INVOKING VI ............................................................................................................. 46

4.5 MODES OF VI ........................................................................................................... 46

4.5.1 Command mode ............................................................................................... 46

4.5.2 Edit mode........................................................................................................... 46

4.5.3 Switching between command mode and edit mode ................................... 47

4.6 POSITIONING TEXT ON THE SCREEN ...................................................................... 47

4.6.1 Scrolling and moving the Screen ................................................................... 47

4.6.2 The GOTO Command ..................................................................................... 48

4.6.3 Searching........................................................................................................... 48

4.7 POSITIONING THE CURSOR : H, L, J, K COMMANDS................................................. 48

4.8 EDITING USING SCOPES .......................................................................................... 49

4.8.1 Delete Text (d, D) ............................................................................................. 50

4.8.2 Change Text (c, C) ........................................................................................... 50

4.8.3 Replace Command (r, R) ................................................................................ 50

4.8.4 Erase Command (x, X) .................................................................................... 51

4.8.5 Undo Command (u, U) .................................................................................... 51

4.9 TEXT INSERTION...................................................................................................... 51

4.9.1 Append Command (a, A) ................................................................................ 51

4.9.2 Insert Command (i, I) ....................................................................................... 52

4.9.3 Open Command (o, O) .................................................................................... 52

4.9.4 Read Command (:r) ......................................................................................... 52

4.10 GLOBAL SEARCH AND REPLACE FOR TEXT ............................................................ 52

4.11 REARRANGING AND DUPLICATING TEXT................................................................. 53

4.11.1 Copying Text and Moving the Copy .............................................................. 53

4.11.2 Deleting Text and Moving It ............................................................................ 54


4.12 NAMED BUFFERS .................................................................................................... 54

4.12.1 Using the named buffers ................................................................................. 55

4.13 MISCELLANEOUS INFORMATION.............................................................................. 56

4.13.1 Creating Line Numbers ................................................................................... 56

4.13.2 Lines and Sentences in VI .............................................................................. 56

4.13.3 Joining Lines ..................................................................................................... 57

4.13.4 Repeating a Command ................................................................................... 57

4.13.5 Editing Multiple Files Using vi......................................................................... 57

4.13.6 Mark Command ................................................................................................ 58

4.14 SAVING OR STORING A FILE.................................................................................... 58

4.14.1 Writing to the file ............................................................................................... 59

4.14.2 Exiting the vi editor ........................................................................................... 59

4.15 SUMMING UP ........................................................................................................... 60

4.16 ANSWERS TO THE SELF-CHECK QUESTIONS ........................................................... 60



45

4. The VI Text Editor

When you write programs, scripts or modify data, write mails, etc., you will need to use text editor. This lesson focuses on the VI text editor; one of the most commonly

used text editors in UNIX systems.

4.0 Objectives


Understand how to open and edit files using vi

Understand various text insertion and deletion methods in vi

Understand the basic structure of vi text editor

Understand the commands to edit text using vi and scopes

Understand miscellaneous other features of vi

4.1 Introduction vi is a visual, non-graphical and interactive text editor which allows a user to

create, modify, and store files on the computer.

Note that in this chapter, the cursor is shown by putting an underscore for a character. For example: The cursor is at the letter ‗n‘ in the following line. This is a line.

There's an editor out there that programmers have been using to edit their programs for the last 24 years. It's called vi (say vee-eye) and it is it is quite

powerful. http://www.websiterepairguy.com/articles/vi/12_learn_vi.html

4.2 Files contain stream of characters When you type characters or numbers, etc. each key goes as an ASCII

character. For example, ‗a‘ gets recorded as ASCII 97. When you write lines like these

This is line 1 This is line 2

These lines are stored as a stream of characters like ―This is line 1 \nThis is line 2‖. Here the \n is a special character which signifies a new line.

http://www.websiterepairguy.com/articles/vi/12_learn_vi.html


46

4.3 How Vi Handles The Files

When you open a file in vi, the file contents are read into a buffer. All text editing jobs are done in memory as the buffer. The file on the disk is not

updated unless vi is explicitly asked to save the changes. This gives an option to change the content of the buffer until you are not satisfied without changing the file on the disk.

4.4 Invoking vi

The vi editor can be invoked using the following command

The figure below shows how the file looks when opened in vi.

4.5 Modes of vi

vi has two modes in which you will work.

4.5.1 Command mode

The command mode is the default mode. All vi commands work only in the

command mode. In the command mode you cannot write text. You can only move around in the text, delete text, modify existing text, search for text, etc.

4.5.2 Edit mode

In edit mode you can add new text in vi. In edit mode you cannot use any commands to search or navigate in the text.

~

~ ~

~ . .

―myfile‖ [new file]

The cursor

Tile(~) in vi represents an

empty line.

File information

$ vi demo.txt


47

4.5.3 Switching between command mode and edit mode

When in command mode, few commands take you to edit mode. For

example, in the command mode, if you press i, you will get to the edit mode and can add text.

When in the edit mode, you can stop editing further and go to the command mode by pressing the <Esc> key.

4.6 Positioning Text on the Screen

vi provides several ways to reach the text you want to edit in a fi le.

4.6.1 Scrolling and moving the Screen

By scrolling the screen we can reach the text desired. The table below

explains how one can scroll the screen.

Command Resulting Action

Cntrl+u Moves window upwards one complete screen

Cntrl+d Moves window downwards one complete screen

H Takes cursor to the top of the screen

L Takes the cursor to the bottom of the screen

M Takes the cursor to the middle of the screen

All these commands work only in the command mode.

Cursor is at same position

but edit mode has started now press ‗d‘

Press

‗esc‘ Now you are in command mode

You are in command mode and cursor is at ‘a’.

press ‗i‘

This is a line

This is a line

This is da

line

This is da line

Cursor is at letter ‗a‘ and

letter ‗d‘ is added.


48

4.6.2 The GOTO Command

Sometimes you already know the line number where you want to reach. You

can use the GOTO in such cases. The table below explains the command and the resulting action.


G Moves cursor to the last line

<N>G Like 33G

Moves the cursor to the Nth line

:<N> Like :65

Moves the cursor to the Nth line

4.6.3 Searching

It is also possible to search for a pattern and by this the screen will be moved to the occurrences of the desired pattern.

Here are the commands that work for search in vi..


‗/pattern‘ Searches the pattern forward from current

cursor position

‗?pattern‘ Searches the pattern backward from current cursor position

:set ic This makes the subsequent searches case

insensitive (ic in set ic stands for ignore case)

:set noic This makes the subsequent searches case sensitive

Once you start a search you can repeat the search in a simple way. On keying in ‗n‘ vi goes to the next instance of pattern in the file and using ‗N‘ it searches in opposite direction.

4.7 Positioning the Cursor : h, l, j, k commands

This section explains finer control of the cursor.

You can move the cursor by use of "arrow" keys. You can also use the

"direction" keys "h" (move left by one character), "j" (move down to next lined), "k" (move up to previous line), and "l" (move right by one character).

The "RETURN" key is similar to the "j" key in that it moves the cursor down

one line. However, the "RETURN" key always positions the cursor at the

beginning of the next line; whereas, the "j" key moves the cursor straight down from its present position, which may be the middle of a line. Moving several

spaces may be accomplished by repeatedly pressing the "RETURN", direction


49

or arrow key; such as, "k" "k" "k" to move upward 3 lines. You can also precede any of these keys with a number and achieve the same results, "3k".


1. If in a file cursor is resting at the 34 line and it is desired to be placed onto the 74 line then the command that is to be issued is _____________G.

2. On searching with ―?‖ and ―/‖, the search respectively will be done ______________ and ____________________. (backwards/forward).

3. To get the file statistics using the VI editor the command required to be issued is

___________. 4. On keying in ―N‖ while searching for a pattern using ―?‖ the cursor will reach the

next instance of the pattern ________________. (backward/forward) 5. To move to the 25 word in the line while the cursor is on 18 line the command

that can be issued is ___________.

6. To move to the beginning of the line on which the cursor is residing in a text file the command that can be issued is __________.

7. The vi editor sets or creates a temporary buffer area while editing a file which is stored on the disk and is used later on for the reference purpose by the editor. (True/False)

4.8 Editing using scopes vi commands have scope built into them. For example, when you say ‗dd‘

then first ‗d‘ indicates the delete operations and the second ‗d‘ tells it to apply the command on a line. Similarly, ‗yy‘ yanks a line. But the commands like ‗d‘ and ‗y‘ can be given a scope and VI commands also have upper case

versions.

Scope Text Unit Encompassed

0 Beginning of line

$ End of line

W w Word right

B b Word left

E e End of word right

With the scopes we can use the operators to get more powerful outcomes. We can further do editing very much locally using the combination of the operators and scopes. In this section we will discuss this combination.


50

4.8.1 Delete Text (d, D)

The delete command is used in command mode to remove portions of text

from the file being edited. The scope must be specified after the delete operator. Some of the most common scopes used with the delete operator shown in the next table.

Delete operator

and scope

Resulting Action

dw Delete word forward

D( Delete complete sentence backward

d) Delete complete sentence forward

dG Delete from current line to end of file

dL Delete from current line to end of screen

d/^xyz Delete from current line to first occurrence of

pattern

dtx Delete from current place to first occurrence of ‗x‘

NOTE: The same scope prefixes can be used with all the scoped text editing

commands so we will not discuss them with any further commands b ut different scopes or operators, if any will be discussed.

NOTE: It is important to remember that the current cursor position serves as the starting point for the scope. This means if you do scoped deletion, it wi ll

happen starting from the current point. For example, typing "2dd" will delete two consecutive lines beginning with the current line.

4.8.2 Change Text (c, C)

You can use the change command to change the text in a line. Scopes are applied in the same manner as they are used with the delete command.

On issuing the change text command, vi gets into the edit mode and after the text insertion on issuing the <ESC> key it returns to the command mode. The

example shows how change command can be used.

4.8.3 Replace Command (r, R)

The replace command is used to replace portions of text on the screen. The

table shows the two variants of the replace command and their usage for replacing text.

This is the line to watch

This is new line to watch

Cursor is positioned

at‗t‘

Text inserted in place of

two words

On issuing the command ‗2cw‘ or change two words

and keying in ―new line‖


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Replace command

Text replacing action

r Used to replace a single character at a time

R Used to replace as many characters as there are keystroke until user issue <ESC>

4.8.4 Erase Command (x, X)

The erase command removes a character.

Erase Command

Erase Action

x Erase character on which cursor is placed

X Erase character left to cursor

4.8.5 Undo Command (u, U)

Undo command reverses the effect of the editing operations done on a file.

‗u‘ reverses the effect of last editing command whereas ‗U‘ reverses the effect of all the editing operations on the file since last save.

4.9 Text Insertion vi editor provides several ways to insert the text in the file. We will be discussing each of these methods in some detail but it is advisable for a newly

inducted candidate to take up one approach and use that to insert the text. 4.9.1 Append Command (a, A)

It is used to add to the existing text. It has two forms ‗a‘ and ‗A‘. These two

forms are explained in the figure below.

This is the line to watch out for.

This is the mine to watch out for.

This is the kite to watch out for.

Cursor positioned at ‗l‘ On issuing ‗r‘ command and typing ‗m‘

‘l‘ is replaced by ‗m‘

On issuing ‗R‘ command, keying in ―kite‖ and <ESC> Complete word is

replaced

The student laughed.

The students laughed.

The students laughed. Aloud.

On issuing ‗a‘ command and typing ‗s‘ and <ESC>

Text appended after the cursor


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4.9.2 Insert Command (i, I)

This command is used to insert the text into a text file. This command has two

forms ‗i‘ and ‗ I‘. In the figure below it is explained how to use this command.

4.9.3 Open Command (o, O)

Open command opens a new line to add text. This has two forms ‗o‘ and ‗O‘,

in the figure below the usage is explained.

4.9.4 Read Command (:r)

The read command is allows the user to copy of another file into the current file. While in command mode and with the cursor on the line above where you want the special file read in, type:

4.10 Global Search and Replace for text

:r <File>

Reads the file specified at cursor location in the current file

Text appended at end of line.


The new student laughed.

Again The student laughed.

On issuing ‗i‘ command and typing ‗new

‘and <ESC>

On issuing ‗ I‘ command and typing

‗Again‘and<ESC>

Text inserted before the

cursor

Text appended in the beginning of line.


A new line is added


A new line added

The student laughed. Another line

On issuing ‘O’ command and typing ‘A new line is added’ and

ESC>

On issuing ‘o’ command and typing ‘Another line ’ and <ESC>

Text inserted above the current line

Text appended in the beginning of the line.


53

The example below shows different commands that can be used for searching and replacing with different purpose.


8. To delete the word on which the cursor is placed ―D‖ command can be issued.

(True/False) 9. The change operator invokes the text insertion mode. (True/False). 10. The operator _______________ changes the text, yet does that in command

mode and not in text insertion mode. 11. The command ______________ replaces the characters on screen one at a time

as the user keys in the new characters. 12. To erase the character on which the cursor is place __________ command is to

be issued, whereas to delete the character prior to the character (left) on which

the cursor is placed _________ command needs to be issued. 13. To replace the name ―shahs‖ with ―mazes‖ in a text file the command to be issued

is ___________.

4.11 Rearranging and Duplicating Text

You can yank text for copying it at another place in the text fi le.

4.11.1 Copying Text and Moving the Copy

Step 1: Copying Text with the Yank Command (y, Y)

The yank command ‗y‘ can be used with the scopes and similar scopes can be used as we have seen in delete command. Yanking places the yanked

content into an unnamed buffer. Some of the examples of yanking are:

:1,$s/oldText/newText/g

:1,15s/oldText/newText/g

:g/oldText/s//newText/gc

This command replaces all the instances of oldText with

newText in the file This command replaces

oldText with newText from line number 1 to 15

This command asks before replacing text each time

This is the line to be yanked.

This is the line to be yanked This is another line to yank

This is yet another line that can be yanked

cursor is character ‗l‘

On issuing the command ‗3yw‘ which means yank

3 words, it yanks 3 words starting from

current cursor position cursor is at first line

Issuing command ‗3yy‘ will

yank 3 lines starting from current line


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Step 2: Put Command (p, P)

The put command is used to place the contents of the unnamed buffer back into the file being edited. Returning whole lines into the text is handled

differently than word and sentence fragments. The lower-case "p" places the line or lines below the current line and the

upper-case "P" places them above the current line.

A handy feature of yank & put is the ability to insert copy repeatedly within the same file. The format for this action is yank, relocate cursor, put, relocate cursor, put, etc. until all needed copies have been placed.

4.11.2 Deleting Text and Moving It

When you delete a text, it gets yanked and thus it can be used to put in another place in the text.

4.12 Named Buffers Named buffers offer another way to copy (yank) or remove (delete) text.

The unnamed buffer only saves the last deleted or yanked text. vi provides 26 named buffers (a-z) are created for your use. Named buffers allow users to

yank multiple text and put them at different places.

These named buffers remain only for the life of the current editing session.

Once you quit vi, these buffers are no longer available.

Here are few examples of how named buffers are used.

Typing "g7yy in command mode, implies the following:

Quote (―) calls for a named buffer ―g gets the buffer named g

7yy implies yanking 7 lines into the named buffer g.

This is the file. It contains text.

This line will be deleted. Below this it wi ll be later

on pasted. This will be the end of file. This is the file. It contains text.

Below this it wi ll be later on pasted.

This line will be deleted. This will be the end of file

This line will be deleted

using ‗dd‘ command.

Currently cursor is placed on this

line

On using the ‗p‘ command the line is placed

below the present cursor

position


55

Now, if you type ―gp, it implies the following:

―g calls for the named buffer g ―gp implies paste the contents of the named buffer g.

You can append more information into a named buffer. When you use the capital letter to yank into a named buffer, the yanked contents are appended

into the named buffer. For example ―g7yy yanks 7 lines into buffer g, now ―G3yy would yank and append the 3 lines after the already yanked 7 lines into

the buffer g. These named buffers are not write-protected. If a named buffer contains

information and it is called a second time with its lower-case name, the original material is over-written.

4.12.1 Using the named buffers

Once you yank contents into a named buffer g, you can paste it anywhere in the file. If you type ―gp, it implies the following:

―g calls for the named buffer g ―gp implies paste the contents of the named buffer g.

p putting the contents below the current line P putting the contents above the current line

It is important to note that VI editor will not tell you which all buffers are defined currently also it cannot tell you which buffer contain what; you must

remember the names of the buffers and what all contents they have.


14. 1To copy 10 lines of text into an unnamed buffer 10_____ command can be used. (Y/y)

15. The text saved in an unnamed buffer created by yanking or deleting can be

placed back into the text below the current line where the cursor is placed by using _________ command.

16. To append 5 more lines to the named buffer ‗a‘, the command to be issued is__________.

17. If a named buffer is called upon again and new information is written into it then

the new information is appended to the buffer. (True/False) 18. It is possible to get the buffer name on the basis of the content stored in the

buffer. (True/False)


56

4.13 Miscellaneous Information

In this section we will discuss about some miscellaneous information which can be used to be more productive in editing the files.

4.13.1 Creating Line Numbers

In vi editor by default the line numbers are not shown. But vi editor allows the line number view. Command for this is:

:%nu

Sometimes depending upon the requirements it is desired that the line

numbers are seen only for the current session. To have line numbers inserted for the current session, type:

:set number Immediately you will see the line numbers appear in your file and they will

remain until you exit the editor or type: :set nonu

The "control s" command stops screen movement. The "control q" command releases frozen screen.

The ―control l‖ command refreshes vi screen without modifying the file.

The .exrc file

There are many setup (set) commands that can be set or changed for vi. It is advisable to put these commands into the ~/.exrc file so that every time vi

automatically loads these settings.

For example:

The following command will show you the available setup commands.

:set all

4.13.2 Lines and Sentences in VI

To be successful in your editing, it is necessary to understand what the editor

considers a line and a sentence. Just for clarity, a line and a sentence are different items to the editor. To the editor, a line begins on the left of a screen and terminates at a carriage return. The carriage return is the invisible

character placed in your file every time you press the "RETURN" key. A sentence to the editor is a string of characters of unspecified length (a few

characters to many lines) terminating with the punctuation marks ―.‖, ―?‖, ―!‖ followed by either a carriage return or two blank spaces.

bash> cat ~/.exrc set nu # Show line numbers

set nows # Do not wrap file while searching. bash>


57

4.13.3 Joining Lines

As you are editing files, you will find it is desirable to combine or join lines. This is easily done using the "J" (join) command. An illustration of joining lines

is given below. The cursor is located on the top line when the "J" command is issued. vi will move the lower line and butt it to the end of the upper line. The editor takes care of necessary spacing for you.

4.13.4 Repeating a Command

To make life a bit easier, vi allows text alteration commands to be repeated by using the ―.‖ (Repeat) command. A handy way to illustrate the repeat

command is with the “cw” command replacing a single word with two new

words throughout a paragraph. In this example, the first occurrence of ―PU‖ is located with the search

command PU”. Then with the cursor on the ―P‖ of ―PU‖, the ―cw” command is issued followed with ―Purdue University‖ and the ―ESC”. The „n ‟ key is pressed to find the next occurrence of ―PU‖. The cursor relocates on the ‗P‘ of the next ―PU‖ and all that is required to change it to ―Purdue University‖ is to type ―.‖

4.13.5 Editing Multiple Files Using vi

The vi editor provides a feature which allows a user to edit multiple files by use of the ":e" (edit) command. This ability to access multiple files without

leaving the editor permits a user to see information in another file without exiting the editor. Additionally, because files are opened within the same

editor invocation they can share the same named buffers, thereby making the transfer of text possible between the files. When vi is invoked, a work area called a buffer is created for editing purposes. It is into this work space that a

copy of a specified disk file is placed. The editor permits only one file copy in


58

this buffer space at a time. Thus after making changes to a file (delete, add, or change), you must inform the editor what you wish done to the current buffer

contents before you will be permitted to bring another file into this space. You do this by use of the ":w" (write current buffer contents to opened file), ":e!\

newfile" (toss current buffer contents, no update to opened file, and place a copy of newly called file in buffer), or ":quit!" (Exit editor and toss buffer and buffer contents).

When you have two files open, VI permits toggling between files by use of ":e\

#". This works because whenever VI sees the character "#" used in a command where a filename is expected, it substitutes the "#" with the name of the previous file.

For example if you had been in fruits then opened vegetables, the command

":e #" would return you to where you were in the fruits file. Repeat ":e #" and you would be back in vegetables.

4.13.6 Mark Command

The mark command sets up a mark in vi and while editing you can go back to the places where you had placed these marks. vi provides 26 marks which are named ‗a‘ to ‗z‘.

You can put a mark ―g‖ in a position using a command like the following:

mg Note that the marks are not visible at all in vi. You have to remember the marks that you have put. To go back to the marked location ―g‖, use the

following command: ‗g

4.14 Saving or Storing a File As mentioned earlier, the VI text editor creates a temporary working area

which can be a copy of the existing file on the disk or a new file. This area is at the disposal of the user until he saves the file. On saving the file, the buffer is removed from storage and changes saved on to the file which gets stored

on the disk. Disk storage on the other hand gets removed with the remove command of UNIX.

The changes made in the buffer are not saved until you specify the command to do so, thus it is advisable to keep on saving the work periodically. We will

discuss how to save our work periodically. Below is a schematic showing how the work is saved on the disk.


59

4.14.1 Writing to the file

It is useful and safe to save the work periodically when typing text. The ‗:w‘

command writes the buffer to the file on the disk thus saving the changes. This works in the command mode.

4.14.2 Exiting the vi editor

To exit the vi editor you can use the quit command ‗:q‘. This command in

conjunction with write command leads to ‗:wq‘ (write and quit). To discard the changes made you can use ‗:q!‖.


19. The text insertion command takes the VI control from command mode to text

insertion mode. (True/False) 20. If some text is required to be added to the current text, such that the new inserted

text is added in the end of the line on which cursor is positioned then text insertion is invoked with the command ____________.

21. If in some application it is required that the same piece of text from one text file is

to be inserted in another text file, user can use the command _______________. 22. When using text insertion command read ‗:r‘, to switch back to the command

mode from text insertion mode the ESC key can be used. (True/False) 23. On issuing the write command once in the complete session we ensure that in

that all the text inserted in the session, includi ng the text inserted after the write

command is issued, is saved. (True/False) 24. If we need to store the editing work done in the editor, the command

___________ is needed to be issued. 25. If one finds out that he does not need the text he has inserted into the editor

window in the present session, then he is required to issue ____________

command. 26. In some application it is required to create a file ‗new‘ from a file ‗old‘ with some

new text and the file ‗old‘ needs to be kept unchanged. The VI commands that

:w <File>

Saves the changes done in the <file>


60

should be issued for writing the new changes is __________________ and exiting the VI session is ____________.

27. The VI editor can operate in two modes. The mode which can let the user change the text in the file is _____________________ mode.

4.15 Summing Up In this chapter we have looked upon Vi text editor quantitatively. We discussed a lot of techniques and viewed examples that can help you in

editing text files very efficiently. With these techniques at hand you will be able to learn other advanced techniques, when you work in actual

environment and situations.

4.16 Answers to self-check questions

1. 74G. 2. backwards and forward.

3. cntrl-g 4. forward 5. 7w

6. 0 (zero) 7. False. 8. False.

9. False. 10. r

11. R. 12. x, X. 13. : g/shahs/s///xyz/g

14. 10yy 15. p

16. ―A5yy 17. False 18. False

19. True 20. A

21. :r 22. True 23. True

24. :w. 25. :q!

26. :w <new>, :q! 27. Edit.


61


1. Explain the processes that are used for changing the text using the VI text

editor 2. Explain the processes that can be used to delete the text using the VI text

editor

3. Write a note about the named buffers and also explain some usage with practical examples

Write briefly about the rearranging and duplicating of text in the VI text 4. Explain how the VI editor functions 5. What are the different modes for operating VI Editor? Explain in brief

6. Explain the append, insert and quit modes of operation of VI editor.

UNIT 2: SHELL SCRIPTING

1: INTRODUCTION TO SHELL ............................................................................... 67

2. SHELL SCRIPTING AND DEBUGGING ........................................................ 85

3. CONDITIONAL STATEMENTS ........................................................................ 101

4. REPETITIVE TASKS ............................................................................................. 113

5. REGULAR EXPRESSIONS................................................................................ 133


LESSON 1 INTRODUCTION TO SHELL

1: INTRODUCTION TO SHELL ........................................................................................ 67

1.1 INTRODUCTION ........................................................................................................ 67

1.2 THE SHELL: COMMAND PROCESSOR ..................................................................... 67

1.3 BASH: BOURNE AGAIN SHELL ............................................................................... 68

1.3.1 Advantages of BASH ....................................................................................... 69

1.4 REDIRECTION .......................................................................................................... 69

1.4.1 Standard Output ............................................................................................... 70

1.4.2 Standard Input .................................................................................................. 71

1.4.3 Standard Error .................................................................................................. 71

1.4.4 Combining Streams ......................................................................................... 72

1.5 VARIABLES .............................................................................................................. 75

1.5.1 Setting strings with the variable names having $ ........................................ 75

1.5.2 Types of variables ............................................................................................ 76

1.5.3 Exporting variables........................................................................................... 76

1.5.4 Using Shell Variables....................................................................................... 77

1.6 COMMAND SUBSTITUTION....................................................................................... 78

1.7 PATTERN MATCHING – THE WILD CARDS .............................................................. 78

1.7.1 The * & ? ............................................................................................................ 79

1.8 THE CHARACTER CLASS......................................................................................... 79

1.9 MATCHING A DOT (.) ................................................................................................ 80

1.10 SUMMING UP ........................................................................................................... 81

1.11 ANSWERS TO THE SELF-CHECK QUESTIONS ......................................................... 81



67

1. Introduction to Shell

The starting point for the unit on Shell Scripting is to first know about Shell. Bash is also introduced in this chapter. In the subsequent lessons further details pertaining to advanced concepts are discussed at length.

1.0 Objectives After going through this lesson, you will be able to:

Know about different types of shell

See how the shell executes commands

Understand and use Redirection, Variables, Pattern matching etc.

1.1 Introduction

The Shell in UNIX is the program which acts as an interface between the user and UNIX system. It understands the user language, interprets it and tells the

kernel what user wants, gets the results of the command execution from the kernel and gets back to the user with the results which he understands. All the wonderful things that we can perform or do using the UNIX system is due

to the virtue of this program, which can understand so less code and execute the commands and user instruction effectively. Shell can also be known as a

command processor it processes the instructions you issue to the machine.

1.2 The Shell: Command Processor

On logging onto the UNIX system you encounter a prompt ($ or % or any user custom prompt). Apparently though it seems that nothing is happening, but a program is running which is waiting for your instructions to execute them, this

is SHELL. When a user logon the shell starts functioning and keeps on doing that unti l the user logs out.

When you issue a command, the shell is the first agency to acquire the information.It accepts and interprets user requests; these are generally the

UNIX commands we key in. The shell examines and rebuilds the command line and then leaves the execution work to the kernel. The kernel handles the

hardware on behalf of these commands and all processes in the system.


68

Users can thus afford to remain ignorant of the happenings behind the scene. This is one of the beauties of UNIX design and phi losophy.

The shell generally is sleeping. It wakes up when input is keyed in at the

prompt. This input is the input to the program that represents the shell. Below is the list of activities that the shell performs typically.

It issues the prompt ($ or otherwise) and sleeps till you enter a command.

After a command has been entered, the shell scans the command line for some special characters (metacharacters, we will have a look further) that have a special meaning for it. Because it permits abbreviated command lines (like the use of * to indicate all files, as in rm *), the shell has to make sure the

abbreviations are expanded before the command can act upon them.

It then creates a simplified command line and passes it on to the kernel for execution.

The shell can‘t do any work while the command is being executed, and has to

wait for its completion. After the job is complete, the prompt reappears and the shell returns to its

sleeping role to start the next ―cycle‖. You are now free to enter some other command.

Note: The command at the lower levels does not know or understand the metacharacters thus the shell has to handle and resolve them to normal

representations before they are parsed to kernel.

1.3 BASH: Bourne Again Shell

Bourne Again shell is the standard GNU shell, intuitive and flexible. Probably most advisable for beginning users while being at the same time a powerful

tool for the advanced and professional user. On Linux, bash is the standard shell for common users. This shell is a so-called superset of the Bourne shell, a set of add-ons and plug-in. This means that the Bourne Again shell is

compatible with the Bourne shell: commands that work in sh, also work in bash. However, the reverse is not always the case.

To know the shell you are using, invoke the command echo $SHELL. The

output could show /bin/sh (Bourne shell), /bin/csh (C shell), /bin/ksh (Korn

shell) or /bin/bash (bash shell).

When BASH is started, it reads its configuration files. The most important are:

/etc/profile - login time for all shelss

~/.bash_profi le – login shell window for bash (eg: printing system details on screen)

~/.bashrc – non-login shell window


69

1.3.1 Advantages of BASH

Bash is an sh−compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh). It is intended to conform to the IEEE POSIX P1003.2/ISO 9945.2 Shell and Tools standard. It offers functional

improvements over sh for both programming and interactive use; these include:

o Command line editing o Unlimited size command history o Job control

o Shell functions and aliases o Indexed arrays of unlimited size

o Integer arithmetic in any base from two to sixty−four Bash can run most Bourne shell scripts without modifications.

In our course, we will work with BASH only. The formats and commands

mentioned in this course will be slightly varied if they are to work in different shells.

1.4 Redirection Many of the UNIX commands that we have came across, sends their outputs to the terminal. There are commands which take their input from keyboard.

So, one can think of that these commands are designed to accept only fixed sources and destinations. These commands are designed to use the

character streams without knowing its source and destination. A character stream is just a sequence of bytes that many commands se as inputs and outputs.

In a UNIX system these streams are dealt to be as files, and a group of UNIX

commands reads from or writes to these fi les. A command is usually not designed to send output to the terminal—but to this file. Likewise, it is not designed to accept input from the keyboard either—but only from a standard

file which it sees as a stream. There‘s a third stream for all error messages thrown out by a program. This stream is the third file.

It‘s here that the shell comes in. The shell sets up these three standard files (for input, output and error) and attaches them to a user‘s terminal at the time

of logging in.Any program that uses streams will find them open and available. The shell also closes these files when the user logs out.

The standard file for input is known as standard input and that for output is known as standard output. The error stream is known as standard error. By

themselves, these standard files are not associated with any physical device, but the shell has set some physical devices as defaults for them:


70

Streams Default sources/destinations

Standard

Input

The default source is Keyboard

Standard Output

The default destination is the terminal screen

Standard Error

The default destination is the terminal screen

1.4.1 Standard Output

There are commands like ―more‖ which sends their output as a character stream, this stream is called the standard output stream and appears on the

terminal screen by default. By using the redirection this stream can be redirected or sent to a disk file.

Examples,

bash>more myFile > newFile The shell looks at the >, understands that standard output has to be

redirected, opens the file new file, writes the stream into it and then closes the file. And all this happens with more knowing nothing about it because more sends the output

to the stream and that stream gets redirected to a disk file.

By using ‗>‘ redirection operator, shell wi ll overwrite and existing file and creates a new file if no file with the name is existing. It is possible alternatively to append to the an existing file by using another redirecting operator ‗>>‘

Operator Action performed

> Creates a new file or if the file is already existing then overwrites

>> Appends to the file if the file is existing or creates a

new file

It is also possible to club the commands together and redirect the output to a

file. A pair of parenthesis groups the files and a redirection can redirect them to a file.

Example,

bash> (ls –l; who) > myFile

It is also possible that the results are redirected to another program, this is the concept of pipelining which we will discuss later on. Thus conclusively the standard output has three possible destinations:

Terminal or the screen and it is the default destination A disk file

A pipe – to another command


71

NOTE: Shell creates the fi le before it redirects the output into it .

1.4.2 Standard Input

Some commands are designed to take their inputs also as streams. This

stream represents the standard input to the command. A classical example for the use of the standard input could be the ―wc‖ command for counting the

words:

With no filename provided the wc tells the user about the number of lines, number of columns and the number of characters used and sends them to the standard output.

With some filename provided and redirected to the commands command takes the input stream to be the disk file.

Conclusively we can say that the standard input has three possible sources: The keyboard – Used as the default standard input

The Pipe – input from the results or output of some other command The fi le – inputs from a file

NOTE: When a fi le is redirected to a command, then it‘s the shell that opens the file and the command does not know as to what is happening. But when

the command is used with the file name as one of the arguments then the command itself opens the file.

1.4.3 Standard Error

When you enter an incorrect command or try to open a nonexistent file, certain diagnostic messages show up on the screen. This is the standard error stream. Like standard output, it too is destined for the terminal. Note

that they are in fact two separate streams, and the shell possesses a mechanism for capturing them individually.

Before we proceed any further, you should know that each of these three standard files has a number, called a file descriptor, which is used for identification:

bash>wc

2 * 4 23 ^ 64

[ctrl-d] 2 10 44 with no filename in output

bash>wc < my

5 9 54


72

0—Standard input ‗<‘ is same as „0<‘ 1—Standard output ‗>‘ is same as „1>‘

2—Standard error Must be „2>‘ only

These descriptors are implicitly prefixed to the redirection symbols. For instance, > and1> mean the same thing to the shell, while < and 0< also are identical. You normally don‘t need to use the numbers 0 and 1 to prefix the

redirect symbols because they are the default values. However, we need to use the descriptor 2> for the standard error:

Without specifying the fi le descriptor with the redirection symbol we don‘t get the errors in the file

This works. You can also append diagnostic output in a manner similar to the one in which you append standard output:

You can now save error messages in a separate file. This enables you to run long programs and save error output to be viewed at the end of the day.

1.4.4 Combining Streams

In UNIX, it is also possible to use both input and output streams at the same time and shell in this case keeps the command ignorant of the source and

destination.

In this case both input and output are redirected. It is also possible to combine < and > operators and the sequence of their use is immaterial for the shell.

bash>cat bar > errorfi le cat: cannot open bar: No such fi le or directory

bash>cat errorfile

bash> cat bar 2>errorfi le bash> cat errorfile

cat: cannot open bar: No such fi le or directory

bash>cat bar 2>> errorfile

bash>cat > my

bash> wc < infile > newfile bash> wc > newfile < infile

bash> newfile < infi le wc


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All the three commands are different commands for the same task. It is also possible to combine the standard output and standard error in the same

command line.

By default, the errors are dumped on the standard error (stderr) and normal output is sent to standard out (stdout). For example, if you simply type the following command to compile some C program, then the only normal output

will be sent to stdout, error will still show up on the terminal.

But if you want both the errors and the usual output (e.g. any warnings, etc.) to go into a single file, then you can use the following command:

2.3 Pipeline

In UNIX, it is desired a lot of times that output of some fi le is fed to another file

and this is used to accomplish a task. For instance, the following set of commands is doing some task:

Now, to count the number of users we can certainly redirect the file user.lst to

make it come from the standard input.

This method of using multiple commands to accomplish tasks has some obvious disadvantages:

bash> cat newfile nofile 2> errorfile > outfile

bash> cc x.c y.c > compile.out variable x is not defined.

variable y is redefined. variable z is not defined.

bash> cc x.c y.c > compile.out 2>&1 # Note there is not output printed on the script

bash> cat compile.out variable x is not defined.

Warning: variable type mismatch. variable y is redefined. variable z is not defined.

bash> who > user.lst

bash> cat user.lst araz tty01 May 18 09:32 amol tty02 May 18 11:18

achint tty03 May 18 13:21

bash> wc -l < user.lst 3


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1. The process is slow. The later command cannot get executed if the earlier ones are not yet executed.

2. An intermediate file is required that has to be removed after the wc command has been executed.

3. When handling large files, temporary files can built up easily and eat up the disk space.

Now, shell has a unique and powerful ability to connect the flow of these three commands, without needing any intermediate files, and each command takes

input from the other. This is accomplished using the pipe (|) operator. By using the pipes the command sequence shown above can be compressed

to the following single command:

Here, ‗who‘ is said to be piped to wc. No intermediate files are created when

they are used. When a sequence of commands is combined together in this way, a pipeline is said to be formed. The name is appropriate as the connection it establishes between programs, resembles a plumbing joint. It‘s

the shell that sets up this interconnection, and, the commands have no knowledge of it.

The pipe is a source and destination of standard input and standard output, respectively. You can now use one to count the number of files in the current

directory:

Note that no separate command was designed to tell you that, though the designers could easily have provided another option to ls to perform this

operation. And because wc uses standard output, you can redirect this output to a fi le:

There‘s no restriction on the number of commands you can use in a pipeline. But you must know the behavioral properties of these commands to place

them there. Consider this generalized command line:

command1 | command2 | command3 | command4 It should be pretty obvious that command2 and command3 must support both

standard input and standard output. Command1 requires to use standard output only, while command4 must be able to read from standard input. If you

can ensure that, then you can have a chain of these tools connected together.

bash> who | wc -l 3

bash> ls | wc -l

15

bash> ls | wc -l > fkount


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The commands command2 and command3 who support both streams are called filters. These will be discussed later.

1.5 Variables It is possible in shell to have shell variables that can have some values stored in then and can be later on referenced to get that value or use that values on

the command line or in shell scripts, we will learn shortly about the shell scripts. The shell variables are of string types, which means the value is

stored in ASCII rather than in binary format. No type declaration is necessary before you can use a shell variable. The shell variables are set using a generalized form of variable=value , and can be referenced by placing a ‗$‘

as a prefix to it. By using the unset command, the variable can be removed.

Example,

NOTE: There should be no space between the variable name, =, and variable

value else, shell will interpret the variable name to be a command and ‗=‘ and the variable value to be the arguments.

By default the shell variables are initialized to null value, but sometimes it is desirable to explicitly set them to a null value by using any one of the following

constructs: x= or x=‘‗ or x=‖‖

It is also possible to assign multiple word string to a shell variable, for this there are two approaches possible: 1. Escape the blank spaces using the escape character ‗\‘

2. Use the quotes.

1.5.1 Setting strings with the variable names having $

There could be strings containing the $ character in them. It could be for two reasons: 1. The string inherently contains the $ sign. Example:

My salary per month is $1000

bash> a=4 bash> echo $a

4 bash> unset a

bash> echo a bash>

bash> a=‗My name is Amrit‘

bash> echo $a My name is Amrit

bash> echo ‗My salary per month is $1000‘ My salary per month is $1000


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In this, $1000 is echoed as it is.

In this it is assumed that $1 is a shell variable and thus this tries to access the value which is undefined, and so replaces it with a null string.

Thus, there is a difference in the way the shell handles the strings if used in the single quotes and double quotes.

2. The string uses a variable name with $ character to replace the variable

with its value.

Example,

My salary per month is \$$x The variable x is to be replaced with the salary amount and preceded with a dollar sign.

1.5.2 Types of variables

As a convention, variables are used with uppercase names. Bash keeps a list of two types of variables:

Global variables

Global variables or environment variables are available in all shells. The env or printenv commands can be used to display environment variables.

Local variables

Local variables are only available in the current shell. Using the set built−in command without any options will display a list of all variables (including

environment variables) and functions. The output will be sorted according to the current locale and displayed in a reusable format.

A local variable is not automatically available to the sub shell unless exported.

1.5.3 Exporting variables

A variable created like the ones in the example above is only available to the current shell. It is a local variable. Child processes of the current shell will not

be aware of this variable. In order to pass variables to a subshell, we need to export them using the export built−in command . Variables that are exported

bash> echo ―My salary per month is $1000‖ My salary per month is 000


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are referred to as environment variables. Setting and exporting is usually done in one step:

export VARNAME="value"

A subshell can change variables it inherited from the parent, but the changes made by the chi ld don't affect the parent. This is demonstrated in the

example:

1.5.4 Using Shell Variables

In UNIX, it is possible to set variables to some path, command and command

substitution to set the output of the command. We will have a look at the usage examples wherein the variables can be set to these values and then can be used as substitutes of the operations.

Setting the path name

Thus, in some variables we can set the pathname and then cd command can be used to access that pathname again and again. NOTE: In practical applications and day to day life, this can be a great

practice to be done, it is because there are sometimes long absolute pathnames that can be actually stored in some variables and can be

accessed again and again without facing the trouble of memorizing them or typing long pathnames.

bash> full_name=―Amrit Swarup" bash> bash

bash> echo $full_name bash> exit

bash> export full_name bash> bash

bash> echo $full_name Amrit Swarup bash> export full_name=―Charan Singh"

bash> echo $full_name Charan Singh

bash> exit bash> echo $full_name

Amrit Swarup

bash> x=‘/home/ganesh/father‘

bash> cd $x bash> pwd /home/ganesh/father


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1.6 Command Substitution It is possible in UNIX systems to connect two commands. It is possible to

connect the standard output of a command to the standard input of another command using the pipelines or using the redirection. The shell allows obtaining the argument of a command from another

command; this feature is called command substitution. In some features, it is sometimes required that the command argument is the output of another

command. For example, we need to print some string which tells us about the number of files in the directory:

There are 24 files in the directory.

So, how will you achieve this? The shell has this feature.

So, you have substituted the command in the string which then acts as an

argument to the other command (echo), by placing the command in between two `` (backquote or backtick). This is a metacharacter that shell looks at (we cover metacharacters ahead). If enclosed in between the back quotes the

shell first executes the command, and then replaces the enclosed command text with the output of the command.

By now, we have seen that all the metacharacters behaves in the similar manner when used with either the double or single quotes. Lets try this one:

So, they are not interpreted by the shell, if placed in between the single quotes.

1.7 Pattern Matching – The Wild Cards While working with the UNIX system we often lands up in the situation when

we have to perform operations which can be used to apply the same operations collectively on a larger group. Typically, listing files starting with name lesson:

ls –l lesson01 lesson02 lesson03….

This can also be represented as:

ls –l lesson*

bash> echo ―There are `ls | wc –l` fi les in the directory.‖ There are 24 files in the directory.

$echo ‗There are `ls | wc – l̀ files in the directory.‘ There are `ls | wc –l` fi les in the directory.


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These are called the metacharacters, these are the special characters that

the shell understands and does some expanding operations based on the character and its intended use. Let‘s now discuss the metacharactes and

their attributes in some details

1.7.1 The * & ?

The *, known as a metacharacter, is one of the characters of the shell‘s

special set. This character matches any number of characters (including none).When the * is appended to the string lesson, the pattern lesson* matches fi lenames beginning with the string lesson—including the file lesson.

It thus matches all the files specified in the previous command line. You can now use this pattern as an argument to ls:

When the shell encounters this command line, it immediately identifies the *

as a metacharacter. It then creates a list of fi les from the current directory that match this pattern. It reconstructs the command line as below:

NOTE: Windows users may be surprised to know that the * may occur

anywhere in a filename, and not merely at the end. Thus, *lesson* matches all the following filenames: lesson newlesson lesson03 lesson03.txt.

The next metacharacter is the ‗?‘ This matches a single character. When used with the same string lesson (as lesson?), the shell matches all five-character

filenames beginning with lesson. Place another? at the end of this string, and you have the pattern lesson??. Use both these expressions separately, and the meaning of the ? will be obvious:

These metacharacters are also called wild cards (to depict something like a joker that can match any card). In the upcoming sessions we will take a look

at other wild cards.

1.8 The Character Class

bash> ls –x lesson*

lesson lesson01 lesson02 lesson03 lesson04 lesson05

lessonA lesson.pl lesson.c lesson.cpp

bash> ls –x lesson lesson01 lesson02 lesson03 lesson04 lesson05 lessonA lesson.pl lesson.c lesson.cpp

bash> ls -x lesson? lessonx lessony lessonz

bash> ls -x lesson?? lesson01 lesson02 lesson03 lesson04 lesson15 lesson16

lesson17


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It can be noted in the previous examples that the patterns which we have

framed in the previous examples are not very restrictive and specific. If we want to list only lessonA and lessonZ amongst the entire lesson we cannot do

that using the patterns, we have studied by now. To do this we need a character class for specific matching.

The character class uses two more metacharacters represented by a pair of brackets [ ]. You can have multiple characters inside this enclosure, but matching takes

place for a single character in the class. For example, a single character expression that can take one of the values 1, 2 or 4, can be represented by

the expression: [124] Either 1, 2 or 4

This can be combined with any string or another wild-card expression, so selecting the files lesson01, lesson02, lesson03, lesson04 becomes a simple matter :

1.9 Matching a dot (.) In UNIX file systems, there are lots of files that start with dots (.). It is sometimes desirable to do some collective wild card operations on these files.

Example can be,

This will not show the files starting with dots. To match the dots in the starting of a file name it is important to use the dot literally.

But it is possible to match as many dots, if they occur in the middle of the

filename.

NOTE: Using * with rm

bash> ls -x .*

.exrc .encrc .profile

bash> ls –x my*c my_file.c my.c my.stored.c

bash> ls –x lesson0[1234] lesson01 lesson02 lesson03 lesson04

bash> ls –x * lesson01 lesson02 ….


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Lets discuss a potential issue which each UNIX user faces at least once in his life that is the use of very beautiful and powerful command

bash> rm *

To remove all the files starting with lesson we can use the command

bash> rm lesson*

But with a bit of carelessness you can type bash> rm lesson *

And you have messed up everything beyond repair. Now be ready to have a

scolding from the system administrator. So be careful while using this command

1.10 Summing up Shell is a core component of the UNIX Operating System. It interprets the user commands and provides powerful features like Redirection, Pipes,

Metacharacters etc.

Bash is the shell, compatible with the Bourne shell and incorporating many useful features from other shells. Bash‘s biggest feature is a powerful history support and command line editing. In our course, we use the BASH shell to

explain the examples. In other shells the implementation is slightly different.


1. While a command is being executed the shell prompts the user for another

command and puts that command in its priority queue. (True/False) 2. Shell is in __________________ (execution/sleep) mode while there is no

command keyed in on the terminal and another command is running.

3. The redirection symbol ‗>‘ appends the redirected text to a file. (True/False) 4. Get the odd one out: The possible sources of standard input are:

a. Pipe b. Keyboard c. Printer

d. file

1.11 Answers to the Self-Check Questions


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1. False

2. Sleep 3. False

4. (c)


1. What is exporting a variable and why is it used? 2. Explain what is a metacharacter? Why do you need it? 3. Explain the difference between pipes and redirection.


LESSON 2 SHELL SCRIPTING AND DEBUGGING

2. SHELL SCRIPTING AND DEBUGGING..................................................................... 85

2.0 OBJECTIVES ............................................................................................................ 85

2.1 INTRODUCTION ........................................................................................................ 85

2.2 CREATING AND RUNNING A SCRIPT ......................................................................... 85

2.2.1 myScript.sh........................................................................................................ 85

2.2.2 Writing and naming .......................................................................................... 86

2.2.3 Executing the Script ......................................................................................... 86

2.3 SCRIPT BASICS ....................................................................................................... 88

2.3.1 Which shell will Run the Script? ..................................................................... 88

2.3.2 Adding comments............................................................................................. 88

2.4 DEBUGGING BASH SCRIPTS ................................................................................... 89

2.4.1 Debugging On the Entire Script ..................................................................... 89

2.4.2 Debugging On Part(s) Of the Script .............................................................. 90

2.5 QUOTING ................................................................................................................. 93

2.5.1 Escape Character............................................................................................. 93

2.5.2 Single Quotes ................................................................................................... 94

2.5.3 Double-Quotes.................................................................................................. 94

2.6 SPECIAL VARIABLES................................................................................................ 95

2.7 SUMMING UP ........................................................................................................... 98

2.8 ANSWERS TO THE SELF-CHECK QUESTIONS.......................................................... 98



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2. Shell Scripting and Debugging

To be able to write effective scripts, it is important to know the structure of a script and also be able to debug it if required. Therefore it is important to understand these concepts as they would form a base for subsequent chapters.

2.0 Objectives

After going through this lesson, you will be able to:

Write a simple script Define the shell type that should execute the script Put comments in a script

Change permissions on a script Execute and debug a script

2.1 Introduction This chapter is to enable the student to indulge in writing scripts with low

complexity. It is also pointed out that debugging is also needed at times. The student would be enabled to debug effectively using the methodology described in this chapter.

2.2 Creating and running a script

2.2.1 myScript.sh

In this example we use the echo Bash built-in to inform the user about what is

going to happen, before the task that will create the output is executed. The script welcomes the user, gives current date and time, lists the directory

contents and searches for the text ―Blue‖ in all files starting with the name ―demo‖ and stores the result in the file - searchResult .txt. For the scripts in this chapter we are assuming they are created in the following directory: ~/scripts


86

2.2.2 Writing and naming

To create a shell script:

Open a new empty file in your editor (vi, vim, gvim, emacs, gedit, dtpad etc.).

Put UNIX commands in the new empty file, like you would enter them on the

command line. As discussed in the previous chapter, commands can be shell functions, shell built-ins, UNIX commands and other scripts.

Give your script a sensible name that gives a hint about what the script does.

Make sure that your script name does not conflict with existing commands. In order to ensure that no confusion can rise, script names often end in .sh; even so, there might be other scripts on your system with the same name as the

one you chose.

Check using which, where is and other commands for finding information about programs and files: which −a script_name

whereis script_name locate script_name

2.2.3 Executing the Script

The script can run like any other command:

myScript.sh

#!/bin/bash

echo "" echo "This is my first shell script."

USERNAME=`whoami` echo "Welcome $USERNAME" echo ""

CURRENT_TIME=`date +%T` CURRENT_DATE=`date +%D`

echo "Date: $CURRENT_DATE Time: $CURRENT_TIME" echo ""

echo "" echo "Here are the files in your current directory."

echo "" ls grep Blue demo* > searchResult.txt


87

The above mentioned scheme is the most common way to execute a script. It is preferred to execute the script like this in a sub shell. The variables,

functions and aliases created in this sub shell are only known to the particular bash session of that sub shell. When that shell exits and the parent shell regains control, everything is cleaned up.

Remember to add the directory to the contents of the PATH variable.

It is essentially a colon separated list of directories. When you execute a command, the shell searches through each of these directories, one by one, until it finds a directory where the executable exists.

export PATH="$PATH:~/scripts"

If you did not put the scripts directory in your PATH, and the current directory is not in the PATH either, you need to specify the path of the script and

activate it. If it is in the current directory activate the script like this: ./script_name.sh

A script can also explicitly be executed by a given shell, but generally we only do this if we want to obtain special behavior, such as checking if the script

works with another shell or printing traces for debugging:

rbash script_name.sh

bash> chmod u+x myScript.sh

bash> ls −l myScript.sh −rwxrw−r−− 1 salil salil 456 Dec 24 17:11

myScript.sh bash> myScript.sh Check that you really

obtained the permissions that you want

This is my first shell script.

Welcome salil Date: 12/21/07 Time: 12:26:40 Here are the files in your current directory.

demo.txt demo2.txt

demo3.txt lab myScript.sh

newfile.txt output.txt

update.ppt

The script should have execute permissions for the correct owners

in order to be runnable.


88

sh script_name.sh bash −x script_name.sh

The specified shell will start as a sub shell of your current shell and executes

the script. This is done when you want the script to start up with specific options or under specific conditions which are not specified in the script.

If you don't want to start a new shell but execute the script in the current shell, you source it:

source script_name.sh

The script does not need execute permission in this case. Commands are executed in the current shell context, so any changes made to your

environment will be available when the script finishes execution

2.3 Script Basics

2.3.1 Which shell will Run the Script?

When running a script in a subshell, you should define which shell should run

the script. Consider for example that your login shell may be C – Shell but your script may be containing bash commands. The shell type in which you

wrote the script might not be the default on your system, so commands you entered might result in errors when executed by the wrong shell.

The first line of the script determines the shell in which the script will run. The first two characters of the first line should be #!, then follows the path to the

shell that should interpret the commands that follow. Blank lines are also considered to be lines, so don't start your script with an empty line.

For the purpose of this course, all scripts will start with the line #!/bin/bash

2.3.2 Adding comments

It is a good practice to add comments into your scripts. Comments help in future when you will need to enhance or fix the script. Comments also make

the scripts more readable.


89

Usually, the initial few lines of script should indicate about the purpose of the script. And then you should put comments in the code too.

2.4 Debugging Bash Scripts

2.4.1 Debugging On the Entire Script

Bash provides extensive debugging features. The most common is to start up the sub shell with the −x option, which will run the entire script in debug mode.

Traces of each command plus its arguments are printed to standard output after the commands have been expanded but before they are executed.

Following is the commented_script1.sh script ran in debug mode. Note again that the added comments are not visible in the output of the script.

commented_script1.sh

#!/bin/bash

# This script clears the terminal, displays a greeting and gives information # about currently connected users. The current directory

contents are # displayed too

clear # clear terminal window

echo "The script starts now."

echo "Hi, $USER!" # dollar sign is used to get content of variable

echo

echo "List of connected users:" echo w # show who is logged on

echo echo "Displaying the contents of this directory"

ls # To list the contents of this directory

The first line of the script determines

the shell to start – BASH in this case

This is a Comment. Everything the shell encounters after a hash mark on a line is ignored.


90

2.4.2 Debugging On Part(s) Of the Script

Using the set Bash built-in you can run in normal mode those portions of the

script of which you are sure they are without fault, and display debugging information only for troublesome zones.

Say we are not sure what the w command will do in the example commented−script1.sh, then we could enclose it in the script like this:

set −x # activate debugging from here

w set +x # stop debugging from here

bash> bash −x commented_script1.sh

+ clear

+ echo 'The script starts now.' The script starts now. + echo 'Hi, sali l!'

Hi, sali l! + echo

+ echo 'List of connected users:' List of connected users:

+ echo

+ w 4:50pm up 18 days, 6:49, 4 users, load average: 0.58, 0.62, 0.40

USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT

root tty2 − Sat 2pm 5:36m 0.24s 0.05s −bash salil :0 − Sat 2pm ? 0.00s ?

− salil pts/2 − Sat 2pm 43:13 0.13s 0.06s /usr/bin/screen

+ echo

+ echo 'Displaying the contents of this directory' Displaying the contents of this directory

+ ls demo1.txt demo2.txt myScript.sh


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Output then looks like this:

The table below gives an overview of other useful Bash options:

Table – Overview of set debugging options Short

notation Long notation Result

set –f set –o noglob Disable file name generation using metacharacters (globbing).

set –v set –o verbose Prints shell input lines as they are

read.

set –x set –o xtrace Print command traces before executing command.

The dash is used to activate a shell option and a plus to deactivate it. In the example below, we demonstrate these options on the command line:

Alternatively, these modes can be specified in the script itself, by adding the desired options to the first line shell declaration. Options can be combined, as

is usually the case with UNIX commands: #!/bin/bash −xv

bash> script1.sh

The script starts now. Hi, sali l!

List of connected users:

+ w 5:00pm up 18 days, 7:00, 4 users, load average: 0.79,

0.39, 0.33 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT

Root tty2 − Sat 2pm 5:47m 0.24s 0.05s −bash

salil :0 − Sat 2pm ? 0.00s ? − salil pts/2 − Sat 2pm 54:02 0.13s 0.06s

/usr/bin/screen + set +x

Displaying the contents of this directory demo1.txt demo2.txt myScript.sh

bash>


92

Once you found the buggy part of your script, you can add echo statements before each command of which you are unsure, so that you will see exactly

where and why things don't work. In the example commented−script1.sh script, it could be done like this, still assuming that the displaying of users

gives us problems:

In more advanced scripts, the echo can be inserted to display the content of variables at different stages in the script, so that flaws can be detected:

bash> set −v

bash> ls ls commented−scripts.sh script1.sh

bash> set +v

set +v bash> ls *

commented−scripts.sh script1.sh

bash> set −f bash> ls *

ls: *: No such file or directory

bash> touch * bash> ls

* commented−scripts.sh script1.sh

bash> rm *

bash> ls

commented−scripts.sh script1.sh

echo "debug message: now attempting to start w command"; w

echo "Variable VARNAME is now set to $VARNAME."


93

2.5 Quoting Quoting is used to remove the special meaning of certain characters or words

to the shell. Quoting can be used to disable special treatment for special characters (to preserve their literal meaning), to prevent reserved words from being recognized as such, and to prevent parameter expansion. The

application should quote the following characters if they are to represent themselves: | & ; < > ( ) $ ` \ " ' <space> <tab> <newline>

There are three quoting mechanisms:

1. The escape character

2. Single quotes 3. Double quotes

2.5.1 Escape Character

A non-quoted backslash ‗\‘ is the Bash escape character. It preserves the literal value of the next character that follows, with the exception of newline. If a \newline pair appears, and the backslash itself is not quoted, the \newline is

treated as a line continuation (that is, it is removed from the input stream and effectively ignored).

The following script shows the effect of backslash on newline

bash> date=26122007

bash> echo $date

26122007 bash> echo \$date

$date

Variable date is created and set to hold a value. The first echo

displays the value of the variable, but for the second, the dollar sign is escaped.

escape.sh

#!/bin/bash

echo "Statement 1: This will print as two lines."

echo "Statement 2: This will print \ as one line."


94

On running this script:

2.5.2 Single Quotes

Enclosing characters in single quotes (' ') preserves the literal value of each

character within the quotes. A single quote may not occur between single

quotes, even when preceded by a backslash. Example:

2.5.3 Double-Quotes

Enclosing characters in double-quotes ( " " ) shall preserve the literal value of all characters within the double-quotes, with the exception of the characters

dollar sign ‗$‘, backquote ‗`‘ and ‗\‘. The characters ‗$‘ and ‗`‘ retain their special meaning within double quotes. The backslash retains its special meaning only when followed by one of the

following characters: ‗$‘, ‗`‘, ‗"‘, ‗\‘, or newline.

bash> escape.sh

Statement 1: This will print as two lines

Statement 2: This will print as one line

bash> echo '$date'

$date

bash> echo "$date"

20021226

bash> echo "`date`" Sun Apr 20 11:22:06 CEST 2003

bash> echo "I'd say: \"Go for it!\"" I'd say: "Go for it!"

bash> echo "In DOS directories are separated by \\ character"

In DOS directories are separated by \ character


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2.6 Special Variables

There are some variables which are set internally by the shell and which are available to the user. The following table lists some of them:

Variable Definition

$0 Expands to the name of the shell script or command currently being executed or the name of the shell

$1 Positional parameter #1. Similarly for 2,3..9. For 10

use ${10}

$*

Expands to the positional parameters, starting from one ($1). When the expansion occurs within double

quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS (Refer note below) special variable.

$@

Expands to the positional parameters, starting from

one ($1). When the expansion occurs within double quotes, each parameter expands to a separate

word.

$# Expands to the total number of positional parameters in decimal.

$? The exit status of the last command executed is given as a decimal string.

$- Flags passed to script (using set)

$$ Expands to the process ID of the shell.

$! Expands to the process ID of the most recently executed background command.

Note: $IFS or the internal field separator is a variable which determines how Bash recognizes fields, or word boundaries, when it interprets character strings. $IFS defaults to whitespace.

A positional parameter is a variable within a shell script whose value is set

from an argument specified on the command line that invokes the script. Positional parameters are numbered and are referred to with a preceding ``$'': $1, $2, $3, and so on. A shell program may reference up to nine positional

parameters. If a shell program is invoked with a command line that appears like this:

my_script.sh pp1 pp2 pp3 pp4 pp5 pp6 pp7 pp8 pp9 then positional parameter $1 within the script is assigned the value pp1,

positional parameter $2 is assigned the value pp2, and so on, at the time the shell script is invoked.


96

Upon execution one could give any numbers of arguments:

When a UNIX command runs, it can return a numeric exit status value to the

process that called (started) it. The status can tell the calling process whether the command succeeded or failed. Many (but not all) UNIX commands return

a status of zero if everything was okay or non-zero (1, 2, etc.) if something went wrong. A few commands, like grep and diff, return a different non-zero

status for different kinds of problems. See your online manual pages to find out.

bash> positional.sh one two three four five one is the first positional parameter, $1.

two is the second positional parameter, $2. three is the third positional parameter, $3.

The total number of positional parameters is 5.

bash> positional.sh one two one is the first positional parameter, $1.

two is the second positional parameter, $2. is the third positional parameter, $3.

The total number of positional parameters is 2.

$3 is empty

#!/bin/bash

# positional.sh

# This script reads 3 positional parameters and prints them out.

PAR1="$1" PAR2="$2"

PAR3="$3" echo "$1 is the first positional parameter, \$1."

echo "$2 is the second positional parameter, \$2." echo "$3 is the third positional parameter, \$3."

echo echo "The total number of positional parameters is $#."


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More examples:

The following script shows the use of ―$*‖ special variable:

Upon execution:

bash> grep dictionary /usr/share/dict/words dictionary

bash> echo $$

10662

bash> mozilla & [1] 11064

bash> echo $! 11064

bash> echo $0 bash

bash> echo $?

0 bash> ls abc

ls: abc: No such file or directory bash> echo $?

1

User rahul starts entering the

grep command. The process ID of his shell is

10662. After putting a job in the background, the ! holds the

process ID of the backgrounded job.

The shell running is bash.

When a mistake is made, ? holds an exit status different from 0 (zero). Else

the status is 0.

spl_var_eg.sh

#!/bin/bash echo ―My Process ID is: $$‖

echo ―The number of Arguments is $#‖ echo ―The Arguments are $*‖ grep ―$1‖ $2

echo ―\Job Over‖

bash> spl_var_eg.sh Blue demo1.txt My Process ID is: 23465 The number of Arguments is 2

The Arguments are Blue demo1.txt My favourite colour is Blue.

Job Over


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2.7 Summing Up A shell script is a reusable series of commands put in an executable text file.

Any text editor can be used to write scripts. Scripts start with #! followed by the path to the shell executing the commands

from the script. Comments are added to a script for your own future reference, and also to make it understandable for other users. It is better to have too

many explanations than not enough. Debugging a script can be done using shell options. Shell options can be

used for partial debugging or for analyzing the entire script. Inserting echo commands at strategic locations is also a common troubleshooting technique.


1. What do you need to add to the first line of the script to indicate Bash shell? 2. Why are comments needed and how do you add them?

3. What happens when a script is executed with the option "bash -x" option?

2.8 Answers to the Self-Check questions

1. #!/bin/bash 2. Comments are useful to enlighten the reader about the script and make it

comprehendible. A comment is added in the format: # <the comment>

3. It will run the entire script in debug mode


1. What are the different steps for creating a shell script? 2. How would you debug a part of the script?

3. What are the different shell debugging options? 4. Why is Quoting used? Give examples.


LESSON 3 CONDITIONAL STATEMENTS


101

3. Conditional statements

One of the advanced concepts, conditional statements are very frequently used in scripts. A clear understanding of this concept is very important.

3.0 Objectives

After going through this lesson, you will learn about:

The if statement

Using the exit status of a command

Comparing and testing input and files

If-then-else constructs

If-then-elif-else constructs

Using and testing the positional parameters

Nested if statements

Using case statements

3.1 Introduction This chapter introduces the use of conditionals in Bash scripts. This would enable the student to write scripts that are more powerful and cater to

different conditions.

3.2 Introduction to if

3.2.1 General

At times you need to specify different courses of action to be taken in a shell script, depending on the success or failure of a command. The if construction allows you to specify such conditions.

The most compact syntax of the if command is:

if TEST−COMMANDS; then CONSEQUENT−COMMANDS; fi

Example: For Checking shell options


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The TEST−COMMAND list is executed, and if its return status is zero, the

CONSEQUENT−COMMANDS list is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

The TEST−COMMAND often involves numerical or string comparison tests, but it can also be any command that returns a status of zero when it succeeds

and some other status when it fails. Unary expressions are often used to examine the status of a fi le. If the FILE argument to one of the primaries is of the form /dev/fd/N, then file descriptor "N" is checked. stdin, stdout and stderr

and their respective file descriptors may also be used for tests.

Expressions used with if

The table below contains an overview of the so−called "primaries" that make

up the TEST−COMMAND command or list of commands. These primaries are put between square brackets to indicate the test of a conditional expression.

Table − Primary expressions

Primary Meaning

[ -a FILE ] True if FILE exists

[ -o OPTIONNAME ]

True if shell option ―OPTIONNAME‖ is enabled

[ -z STRING ] True of the length of ―STRING‖ is non-zero.

[ -n STRING ]or [

STRING]

True of the length of ―STRING‖ is non-Zero

[ STRING1 == STRING2 ]

True if the strings are equal. ―=‖may be used instead of ‖==‖ for strict POSIX

compliance

[STRING1! = STRING2]

True if the strings are not equal

[ STRING1<

STRING2 ]

True if ―STRING1‖ sorts before ―STRING2‖

lexicographically in the current locale.

[ STRING1> STRING2 ]

True if ―STRING1‖ sorts after ―STRING2‖ lexicographically in the current locale.

[ ARG1 OP

ARG2 ]

―OP‖ is one of – eq, -ne, -lt, -le,-gt or –ge.

These arithmetic binary operators return true if ―ARG1‖ is equal to, not equal to, less than, less than or equal to, greater

than, or greater than or equal to ―ARG2‖. ―ARG1‖ and ―ARG2‖ are integers.

# These lines will print a message if the noclobber option is set

if [ −o noclobber ]

then echo "Your files are protected against accidental

overwriting using redirection."

fi


103

Expressions may be combined using the following operators, listed in

decreasing order of precedence:

Table – Combining expressions

Operation Effect

[ ! EXPR ] True if EXPR is false

[ (EXPR) ] Returns the value of EXPR. This may be used to override the normal precedence of operators.

[ EXPR1 –a

EXPR2 ] True if both EXPR1 and EXPR2 are True

[ EXPR1 –o EXPR2 ]

True if either EXPR1 and EXPR2 is true.

The [ (or test) built−in evaluates conditional expressions using a set of rules

based on the number of arguments. More information about this subject can be found in the Bash documentation. Just like, the if is closed with fi, the opening angular bracket should be closed after the conditions have been

listed.

Commands following the then statement

The CONSEQUENT−COMMANDS list that follows the then statement can be

any valid UNIX command, any executable program, any executable shell script or any shell statement, with the exception of the closing fi. It is important

to remember that the then and fi are considered to be separated statements in the shell. Therefore, when issued on the command line, they are separated by a

semi−colon. In a script, the different parts of the if statement are usually well−separated.

Below are a couple of simple examples.

Checking files

The first example checks for the existence of a file:


104

3.2.2 Simple applications of if

Testing exit status

Numeric comparisons

:

#!/bin/bash

echo "This scripts checks the existence of the demo file."

echo "Checking..." if [ −f /usr/guest/demo.txt ] then

echo "/usr/guest/demo.txt fi le exists." fi

echo echo "...done."

bash> ./filecheck.sh This scripts checks the existence of the messages file.

Checking... /usr/guest/demo.txt file exists.

...done.

bash> if [ $? −eq 0 ] > then echo 'That was a good job!' > fi

That was a good job!

bash>

bash> num=`wc −l demo1.txt`

bash> echo $num 201

bash> if [ "$num" −gt "150" ]

> then echo ; echo "This is a big file." > echo ; fi

This is a big file.

bash>

filecheck.sh


105

String comparisons

3.3 More advanced if usage

3.3.1 if-then-else constructs

Like the CONSEQUENT−COMMANDS list following the then statement, the

ALTERNATE−CONSEQUENT−COMMANDS list following the else statement can hold any UNIX−style command that returns an exit status. Example 1

On executing the script we get:

Example 2

dir=`pwd` # /tmp/proc

updir=`basename $dir` # /tmp if [ "$updir"‖X‖ != ―/tmpX'' ]; then

echo "You need to be in a subdirectory of /tmp." exit 1; fi

bash> bash −x fun_weigh.sh 55 169

+ weight=55 + height=169

+ idealweight=59 + '[' 55 −le 59 ']' + echo 'You should eat a bit more fat.'

You should eat a bit more fat.

fun_weigh.sh

fun_weigh.sh

#!/bin/bash # This script prints a message about your weight if you

give it your # weight in kilos and hight in centimeters.

weight="$1" height="$2"

idealweight=$[$height − 110]

if [ $weight −le $idealweight ] ; then echo "You should eat a bit more fat." else

echo "You should eat a bit more fruit." fi


106

Testing the number of arguments - The previous script is modified so that it prints a message if more or less than 2 arguments are given:

The first argument is referred to as $1, the second as $2 and so on. The total number of arguments is stored in $#.

3.3.2 if-then-elif-else constructs

This is the full form of the if statement: if TEST−COMMANDS; then

CONSEQUENT−COMMANDS; elif MORE−TEST−COMMANDS; then

MORE−CONSEQUENT−COMMANDS; else ALTERNATE−CONSEQUENT−COMMANDS; fi

#!/bin/bash

# This script prints a message about your weight if you give it your

# weight in kilos and hight in centimeters. if [ ! $# == 2 ]; then

echo "Usage: $0 weight_in_kilos length_in_centimeters"

exit fi

weight="$1" height="$2"

idealweight=$[$height − 110] if [ $weight −le $idealweight ] ; then

echo "You should eat a bit more fat." else

echo "You should eat a bit more fruit." fi

bash> fun_weigh.sh 70 150 You should eat a bit more fruit.

bash> fun_weigh.sh 70 150 33 Usage: ./weight.sh weight_in_kilos

length_in_centimeters

fun_weigh.sh


107

3.3.3 Returning the exit status using if

Sometimes, you test for a condition and find that it fails. You would rather like the program to terminate since there is no point in continuing further if an essential resource is missing—say the file you want to search. The exit

statement is used to prematurely terminate a program.

The exit statement takes an optional argument. This argument is the integer exit status code, which is passed back to the parent and stored in the $? variable.

In this example if the number of arguments is not 2 then the execution is

exited (with a code 2) and a message about the usage is printed.

#!/bin/bash # This script will test if we're in a leap year or not.

year=`date +%Y`

if [ $[$year % 400] −eq "0" ]; then echo "This is a leap year. February has 29 days." elif [ $[$year % 4] −eq 0 ]; then

if [ $[$year % 100] −ne 0 ]; then echo "This is a leap year, February has 29

days." else echo "This is not a leap year. February has 28

days." fi

else echo "This is not a leap year. February has 28 days." fi

bash> date Fri Dec 21 17:14:28 IST 2007

bash> testleap.sh

This is not a leap year.

testleap.sh

Also note nested ifs here. You may

use as many levels of nested ifs

as you can logically manage.

#!/bin/bash if [ $# -ne 2 ]; then

echo "Usage $0 \<file1\> \<file2\>"; exit 2

fi ...<rest of script>


108

3.4 Using case statements

Nested if statements might be nice, but as soon as you are confronted with a couple of different possible actions to take, they tend to confuse. For the more

complex conditionals, use the case syntax: case EXPRESSION in CASE1) COMMAND−LIST;; CASE2)

COMMAND−LIST;; ... CASEN) COMMAND−LIST;; esac

Each case is an expression matching a pattern. The commands in the COMMAND−LIST for the first match are executed. The "|" symbol may be

used for separating multiple patterns, and the ")" operator terminates a pattern list. Each case plus its according commands are called a clause. Each clause

must be terminated with ";;". Each case statement is ended with the esac statement.

In the example, we demonstrate use of case for getting the disk usage.

Echo interprets and treats the character c as special because of the backslash. The \c here represents an escape sequence, which positions the

cursor immediately after the argument instead of the next line. The read statement takes input from the user, thereby making the script

interactive. The input is read into a variable (selection in this case). The output is as follows:

disk_utility.sh

#!/bin/bash

echo ―\n 1. The free disk space\n 2. Space consumed by

this user 3. Exit\n\n SELECTION: \c‖

read selection case $selection in

1) df ;; 2) du –s $HOME ;; 3) exit ;;

*) echo ― Not a valid option‖ esac


109

3.5 Summary

In this chapter we learned how to build conditions into our scripts so that different actions can be undertaken upon success or failure of a command. The actions can be determined using the if statement. This allows you to

perform arithmetic and string comparisons, and testing of exit code, input and files needed by the script.

A simple If-then-fi test often precedes commands in a shell script in order to prevent output generation, so that the script can easily be run in the

background or through the cron facility. More complex definitions of conditions are usually put in a case statement.


1. What is the use of the "if" statement? 2. What is the exit status of a command? What is its normal value and where is the

value stored?


1. The "if" statement takes two-way decisions depending on the fulfillment of a

certain condition. 2. The exit status is an integer that represents the success or failure of a

command. It has the value 0 when the command executes successfully and is

stored in the parameter $?


1. List some applications of the ―if-then-elif-else‖ statement. 2. Give an example of ―Case‖ usage.

bash> disk_utility.sh

1. The free disk space 2. Space consumed by this user

3. Exit SELECTION: 2

456100 /home/pallavi


LESSON 4 REPETITIVE TASKS

4. REPETITIVE TASKS .................................................................................................... 113

4.0 OBJECTIVES .......................................................................................................... 113

4.1 INTRODUCTION ...................................................................................................... 113

4.2 THE FOR LOOP....................................................................................................... 113

4.2.1 How does it work? .......................................................................................... 113

4.2.2 Examples ......................................................................................................... 114

4.3 THE WHILE LOOP ................................................................................................... 115

4.3.1 What is it? ........................................................................................................ 115

4.3.2 Examples ........................................................................................................... 115

4.4 THE UNTIL LOOP .................................................................................................... 117

4.4.1 What is it? ........................................................................................................ 117

4.4.2 Example ........................................................................................................... 118

4.5 I/O REDIRECTION AND LOOPS ............................................................................... 118

4.5.1 Input redirection .............................................................................................. 119

4.5.2 Output redirection ........................................................................................... 119

4.6 BREAK AND CONTINUE ................................................................................................ 119

4.6.1 The break built−in........................................................................................... 120

4.6.2 The continue built−in...................................................................................... 121

4.6.3 Examples ......................................................................................................... 121

4.7 MAKING MENUS WITH THE SELECT BUILT−IN ........................................................ 123

4.7.1 General ............................................................................................................ 123

4.7.2 Submenus ....................................................................................................... 126

4.8 THE SHIFT BUILT−IN .............................................................................................. 126

4.8.1 What does it do?............................................................................................. 126

4.8.2 Examples ......................................................................................................... 126

4.9 SUMMARY .............................................................................................................. 127

4.10 ANSWERS TO THE SELF-CHECK QUESTIONS........................................................ 128

4.11 TERMINAL QUESTIONS .......................................................................................... 128


113

4. Repetitive tasks

It is important to appreciate the need of loops in scripts. It takes scripting to the next level and comes very handy in a wide variety of applications.

4.0 Objectives Upon completion of this chapter, you will be able to

Use for, while and until loops, and decide which loop fits which occasion.

Use the break and continue Bash built−ins.

Write scripts using the select statement.

Write scripts that take a variable number of arguments.

4.1 Introduction This chapter teaches the student to write different types of loops as per any

application that requires repetitive tasks. This is very helpful in writing useful scripts that require something to be done repeatedly.

4.2 The for loop

4.2.1 How does it work?

The for loop is the first of the three shell looping constructs. This loop allows for specification of a list of values. A list of commands is executed for each

value in the list.

The syntax for this loop is: for NAME [in LIST ]; do COMMANDS; done

If [in LIST] is not present, it is replaced with $@ and for executes the

COMMANDS once for each positional parameter that is set. The return status is the exit status of the last command that executes. If no commands are executed because LIST does not expand to any items, the return status is

zero.

NAME can be any variable name, although it is used very often. LIST can be any list of words, strings or numbers, which can be literal or generated by any command. The COMMANDS to execute can also be any operating system


114

commands, script, program or shell statement. The first time through the loop, NAME is set to the first item in the LIST. The second time, its value is set to

the second item in the list, and so on. The loop terminates when NAME has taken on each of the values from LIST and no items are left in the LIST.

4.2.2 Examples

Using command substitution for specifying LIST items

The first is a command line example, demonstrating the use of a for loop that makes a backup copy of each .xml fi le. After issuing the command, it is safe to start working on your sources:

This one lists the files in /sbin that are just plain text files, and possibly scripts:

Using the content of a variable to specify LIST items

The following is a specific application script for converting HTML files,

compliant with a certain scheme, to PHP files. The conversion is done by taking out the first 25 and the last 21 lines, replacing these with two PHP tags

that provide header and footer lines:

bash> ls *.xml file1.xml fi le2.xml file3.xml

bash> ls *.xml > list

bash> for i in `cat list`; do cp "$i" "$i".bak ; done

bash> ls *.xml*

file1.xml file1.xml.bak file2.xml fi le2.xml.bak file3.xml file3.xml.bak

#!/bin/bash # specific conversion script for my html files to php LIST="$(ls *.html)"

for i in "$LIST"; do NEWNAME=$(ls "$i" | sed −e 's/html/php/')

cat beginfi le > "$NEWNAME" cat "$i" | sed −e '1,25d' | tac | sed −e '1,21d'| tac >> "$NEWNAME"

cat endfile >> "$NEWNAME" done

html2php.sh

for i in `ls /sbin`; do file /sbin/$i | grep ASCII;done


115

Since we don't do a line count here, there is no way of knowing the line number from which to start deleting lines until reaching the end. The problem

is solved using tac, which reverses the lines in a file.

4.3 The while loop

4.3.1 What is it?

The while construct allows for repetitive execution of a list of commands, as long as the command controlling the while loop executes successfully (exit

status of zero). The syntax is:

while CONTROL−COMMAND; do CONSEQUENT−COMMANDS; done CONTROL−COMMAND can be any command(s) that can exit with a success

or failure status. The CONSEQUENT−COMMANDS can be any program, script or shell construct.

As soon as the CONTROL−COMMAND fails, the loop exits. In a script, the command following the done statement is executed.

The return status is the exit status of the last CONSEQUENT−COMMANDS

command, or zero if none was executed.

4.3.2 Examples

Simple example using while

Here is an example for the impatient:

Nested while loops

The example below was written to copy pictures that are made with a webcam to a web directory. Every five minutes a picture is taken. Every hour, a new

directory is created, holding the images for that hour. Every day, a new

#!/bin/bash

# This script opens 4 terminal windows.

i="0"

while [ $i −lt 4 ] do xterm &

i=$[$i+1] done


116

directory is created containing 24 subdirectories. The script runs in the background.

Note the use of the true statement. This means: continue execution until we

are forcibly interrupted (with kill or Ctrl+C). This small script can be used for simulation testing; it generates files:

Note the use of the date command to generate all kinds of file and directory names. See the man page for more information on date command

Calculating an average

#!/bin/bash

# This script copies files from my homedirectory into the webserver directory.

# (use scp and SSH keys for a remote directory) # A new directory is created every hour.

PICSDIR=/home/mohan/pics WEBDIR=/var/www/mohan/webcam

while true; do

DATE=`date +%Y%m%d` HOUR=`date +%H` mkdir $WEBDIR/"$DATE"

while [ $HOUR −ne "00" ]; do

DESTDIR=$WEBDIR/"$DATE"/"$HOUR" mkdir "$DESTDIR" mv $PICDIR/*.jpg "$DESTDIR"/

sleep 3600 HOUR=`date +%H`

done done

#!/bin/bash

# This generates a fi le every 5 minutes

while true; do touch pic−`date +%s`.jpg sleep 300

done


117

This script calculates the average of user input, which is tested before it is processed: if input is not within range, a message is printed. If q is pressed,

the loop exits:

Note how the variables in the last lines are left unquoted in order to do arithmetic.

4.4 The until loop

4.4.1 What is it?

The until loop is very similar to the while loop, except that the loop executes until the TEST−COMMAND executes successfully. As long as this command fails, the loop continues. The syntax is the same as for the while loop:

until TEST−COMMAND; do CONSEQUENT−COMMANDS; done

The return status is the exit status of the last command executed in the CONSEQUENT−COMMANDS list, or zero if none was executed.

TEST−COMMAND can, again, be any command that can exit with a success or failure status, and CONSEQUENT−COMMANDS can be any UNIX

command, script or shell construct.

#!/bin/bash

# Calculate the average of a series of numbers.

SCORE="0" AVERAGE="0" SUM="0"

NUM="0"

while true; do echo −n "Enter your score [0−100%] ('q' for quit): ";

read SCORE; if (("$SCORE" < "0")) || (("$SCORE" > "100")); then

echo "Be serious. Common, try again: " elif [ "$SCORE" == "q" ]; then echo "Average rating: $AVERAGE%."

break else

SUM=$[$SUM + $SCORE] NUM=$[$NUM + 1] AVERAGE=$[$SUM / $NUM]

fi done

echo "Exiting."


118

As was previously explained, the ";" may be replaced with one or more

newlines wherever it appears.

4.4.2 Example

An improved picturesort.sh script (see Section 4.2.2.2), which tests for available disk space. If disk space is not enough, remove pictures from the

previous months:

Note the initialization of the HOUR and DISKFUL variables and the use of options with ls and date in order to obtain a correct listing for TOREMOVE. (Not Clear)

4.5 I/O redirection and loops

#!/bin/bash

# This script copies files from my homedirectory into the webserver directory. # A new directory is created every hour.

# If the pics are taking up too much space, the oldest are removed.

while true; do DISKFUL=$(df −h $WEBDIR | grep −v File |

awk '{print $5}' | cut −d "%" −f1 −)

until [ $DISKFUL −ge "90" ]; do DATE=`date +%Y%m%d` HOUR=`date +%H`

mkdir $WEBDIR/"$DATE"

while [ $HOUR −ne "00" ]; do DESTDIR=$WEBDIR/"$DATE"/"$HOUR"

mkdir "$DESTDIR" mv $PICDIR/*.jpg "$DESTDIR"/ sleep 3600

HOUR=`date +%H` done

DISKFULL=$(df −h $WEBDIR | grep −v File | awk '{ print $5 }' | cut −d "%" −f1 −) done

TOREMOVE=$(find $WEBDIR −type d −a

−mtime +30) for i in $TOREMOVE; do rm −rf "$i";

done

done


119

4.5.1 Input redirection

Instead of controlling a loop by testing the result of a command or by user

input, you can specify a file from which to read input that controls the loop. In such cases, read is often the controlling command. As long as input lines are fed into the loop, execution of the loop commands continues. As soon as, all

the input lines are read the loop exits.

Since the loop construct is considered to be one command structure (such as while TEST−COMMAND; do CONSEQUENT−COMMANDS; done), the redirection should occur after the done statement, so that it complies with the

form

command < file This kind of redirection also works with other kinds of loops.

4.5.2 Output redirection

In the example below, output of the find command is used as input for the read command controlling a while loop:

Files are compressed by gzip command before they are moved into the archive directory.

4.6 Break and continue

#!/bin/bash

# This script creates a subdirectory in the current directory, to which old

# files are moved. # Might be something for cron (if slightly adapted) to execute weekly or

# monthly.

ARCHIVENR=`date +%Y%m%d` DESTDIR="$PWD/archive−$ARCHIVENR"

mkdir $DESTDIR

find $PWD −type f −a −mtime +5 | while read file do gzip "$file"; mv "$file".gz "$DESTDIR"

echo "$file archived" done

archiveoldstuff.sh


120

4.6.1 The break built−in

The break statement is used to exit the current loop before its normal ending.

This is done when you don't know in advance how many times the loop will have to execute, for instance because it is dependent on user input.

The example below demonstrates a while loop that can be interrupted. This is a slightly improved version of the wisdom.sh script from Section 4.3.2

#!/bin/bash

# This script provides wisdom # You can now exit in a decent

way.

FORTUNE=/usr/games/fortune while true; do

echo "On which topic do you want advice?"

echo "1. politics" echo "2. startrek" echo "3. kernelnewbies"

echo "4. sports" echo "5. bofh−excuses"

echo "6. magic" echo "7. love" echo "8. literature"

echo "9. drugs" echo "10. education"

echo echo −n "Enter your choice, or 0 for

exit: " read choice echo

case $choice in

1) $FORTUNE politics ;;

2) $FORTUNE startrek

;; 3) $FORTUNE

kernelnewbies ;;

4) echo "Sports are a waste of time, energy and money."

echo "Go back to your keyboard."


121

Mind that break exits the loop, not the script. This can be demonstrated by

adding an echo command at the end of the script. This echo will also be executed upon input that causes break to be executed (when the user types

"0"). In nested loops, break allows for specification of which loop to exit. See the Bash info pages for more.

4.6.2 The continue built−in

The continue statement resumes iteration of an enclosing for, while, until or select loop. When used in a for loop, the controlling variable takes on the value of the next

element in the list. When used in a while or until construct, on the other hand, execution resumes with TEST−COMMAND at the top of the loop.

4.6.3 Examples

In the following example, file names are converted to lower case. If no conversion needs to be done, a continue statement restarts execution of the

loop. These commands don't eat much system resources, and most likely,

5) $FORTUNE bofh−excuses ;;

6) $FORTUNE magic

;; 7) $FORTUNE love

;; 8)

$FORTUNE literature ;; 9)

$FORTUNE drugs ;;

10) $FORTUNE education ;;

0) echo "OK, see you!"

break ;; *)

echo "That is not a valid choice, try a number from 0 to 10." ;;

esac done


122

similar problems can be solved using sed and awk. However, it is useful to know about this kind of construction when executing heavy jobs, that might

not even be necessary when tests are inserted at the correct locations in a script, sparing system resources.

This script has at least one disadvantage: it overwrites existing files. The noclobber option to Bash is only useful when redirection occurs. The −b

option to the mv command provides more security, but is only safe in case of one accidental overwrite, as is demonstrated in this test:

#!/bin/bash

# This script converts all file names containing upper case characters into

file

# names containing LIST="$(ls)" for name in "$LIST"; do

if [[ "$name" != *[[:upper:]]* ]]; then

continue fi

ORIG="$name" NEW=ècho $name | tr 'A−Z' 'a−z'`

mv "$ORIG" "$NEW" echo "new name for $ORIG is $NEW"

done

tolower.sh

bash> rm *

bash> touch test Test TEST

bash> bash −x tolower.sh

++ ls

+ LIST=test Test TEST

+ [[ test != *[[:upper:]]* ]] + continue

+ [[ Test != *[[:upper:]]* ]] + ORIG=Test


123

The tr is part of the textutils package; it can perform all kinds of character transformations.

4.7 Making menus with the select built−in

4.7.1 General

Use of select

The select construct allows easy menu generation. The syntax is quite similar to that of the for loop:

select WORD [in LIST]; do RESPECTIVE−COMMANDS; done

LIST is expanded, generating a list of items. The expansion is printed to standard error; each item is preceded by a number. If in LIST is not present,

the positional parameters are printed, as if in $@ would have been specified. LIST is only printed once.

Upon printing all the items, the PS3 prompt is printed and one line from standard input is read. If this line consists of a number corresponding to one

of the i tems, the value of WORD is set to the name of that item. If the line is empty, the items and the PS3 prompt are displayed again. If an EOF (End Of

++ echo TEST ++ tr A−Z a−z

+ NEW=test + mv −b TEST test

+ echo 'new name for TEST is test' new name for TEST is test bash> ls −a

./ ../ test test~


124

File) character is read, the loop exits. Since most users don't have a clue which key combination is used for the EOF sequence, it is more user−friendly

to have a break command as one of the items. Any other value of the read line will set WORD to be a null string.

The read line is saved in the REPLY variable. The RESPECTIVE−COMMANDS are executed after each selection until the

number representing the break is read. This exits the loop.

Examples

This is a very simple example, but as you can see, it is not very user−friendly:


125

Setting the PS3 prompt and adding a possibility to quit makes it better:

#!/bin/bash echo "This script can make any of the files in this directory

private." echo "Enter the number of the file you want to protect:"

select FILENAME in *; do

echo "You picked $FILENAME ($REPLY), it is now only accessible to you."

chmod go−rwx "$FILENAME" done

bash>./private.sh This script can make any of the fi les in this directory

private. Enter the number of the file you want to protect: 1) archive−20030129

2) bash 3) private.sh #? 1

You picked archive−20030129 (1) #?

#!/bin/bash

echo "This script can make any of the files in this directory private." echo "Enter the number of the file you want to protect:"

PS3="Your choice: "

QUIT="QUIT THIS PROGRAM − I feel safe now." touch "$QUIT"

select FILENAME in *; do

case $FILENAME in "$QUIT") echo "Exiting."

break ;;

*) echo "You picked $FILENAME ($REPLY)" chmod go−rwx "$FILENAME"

;; esac

done rm "$QUIT"

private.sh


126

4.7.2 Submenus

Any statement within a select construct can be another select loop, enabling (a) submenu(s) within a menu.

By default, the PS3 variable is not changed when entering a nested select loop. If you want a different prompt in the submenu, be sure to set it at the

appropriate time(s).

4.8 The shift built−in

4.8.1 What does it do?

The shift command is one of the Bourne shell bui lt−ins that comes with Bash. This command takes one argument, a number. The positional parameters are shifted to the left by this number, N. The positional parameters from N+1 to $#

are renamed to variable names from $1 to $# − N+1. Say you have a command that takes 10 arguments, and N is 4, then $4 becomes $1, $5

becomes $2 and so on. $10 becomes $7 and the original $1, $2 and $3 are thrown away. If N is zero or greater than $# (the total number of arguments, see Section

7.2.1.2). If N is not present, it is assumed to be 1. The return status is zero unless N is greater than $# or less than zero; otherwise it is non−zero.

4.8.2 Examples

A shift statement is typically used when the number of arguments to a command is not known in advance, for instance when users can give as many

arguments as they like. In such cases, the arguments are usually processed in a while loop with a test condition of (($# )). This condition is true as long as the number of arguments is greater than zero. The $1 variable and the shift

statement process each argument. The number of arguments is reduced each time shift is executed and eventually becomes zero, upon which the while

loop exits. The example below, cleanup.sh, uses shift statements to process each file in

the list generated by find:


127

The above find command can be replaced with the following:

find options | xargs [commands_to_execute_on_found_files]

The xargs command builds and executes command lines from standard input. This has the advantage that the command line is filled unti l the system limit is

reached. Only then will the command to execute be called, in the above example this would be rm. If there are more arguments, a new command line

will be used, until that one is full or until there are no more arguments. The same thing using find −exec calls on the command to execute on the found files every time a file is found. Thus, using xargs greatly speeds up your

scripts and the performance of your machine.

4.9 Summary

In this chapter, we discussed how repetitive commands can be incorporated in loop constructs. Most common loops are built using the for, while or until

statements, or a combination of these commands. The for loop executes a task a defined number of times. If you don't know how many times a command should execute, use either until or while to specify when the loop

should end.

Loops can be interrupted or reiterated using the break and continue statements. A file can be used as input for a loop using the input redirection

#!/bin/bash

# This script can clean up fi les that were last accessed

over 365 days ago. USAGE="Usage: $0 dir1 dir2 dir3 ... dirN"

if [ "$#" == "0" ]; then

echo "$USAGE" exit 1 fi

while (( "$#" )); do

if [[ "$(ls $1)" == "" ]]; then echo "Empty directory, nothing to be done."

else find $1 −type f −a −atime +365 −exec rm −i {} \;

fi shift

done


128

operator, loops can also read output from commands that is fed into the loop using a pipe.

The select construct is used for printing menus in interactive scripts. Looping

through the command line arguments to a script can be done using the shift statement.


1. What is the use of Loops?

2. List the different types of Loops in shell? 3. What is the use of the "break" statement?

4. What will the following construct do and why? while [ 5 ]


1. Loops let the user perform a set of instructions repeatedly.

2. For, While and Unti l. 3. The break statement is used to exit the current loop before its normal ending. 4. This sets up an infinite loop since a value greater than 0 is considered to be

true.


1. How would you decide which type of loop to use? 2. Explain why it is so important to put the variables in between double quotes in

the example from Section 4.4.2? 3. Describe the ―shift‖ built-in command. 4. There are at least 6 syntactical mistakes in the following program. Locate

them.


129

1 ppprunning = yes 2 while $ppprunning = yes ; do

3 echo ― INTERNET MENU\n 4 1. Dial out 5 2. Exit

6 7 Choice:

8 read choice 9 case choice in 10 1) i f [ -z ―$ppprunning‖ ]

11 echo ―Enter your username and password‖

12 else 13 chat.sh 14 endif ;

15 *) ppprunning=no 16 endcase

17 done


LESSON 5 REGULAR EXPRESSIONS

5. REGULAR EXPRESSIONS......................................................................................... 133

5.0 OBJECTIVES .......................................................................................................... 133

5.1 INTRODUCTION ...................................................................................................... 133

5.2 REGULAR EXPRESSIONS ....................................................................................... 133

5.2.1 What are regular expressions? .................................................................... 133

5.2.2 The Structure of a Regular Expression....................................................... 134

5.2.3 Regular expression metacharacters ........................................................... 135

5.2.4 Creating complex regular expressions by concatenating other regEx .. 136

5.2.5 Using metacharacters on regEx to create complex regEx ..................... 136

5.3 THE GREP .............................................................................................................. 137

5.3.1 Grep and regular expressions ...................................................................... 138

5.4 PATTERN MATCHING USING SHELL........................................................................ 140

5.4.1 Character ranges............................................................................................ 140

5.4.2 Character classes........................................................................................... 141

5.5 SUMMARY .............................................................................................................. 141

5.6 ANSWERS TO THE SELF-CHECK QUESTIONS........................................................ 142



133

5. Regular Expressions

Regular expressions are very helpful in creating powerful scripts. Regula r expressions are also used heavily in advanced Unix utilities that we will be studying further, like sed, AWK and perl language.

5.0 Objectives After going through this lesson, you will learn about:

Using regular expressions

Regular expression metacharacters

Finding patterns in files or output

Character ranges and classes in Bash

5.1 Introduction This chapter introduces the concept of regular expressions. A regular

expression is a pattern that describes a set of strings. This is a very powerful concept and can be used effectively in scripting.

5.2 Regular expressions

5.2.1 What are regular expressions?

Often you will encounter conditions where you need to match specific patterns in scripts. For example, given a list of cricket players you may need to find out

all those players whose names begin with A or B. In other words, you need to match with a pattern set. A regular expression helps you define a pattern

space in a terse way. For example, if you want to match any number where no other digit used other than 9 (e.g., 9, 99, 999, 9999, …), then it is impossible to write out the entire pattern set. But a regular expression can

express the same set very easily. Lets see what is a regular expression and how are they used.

Here are few examples of regular expressions. You will begin to understand how they represent their patter set as you study this chapter.

9* => Any number that contains only digit 9 (e.g., 99, 9999, etc.)

India.* => Any string beginning with India (e.g., India, Indian, Indiana, etc.)


134

A regular expression is a sequence of characters that represents patterns.

The pattern can be a simple word, like, ―India‖, or can describe more general set of patterns like ―India‖, ―Indian‖, ―Indiana‖, etc. Using regular expression

you can create general patterns like any 3 digit number that does not contain the digit 2.

What is meant by ―regular‖ in the term regular expression? The term ―regular‖ refers to the fact that there is a pre-defined repetition that it denotes. If the

repetitions are irregular, then you cannot denote the pattern with a regular expression. For example, a set of all the prime numbers cannot be denoted using a regular expression!

What is meant by ―expression‖ in the term regular expression? The

―expression‖ in regular expression refers to the fact that, just like mathematical expressions, regular expressions can be combined together to form new and more complex regular expression.

By the way, regular expressions are often referred to as regEx by developers.

5.2.2 The Structure of a Regular Expression

All single characters, including characters like ‗a‘, ‗=‘, ‗3‘, etc., are fundamental regular expressions. They match the single character they represent. Most

characters, including all letters and digits, are regular expressions that match themselves. The fundamental regular expressions can be combined to create more complex regular expressions. Lets see how we create more complex

regEx.

There are three important parts to a regular expression:

Anchors

Character sets

Modifiers

Anchors are used to specify the position of the pattern in relation to a line of text.

Character Sets match one or more characters in a single position.

Modifiers specify how many times the previous character set is repeated.

A simple example that demonstrates all three parts is the regular expression "^#*." The up arrow is an anchor that indicates the beginning of the line. The character "#" is a simple character set that matches the single character "#".

The asterisk is a modifier. In a regular expression asterisk specifies that the character set can appear any number of times.


135

5.2.3 Regular expression metacharacters

There are few special characters that specify repetition styles for the

preceding character or the preceding expression. These special characters that denote the repetition types are called Meta Characters.

The table below lists various metacharacters and their meanings.

Table – Regular expression metacharacters

Operator Effect

. (single dot) Matches any single character

? The preceding item is optional and will be matched, at most, once.

* The preceding item will be matched zero or more

times.

+ The preceding item will be matched one or more times.

{N} The preceding item will be matched exactly N times.

{N,} The preceding item will be matched exactly N or

more times.

{N,M} The preceding item will be matched at least N times, but not more than M times.

- Represents the range if it‘s not first or last in a list or

the ending point of a range in a list.

^ Matches the empty string at the beginning of a line; also represents the characters in the range of a list.

$ Matches the empty string at the end of a line.

\b Matches the empty string at the edge of a word.

\B Matches the empty string provided it‘s not at the

edge of word.

\< Match the empty string at the beginning of a word.

\> Match the empty string at the end of word.

In the example below, the * indicates zero or more

repetitions of 9. 9* => Any number that contains only digit 9 (e.g., 99, 9999, etc.)

In the example below, the . indicates any character and

therefore .* indicates any number of repetitions of any characters. India.* => Any string beginning with India (e.g., India,

Indian, Indiana, etc.)

So, for example, India.* will also match India123, IndiaZZZ, etc.


136

5.2.4 Creating complex regular expressions by concatenating other regEx

Suppose you want to use a regular expression to match any string in which

letter ‗A‘ repeats one or number of times. (e.g., A, AA, AAA, etc.). Then the regular expression for this is

Now suppose you want to use a regular expression to match any string in which the digit 4 repeat any number of times.

Now, suppose you want to create a regular expression to match any string in

which first the letter ‗A‘ repeats one ore more number of times and then the digit 4 repeats any number of times (e.g., A4, A444, AA4, etc). So, you can

combine the regular expression created earlier:

5.2.5 Using metacharacters on regEx to create complex regEx

Now, suppose you want to create a regular expression that denotes an unsigned real number. You can use the following regEx for it:

Lets dissect this example to understand better:

First [0-9]+ will match one or more occurrence of a digit.

To make the fractional part, we need to allow a dot (e.g., dot in .32) . So we have \. there.

The fractional part, if present needs to again have at least one digit, so have the complete fractional part written as \.[[0-9]+ there.

However we need to make sure that the fractional part should be optional (it

should match numbers without the fractional parts too). So, the fractional part is made optional by putting a question mark for it. Thus making the entire regEx as [0-9]+(\.[0-9]+)?

A+ => will match A, AA, AAA, etc. but will not match empty string.

4* => will match 4, 44, 444, etc. and will also match an empty string.

A+4* => will match A4, A44, AA4, etc. but will not match 4AA.

[0-9]+(\.[0-9]+)? => will match 4, 0.32, 4, etc. but will not match -5, .33 or 7e-3.


137

5.3 The grep command Unix has a command to that performs regular expressions based search. This

command is called grep. grep searches the input for lines containing a match to a given pattern list. When it finds a match in a line, it prints the line.

Note that grep command does not match patterns across multiple lines. Here are few examples on grep.

With the first command, user displays the lines from /etc/passwd containing

the string root. Then displays the line numbers containing this search string. With the third command the user checks which users are not using bash, but

accounts with the nologin shell are not displayed.

Then the user counts the number of accounts that have /bin/false as the shell. The last command displays the lines contining root or Root or ROOT, etc..

Now let's see what else we can do with grep, using regular expressions.

bash> grep root /etc/passwd root:x : 0 : 0 : root:/root:/bin/bash operator:x : 11 : 0 : operator:/root:/sbin/nologin

bash> grep −n root /etc/passwd # prints line

numbers of matches 1: root:x : 0 : 0 : root:/root:/bin/bash 12 : operator:x : 11 : 0 : operator:/root:/sbin/nologin

bash> grep −v bash /etc/passwd | grep −v nologin #

matching reverted sync : x : 5 : 0 : sync : /sbin:/bin/sync shutdown : x : 6 : 0 : shutdown : /sbin:/sbin/shutdown

halt:x : 7: 0 : halt:/sbin:/sbin/halt news : x : 9 : 13 : news : /var/spool/news:

apache : x : 48 : 48 : Apache : /var/www : /bin/false

bash> grep −c false /etc/passwd # returns number of

matches 7

bash> grep −i root /etc/passwd # match regardless of the case Root:0:0:/root

root:0:0:/sysadm


138

5.3.1 Grep and regular expressions

a. Line and word anchors

From the previous example, we now exclusively want to display lines starting with the string "root":

If we want to see which accounts have no shell assigned whatsoever, we

search for lines ending in ":":

To check that PATH is exported in ~/.bashrc, first select "export" lines and then search for lines starting with the string "PATH", so as not to display

MANPATH and other possible paths:

If you want to find a string that is a separate word (enclosed by spaces), it is better to use the −w, as in this example where we are displaying information

for the root partition:

If this option is not used, all the lines from the file system table will be displayed.

b. Character classes

A bracket expression is a list of characters enclosed by "[" and "]". It matches

any single character in that list; if the first character of the list is the caret, "^", then it matches any character NOT in the list. For example, the regular expression "[0123456789]" matches any single digit. You can also write it like

[0-9].

bash> grep ^root /etc/passwd root:x:0:0:root:/root:/bin/bash

bash> grep :$ /etc/passwd news:x:9:13:news:/var/spool/news:

bash> grep export ~/.bashrc | grep „'\<PATH' export

PATH="/bin:/usr/lib/mh:/lib:/usr/bin:/usr/local/bin:/usr/ucb:/usr/dbin:$PATH"

bash> cat myFile.txt Neil Armstrong was the first man to walk on the moon.

He had said, ―this is a small step for me but a huge step for mankind‖.

bash> grep –w man myFile.txt Neil Armstrong was the first man to walk on the moon.

Note here that the other line is not matched because

mankind is a single word hence will not match for the word man because –w option is used.


139

Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between

the two characters, inclusive, using the locale's collating sequence and character set. For example, in the default C locale, "[a−d]" is equivalent to

"[abcd]". Many locales sort characters in dictionary order, and in these locales "[a−d]" is typically not equivalent to "[abcd]"; it might be equivalent to "[aBbCcDd]", for example. To obtain the traditional interpretation of bracket

expressions, you can use the C locale by setting the LC_ALL environment variable to the value "C".

Finally, certain named classes of characters are predefined within bracket expressions. See the grep man or info pages for more information about

these predefined expressions.

In the example, all the lines containing either a "y" or "f" character are first displayed, followed by an example of using a range with the ls command.

c. Wildcards

Use the "." for a single character match. If you want to get a list of all

five−character English dictionary words starting with "c" and ending in "h" (handy for solving crosswords):

If you want to display lines containing the literal dot character, use the −F

option to grep.

bash> grep [yf] /etc/group sys:x : 3 : root,bin,adm tty : x : 5 :

mail : x : 12 :mail,postfix ftp : x : 50 :

nobody : x : 99 : floppy:x : 19 : xfs : x : 43 :

nfsnobody : x : 65534 : postfix : x : 89 :

bash> ls *[1−9].xml app1.xml chap1.xml chap2.xml chap3.xml chap4.xml

bash> grep „'\<c...h\>' /usr/share/dict/words catch

clash cloth

coach couch cough

crash crush


140

For matching multiple characters, use the asterisk. This example selects all

words starting with "c" and ending in "h" from the system's dictionary:

5.4 Pattern matching using shell

5.4.1 Character ranges

Apart from grep and regular expressions, there's a good deal of pattern

matching that you can do directly in the shell, without having to use an external program.

As you already know, the asterisk (*) and the question mark (?) match any string or any single character, respectively. Quote these special characters to match them literally:

But you can also use the square braces to match any enclosed character o r range of characters, if pairs of characters are separated by a hyphen. An

example:

Lists all files in radha's home directory, starting with "a", "b", "c", "x", "y" or "z". If the first character within the braces is "!" or "^", any character not enclosed

will be matched. To match the dash ("−"), include it as the first or last character in the set. The sorting depends on the current locale and of the

value of the LC_COLLATE variable, if it is set. Mind that other locales might interpret "[a−cx−z]" as "[aBbCcXxYyZz]" if sorting is done in dictionary order. If you want to be sure to have the traditional interpretation of ranges, force this

behavior by setting LC_COLLATE or LC_ALL to "C".

bash> ls −ld [a−cx−z]*

drwxr−xr−x 2 radha radha 4096 Jul 20 2002 app−defaults/ drwxrwxr−x 4 radha radha 4096 May 25

2002 arabic/ drwxrwxr−x 2 radha radha 4096 Mar 4

18:30 bin/ drwxr−xr−x 7 radha radha 4096 Sep 2 2001 crossover/

drwxrwxr−x 3 radha radha 4096 Mar 22 2002 xml

bash> grep „'\<c.*h\>' /usr/share/dict/words caliph cash

catch cheesecloth

cheetah

bash> ls "*" This will not list all the files. It will list the file named *.


141

5.4.2 Character classes

Character classes can be specified within the square braces, using the syntax

[:CLASS:], where CLASS is defined in the POSIX standard and has one of the values "alnum", "alpha", "ascii", "blank", "cntrl", "digit", "graph", "lower", "print",

"punct", "space", "upper", "word" or "xdigit".

When the extglob shell option is enabled (using the shopt built−in), several

extended pattern matching operators are recognized.

5.5 Summary Regular expressions are powerful tools for selecting particular lines from fi les or output. A lot of UNIX commands use regular expressions: vim, perl, the

PostgreSQL database and so on. They can be made available in any language or application using external libraries, and they even found their way

to non−UNIX systems. For instance, regular expressions are used in the Excell spreadsheet that comes with the MicroSoft Windows Office suite. In this chapter we got the feel of the grep command, which is indispensable in

any UNIX environment.

Bash has built−in features for matching patterns and can recognize character classes and ranges.


1. What are regular expressions 2. What will be the result of ls -l | grep '^.....w' 3. What does the expression gg* signify?

4. How do you locate lines in a file foo containing ram and raman using grep?

bash> ls −ld [[:digit:]]* drwxrwxr−x 2 radha radha 4096 Apr 20 13:45

2/

bash> ls −ld [[:upper:]]* drwxrwxr−− 3 radha radha 4096 Sep 30 2001 Nautilus/

drwxrwxr−x 4 radha radha 4096 Jul 11 2002 OpenOffice.org1.0/

−rw−rw−r−− 1 radha radha 997376 Apr 18 15:39 Schedule.sdc


142


1. A regular expression is a pattern that describes a set of strings. 2. This locates all files which have write permission for the group (e.g. drwxrw-r-

x) 3. One or more occurrences of g. 4. Use grep ―rama*n*‖ foo


1. Describe the structure of a regular expression. 2. Describe some regular expression operators. 3. What is the difference between a wild card and a regular expression?

4. What is the difference between basic and extended regular expression

UNIT 3: Advanced Shell Scripting, sed, and

awk

1. FUNCTIONS IN SHELL SCRIPTS .................................................................. 147

2. SED – STREAM EDITOR .................................................................................... 159

3. AWK BASICS ........................................................................................................... 169

4. AWK PROGRAMMING ........................................................................................ 177


LESSON 1 FUNCTIONS IN SHELL SCRIPTS

1. FUNCTIONS IN SHELL SCRIPTS ............................................................................. 147

1.0 OBJECTIVES .......................................................................................................... 147

1.1 INTRODUCTION TO SHELL FUNCTIONS................................................................... 147

1.1.1 When to use functions? ................................................................................. 147

1.1.2 Benefits of using functions ............................................................................ 149

1.1.3 Where you cannot create functions?........................................................... 150

1.2 WRITING A SHELL FUNCTION ................................................................................. 150

1.2.1 Function header.............................................................................................. 150

1.2.2 Function body ................................................................................................. 151

1.2.3 Returning from a function.............................................................................. 152

1.2.4 Function arguments ....................................................................................... 152

1.2.5 IFS (internal field separators) ....................................................................... 153

1.2.6 Creating a utility library of shell functions ................................................... 154

1.2.7 Things to keep in mind while writing shell functions ................................. 154

1.3 SUMMARY .............................................................................................................. 155

1.4 ANSWERS TO THE SELF CHECK QUESTIONS ......................................................... 155



147

1. Functions in Shell Scripts

So far you have learnt various unix commands, plumbing commands together using

pipes and creating shell scripts for programming to carry out useful and routine, repetitive tasks. Shell scripting in unix can never be complete without knowing how to write and use functions.

1.0 Objectives After going through these lessons you will know

When to use functions in shell scripts

How to write and use functions in shell scripts

1.1 Introduction to shell functions Often there are few lines of code that need to be used at several places in the shell scripts. For example, if you are creating a shell script that will read a 3

digit STD code and 7 digit phone number and you need to ensure that user types in exactly 3 numeric characters for STD code and exactly 7 numeric

characters for phone number, then it will be better to create and use a function instead of replicating the same code at multiple places.

A function is like a mini script. It can take parameters, can define its own variables, can return a value, etc. Unlike a script‘s call, a function executes in

the same shell. Functions in shell scripts look and work similar to functions in C language.

1.1.1 When to use functions?

Consider the example listed in 1.1 above, without using functions:


148

Script 1

You will find that apart from the marked text below, the rest of the code is repeated.

#!/bin/bash

stdOK=0 do

echo ―Please enter 3 digit STD code: ― read std chkSTD=ècho $std | grep ―^[0-9][0-9][0-

9]$‖` if [ ―$chkSTD‖‖X‖ != ―X‖ ]; then

stdOK=1 else echo ―Please enter exactly 3 digit STD

code here‖ fi

while [ $stdOK –neq 1 ] phoneOK=0

do echo ―Please enter 7 digit phone number: ―

read phone chkPH=ècho $phone | grep ―^[0-9][0-9][0-9][0-9][0-9][0-9][0-9]$‖`

if [ ―$chkPH‖‖X‖ != ―X‖ ]; then phoneOK=1

else echo ―Please enter exactly 7 digit phone number here‖

fi while [ $phoneOK –neq 1 ]

callup $std $phone


149

Script 2

See how much simpler it would be if you had a function that got you the desired numbers!!

Script 3

1.1.2 Benefits of using functions

Functions provide several benefits as listed below:

#!/bin/bash

stdOK=0 do echo ―Please enter 3 digit STD code: ―

read std chkSTD=ècho $std | grep ―^[0-9][0-9][0-

9]$‖` if [ ―$chkSTD‖‖X‖ != ―X‖ ]; then stdOK=1

else echo ―Please enter exactly 3 digit STD

code here‖ fi while [ $stdOK –neq 1 ]

phoneOK=0

do echo ―Please enter 7 digit phone number: ― read phone

chkPH=ècho $phone | grep ―^[0-9][0-9][0-9][0-9][0-9][0-9][0-9]$‖`

if [ ―$chkPH‖‖X‖ != ―X‖ ]; then phoneOK=1 else

echo ―Please enter exactly 7 digit phone number here‖

fi while [ $phoneOK –neq 1 ] callup $std $phone

#!/bin/bash

std=`getNumber 3 ―STD code‖` phone=`getNumber 7 ―phone number‖ callup $std $phone


150

Functions simplify and modularize your scripts. Your scripts become better readable (compare script 1 and script 3 above).

Modularize scripts are easier to maintain and enhance.

Functions provide you easier debugging.

Once you enhance a function, the enhanced effect is automatically available at all places where the function is used.

You can even create a utility file containing functions and source it in your other scripts so that uti lity functions are directly available for use, instead of

writing them over and over again.

1.1.3 Where you cannot create functions?

Be aware that not all shells provide support for functions. For example csh (C-

shell) does not provide support for functions. But most other shells have this support, including sh (Bourne shell), ksh (korn

shell), tsh, bash (born again Bourne shell), etc.

Self Check Questions

1. When few lines of code needs to be repeated at several places a ______ should

be created for it (select one): a. script b. program

c. function 2. A function helps in improving the script by making it (select one or many as

apply): d. more readable e. more debug gable

f. modular g. more maintainable

1.2 Writing a shell function A shell function in bash has the following syntax. Text in bold indicates keywords.

<yourFunctionName>() { <commands>; }

Or function <yourFunctionName> { <commands>; }

1.2.1 Function header


151

You can define a function by using the function keywords or you can define a function by putting braces after the function name. For example:

Following defines a function named ―aaa‖.

Following defines a function named ―bbb‖.

Note that parameters to functions are not passed like C. Therefore, in

function‘s header you will not declare any parameters. See the definition of the function ―bbb‖ above. No parameters are ever listed within the braces.

1.2.2 Function body

Set of commands comprise of the function body. A function can contain any set of shell scripting commands, including flow

control commands like while and conditional commands like if, etc. Commands can also contain calls to other functions and even other shell

scripts. For example:

The above script uses the call to ―date‖ shell command.

function aaa { a = 1 }

bbb() { a = 1 }

getDateString() { echo ―Date format is dd/mm/yy ?:‖ read x

if [ ―$x‖‖X‖ = ―yX‖ ];then str=`date ‗+%dd%mm%yy‘`

else str=`date ‗+%yy%mm%dd‘` fi

echo $str }


152


3. The function keyword is must for writing a function (true/false). 4. You must declare arguments to a function in the function header (true/false).

5. You cannot declare arguments to a function in the function header (true/false). 6. Function body can contain any of the shell commands (true/false).

1.2.3 Returning from a function

If your function reaches the end of its body and it has an echo command, it

echoes the return value. Alternatively, you can return without completing the execution of the function body by using the return keyword.

For example:

1.2.4 Function arguments

Parameters can be passed when calling a function by listing them in front of

the function. When inside the function, these parameters can be accessed as shell variables, $1, $2, etc. Even $# (number of arguments passed) is

available inside the function. Example:

aaa() { a=1

b=2 echo $a

} ret=àaa` # ret will be 1

bbb() {

a=1 b=2 return $a

}

ret=àaa` # ret will be 1


153

1.2.5 IFS (internal field separators)

You need to be careful while passing arguments to a function or a shell

command in a shell script. Shell interprets the values that you supply. As a result, a string passed as a parameter can get interpreted as multiple parameters if it contains spaces.

For example:

paintObj “greenish blue” Here you would expect to see $1 inside the function as ―greenish blue‖ but

you will get $1 as ―greenish‖ and $2 as ―blue‖.

You can tell shell to interpret newline as a field separator by declaring in your script IFS=”

― # Yes, the closing quote is on the next line!

Therefore, if you use the following: IFS=” “

paintObj “greenish blue”

Now, here you will get $1 inside the function as ―greenish blue‖.


7. Parameters passed to a function are accessible using $1, $2, variables. (true/false).

8. The $# inside a function indicates the number of parameters passed to the script (true/false).

addTwoNums() {

sum=0 sum=èxpr $1 + $2`

return $sum }

addAllNums() { sum=0

if [ ―$1‖‖X‖ = ―X‖ ];then return $sum else

sum=`$sum + $1` fi

}


154

1.2.6 Creating a utility library of shell functions

When you create shell functions, you would typically want to make them

somewhat generic so that they can be reused in other shell scripts as well. In such cases, you can simply collect your shell functions into a single file. Such a file containing utility shell functions can be used as a library and can be

sourced in other shell scripts.

For example:

1.2.7 Things to keep in mind while writing shell functions

Just like other shell commands, there are restrictions when writing shell

functions.

The starting curly bracket must be right on the same line as the function

header.

There must be spaces on both sides of curly brackets.

There must be either a semi colon or a new line before the closing curly bracket.

bash>cat a_simple_utility_library.sh

#!/bin/sh #--------------------------------- a_simple_utility_library

--------------------------- IFS=‖

― # myecho function echoes the input and also writes it into multiple files

myecho() { for i in $FILE_LIST

do echo $* >> $i done

echo $* }

mykill() { pid=`ps –ef | grep $1 | grep –v ―grep‖ | awk

‗{print $2}‘` kill $pid

} #------------------------- a_simple_utility_library ends ---------------------------

bash>cat my_application.sh

#!/bin/sh . a_simple_utility_library.sh # The dot in the

beginning sources it mykill junkjob # will kill the process running ―junkjob‖


155

1.3 Summary

Functions help in modularizing the scripts for repetitive tasks. If you use functions, scripts become better readable and maintainable.


1. (c)

2. all. 3. false.

4. false. 5. true. 6. false.

7. true. 8. false.


1. Discuss among your peers how functions are different from aliases.

2. Write a function that gets you a non-empty string. 3. Write a function that uses the function created in assignment 2 above to read

and convert a string into all uppercase.

4. Write a script that takes name, middle name and family name of a person and prints them out in all uppercase or all lowercase depending on a shell

variable‘s value. 5. Write a function that takes number of digits as an input and gets a number

containing those many digits. The function must check that user has to

provide a number. 6. Write a function that takes number of digits as an input and gets a number

containing at most those many digits.


LESSON 2 SED – STREAM EDITOR

2. SED – STREAM EDITOR ............................................................................................ 159

2.0 OBJECTIVES .......................................................................................................... 159

2.1 INTRODUCTION TO SED ......................................................................................... 159

2.2 HOW SED OPERATES ............................................................................................. 159

2.3 SYNTAX OF THE SED COMMAND ............................................................................ 160

2.3.1 Options for the sed ......................................................................................... 160

2.4 COMMANDS IN SED................................................................................................ 161

2.4.1 Syntax of the commands in sed ................................................................... 162

2.5 SUMMARY .............................................................................................................. 164




159

2. SED – Stream Editor

Sed (Stream editor) is a utility program available in unix. sed is a powerful utility that

can be used to transform the input, line-by-line. sed is commonly used in scripting.

2.0 Objectives After going through these lessons you will know

What is sed? Its options and commands

What are regular expressions

Interactive use of sed

Using sed commands in scripts

2.1 Introduction to sed A Stream Editor is used to perform transformations on text read from a file or

a pipe. Sed sends the result to the standard output which can be redirected and collected into another file, if needed.

Sed does not modify the original input file. Unlike other editors, vi and ed, which are interactive editors, sed works on an input stream. Sed therefore is

suitable in scripts when you need text transformations, like in conversion programs.

For example: If you have a fi le where ―error‖ is misspelt as ―erorr‖, you can correct them by using sed command:

sed „s/erorr/error/g‟ myfile > myfile_corrected

2.2 How sed operates It often comes handy to know how a utility works. Here is the detail on how sed works:

A line of input is copied into a pattern space.

All editing commands in a sed script are applied in order to the copied line.

The copied (and modified) line is sent to standard output.

By default, sed works on all the lines of input. However, its scope can be

controlled by line addressing.

Editing commands are applied to all lines (globally) unless line addressing restricts the lines affected.


160

If a command changes the input, subsequent command-addresses will be applied to the current line in the pattern space, not the original input line.

2.3 Syntax of the sed command

Sed can be invoked in one of the following forms: sed [options] 'command' file(s)

Or sed [options] -f scriptfile file(s)

The first form allows you to specify an editing command on the command line, surrounded by single quotes.

The second form allows you to specify a scriptfile, a file containing sed commands. If no files are specified, sed reads from standard input.

2.3.1 Options for the sed

The –e option

-e <script> option tells sed to add the commands in <script> to the set of commands to run. You can give a series of commands using –e option. For example:

sed -e 's/a/A/' -e 's/b/B/' < oldFile >newFile

The –f option

-f <scriptFile> : Tells sed to add the commands from <scriptFile> to the set of

commands to run. For example, instead of just replacing ‗a‘ and ‗b‘, it you want to uppercase all vowels in the input, you can write an sed script file:

sed -f sed_script < oldFile > newFile will uppercase all vowels.

Note that in sed script files, each command must be on a separate line. No

trailing white spaces can exist at the end of lines. No quotes can be used.

The –n option

bash>cat sed_script # sed comment - This script changes lower case

vowels to upper case s/a/A/g s/e/E/g

s/i/I/g s/o/O/g

s/u/U/g


161

-n : This option tells sed not to print by default. Only when specific sed commands for print are used, those specific items will be printed. For

example, sed –n „s/pattern/&/p‟ file

will act like grep looking for ―pattern‖.


1. sed is an interactive editor like vi (true/false) 2. sed can be used in scripts (true/false)

2.4 Commands in sed Sed supports grep like regular expressions to find the text for pattern

substitution and deletion. Sed uses vi like commands:

a appends text below the current line

i Insert text above the current line

c change text in the current line with new text

s search and replace text

d Delete text

p Prints text

For example, if there is a file that lists tasks like:

To delete all lines in a file that are marked DONE, you can use

bash>cat tasks

DONE: functions TODO: sed TODO: awk

DONE: password change

sed ‗/DONE/d‘ tasks > new_tasks

bash>cat new_tasks TODO: sed TODO: awk


162

2.4.1 Syntax of the commands in sed

The sed commands have the general form as listed below:

[address][,address][!]operation [arguments] Sed commands consist of addresses and operation. Each operation consists of a

single letter.

Let‘s take the following input file for the examples given below:

1. If no address is specified, the operation is applied to each line. For example:

2. Only the first pattern is matched by default. For example,

The second “the” is not modified.

To tell sed to work on all the matched patterns on a line, use ―g‖.

bash>cat input_file

This is the first line This is the second line of text

This is the third line of input_file This is the fourth and the last line

sed ‗s/This/this/g‘ < input_file > output_file

bash>cat output_file

this is the first line this is the second line of text

this is the third line of input_file this is the fourth and the last line

sed ‗s/the/a/‘ < input_file > output_file


This is a first line This is a second line of text

This is a third line of input_file This is a fourth and the last line

sed ‗s/the/a/g‘ < input_file > output_file


This is a first line This is a second line of text

This is a third line of input_file This is a fourth and a last line


163

The second “the” is also modified now.

3. Only one address can be given. For example:

4. Two addresses can be given to make a block. For example:

5. $ can be used to denote end of file in specifying addresses For example:

6. Address can also be given using patterns. For example:

7. Address can also be inverted.

sed „/SAVE/!d‟ this will delete all lines that do not have SAVE on them

sed „/BEGIN/,/MID/s/error/error/g‟

sed ‗2s/second/SECOND/g‘ < input_fi le >

output_file

bash>cat output_file This is the first line

This is the SECOND line of text This is the third line of input_file This is the fourth and the last line

sed ‗1,2s/line/input/g‘ < input_file > output_file


This is the first input This is the second input of text

This is the third line of input_file This is the fourth and the last line

sed ‗3,$d‘ < input_file > output_file


This is the first input This is the second input of text

sed ‗/input_file/d‘ < input_file > output_file


This is the first line This is the second line of text

This is the fourth and the last line


164

this will replace erorr by error from BEGIN to MID.

sed „/^BEGIN/,/ÊND/!s/done//g‟

will delete the word done for all lines except for those lines between BEGIN and END.

Address and patterns can include grep like regular expressions as well. For example:


3. What argument can be used to tell sed to apply operations on all the matched patterns on a line: a. none. Sed already does that by default.

b. g c. i

4. What character can be used to invert the address in sed? a. none b. i

c. x

2.5 Summary The sed stream editor is a powerful command line tool, which can handle

streams of data: it can take input lines from a pipe. This makes it fit for non−interactive use. The sed editor uses vi−like commands and accepts regular expressions. The sed tool can read commands from the command line

or from a script. It is often used to perform find−and−replace actions on lines containing a pattern.


1. false.

2. true. 3. (b)

4. (b)

sed ‗/This.*first/p‘ input_file > output_fi le


This is the first line


165


1. Use sed to implement a head like uti lity of unix (prints only first 5 lines). 2. Use sed to implement tail like utility of unix (prints only the last 5 lines).

3. Print a list of files in your scripts directory, ending in ".sh". Mind that you might have to unalias ls. Put the result in a temporary file.

4. Make a list of files in /usr/bin that have the letter "a" as the second character.

Put the result in a temporary file. 5. Delete the first 3 lines of each temporary file.

6. Print to standard output only the lines containing the pattern "an". 7. Create a file holding sed commands to perform the previous two tasks. Add

an extra command to this file that adds a string like "*** This might have

something to do with man and man pages ***" in the line preceding every occurrence of the string "man". Check the results.

8. A long listing of the root directory, /, is used for input. Create a file holding sed commands that check for symbolic links and plain fi les. If a file is a symbolic link, precede it with a line like "−−This is a symlink−−". If the file is a plain file,

add a string on the same line, adding a comment like "<−−− this is a plain file". 9. Create a script that shows lines containing trailing white spaces from a file.

This script should use a sed script and show sensible information to the user 10. Can sed be used to create tail –f kind of utility? 11. Search the internet to find how newline can be replaced.

12. Top 4 lines of a file contain names of students and rest 4 lines contain their marks :

bash>cat file Mohit verma Sushobhit sinha

Mukul Khan Naina Suman

20 25 35

28 Using sed and paste command, create another file that will have

Mohit verma 20 Sushobhit sinha 25 Mukul Khan 35

Naina Suman 28


LESSON 3 AWK BASICS

3. AWK BASICS ................................................................................................................ 169

3.0 OBJECTIVES .......................................................................................................... 169

3.1 INTRODUCTION AND BRIEF HISTORY ..................................................................... 169

3.2 THE SYNTAX OF AWK ........................................................................................... 169

3.3 USING AWK .......................................................................................................... 170

3.3.1 The print command in AWK.......................................................................... 171

3.3.2 Accessing fields on a line.............................................................................. 172

3.4 SUMMARY .............................................................................................................. 174




169

3. AWK Basics

AWK is a utility for performing simple text-processing tasks. Awk also

provides a small but powerful language that allows the user to manipulate fi les containing columns of data and strings, to print reports from the data.

3.0 Objectives

After going through this lesson you will know

What is AWK, the syntax of AWK

How is AWK useful

Print command in AWK

How to access fields in AWK

3.1 Introduction and brief history AWK stands for the names of its authors: "Aho, Weinberger, &

Kernighan".

The original version of AWK was developed in 1977. In Unix it is available as awk. Advanced versions exist (e.g, nawk, gawk) that support user defined functions, multidimensional arrays, ?: operator,

deleting elements in an array, etc.

Awk operates in a cycle: get a line, process it, get the next line, process it, and so on. It is an "interpreted" language -- that is, an Awk program cannot run on its own, it must be executed by the Awk uti lity

itself.

Like sed, AWK reads an input file or reads from a pipe. It does not modify the input file and writes its output onto the standard output. In addition, because AWK is a programming language in itself, awk is

very useful in processing data and printing reports.

3.2 The syntax of AWK

'

awk [options] ‗

[ BEGIN {<initializations>} ] [ <program> ]

[ [ <program>] ] ... [ END {<final actions>} ]

' <File Name>


170

Where each <program> has the format:

[ <search pattern 1> ] [ {<program actions>} ]

Awk operates as listed below: 1. Perform initialization if BEGIN is given

2. Read a line of text, break it into fields 3. For each <program>

4. Perform the program as given by user 5. Goto step2. 6. Perform END calculations if specified by the user

The optional BEGIN clause performs any initializations required before

Awk starts scanning the input file. The subsequent body of the Awk program consists of a series of

search patterns, each with its own program action. Awk scans each line of the input fi le for each search pattern, and performs the

appropriate actions for each string found. Once the file has been scanned, an optional END clause can be used

to perform any final actions required.

3.3 Using AWK

We will use the following example data to see how to use awk. This data is a fi le containing the top marks for some of the subjects along

with the topper names and years.

Example 1: Since almost all of the awk syntax is optional, at the minimum, the simplest awk command can be written as

awk „‟ input_file

bash>cat toppers.txt

Physics 92 2003 Abhay Malhotra Chemistry 97 2003 Suman Gupta Maths 99 2003 Suresh Yadav

Physics 94.5 2004 Shriesh Jadhav Chemistry 98.5 2004 Shriesh Jadhav

Maths 96 2004 Lokesh Arora Physics 89 2005 Vandana Agarwal Chemistry 92 2005 Srinivas Vardharajan

Maths 99 2005 Anup Mathur Physics 98 2006 Ramakant

Chemistry 88 2006 Raju Pandy Chemistry 89 2006 Rajni Kumar Maths 98 2006 Javed M. K. Akthar


171

This will work like the cat command and print the entire input_file as is.

Note that here we are running an AWK code using awk command. The

code is always kept within quotes. Example 2: You can ask awk to work on specific lines. For example,

you can give a search pattern.

awk '/Physics/' toppers.txt > phy_toppers.txt Note that AWK does not modify the input.

Also note that AWK writes output to the standard output.

Here we have redirected the output into a file phy_toppers.txt. Now let‘s see the contents of this output fi le:

Example 3: Pattern matching is based on case. For example, here if you gave ―physics‖ in place of ―Physics‖ here as a search pattern, it

would not match the lines containing ―Physics‖.

awk '/physics/' toppers.txt > no_match.txt The fi le no_match.txt will come out an empty file.


1. Awk is useful for processing text containing columns of data. (true/false). 2. Awk is a small programming language in itself (true/false).

3. Awk does not modify the input fi le (true/false). 4. Awk program cannot run on its own but needs which one to run:

(a) awk (b) sed (c) grep

3.3.1 The print command in AWK

A simple print command is available in AWK. This command does not need any format specifications and values can be printed in a simple way.

bash>cat phy_toppers.txt Physics 92 2003 Abhay Malhotra

Physics 94.5 2004 Shriesh Jadhav Physics 89 2005 Vandana Agarwal

Physics 98 2006 Ramakant


172

Example 4: If you use print with no arguments, it prints the input text as is.

Example 5: You can give arguments to print

Note an important point here. The print command prints the arguments as is, so if you need any text like spaces, you will need to add that in

the print command itself as shown above. We will see more concrete examples of print command in subsequent examples.

3.3.2 Accessing fields on a line

The power of AWK lies in the fact that it treats each input line as a record consisting of fields. Which means, as it reads lines, it breaks up the line into fields and lets you access and manipulate fields and the

output.

By default AWK uses spaces as the separator for fields which means when it reads a line, it breaks it up into words for you. The separator can be changed easily as we will see later in this unit.

To access the fields of input line, awk provides the following built in

variables: $0, $1, $2 … $9. The first one, $0, gives you the entire line, as is. $1 is the first field, $2 is the second field, .. and so on.

Example 6: If the input line just read in by awk is

Physics 92 2003 Abhay Malhotra

awk „/Physics/ { print } /Maths/ {print }‟ toppers.txt

will print

Physics 92 2003 Abhay Malhotra Maths 99 2003 Suresh Yadav Physics 94.5 2004 Shriesh Jadhav

Maths 96 2004 Lokesh Arora Physics 89 2005 Vandana Agarwal

Maths 99 2005 Anup Mathur Physics 98 2006 Ramakant Maths 98 2006 Javed M. K. Akthar

awk ‗/Maths/ {print ―This is a math topper‖}‘ toppers.txt

will print This is a math topper

This is a math topper This is a math topper This is a math topper


173

then, $1 will contain ―Physics‖

$2 will contain 92 $3 will contain 2003

$4 will contain ―Abhay‖ $5 will contain ―Malhotra‖

Note that because awk is treating space as the separator, it breaks up the name too into two separate fields.

Example 7: To print just the names of chemistry toppers, you can use the following command:

Note that we have used $5 to $9, though the names that we got in the output would have come even with $5 and $6 alone because it seems from the output that names are occupying only two fields. However, we

do have a longer name (Javed M. K. Akhtar) also in the names which is occupying 3 fields. Therefore we need to be aware of the data when

printing multiple fields. AWK does not have a way to say things like ―print all fields from $5 onwards‖ so we need to use additional fields. However, if you simply want to print the entire line, then you do not

need to use these fields. For example,

Example 8: To print all data for math toppers, use the following

The examples so far were solving things that can be solved by a combination of grep, sed, cut etc., as well. However AWK is much more capable. We will

see the other features in subsequent chapters.

awk '/Chemistry/ {print $4, ― ―, $5, ― ―,$6, ― ―,$7,‖ ―, $8);}' toppers.txt > chem_topper.txt

bash>cat chem_toppers.txt

Suman Gupta

Shriesh Jadhav Srinivas Vardharajan Raju Pandy

Rajni Kumar

awk '/Maths/ {print}' toppers.txt > math_toppers.txt #Note no $1, $2 used

bash>cat math_toppers.txt

Maths 99 2003 Suresh Yadav

Maths 96 2004 Lokesh Arora Maths 99 2005 Anup Mathur Maths 98 2006 Javed M. K. Akthar


174


5. awk processes how many line(s) of input at a time? (a)1, (b) 2, (c) depends on the available memory, (d) all lines in input.

6. awk breaks inputs into columns or words (true/false)

7. awk uses spaces to break inputs (true/false) 8. The print command can be used to print the fields of input with added text

(true/false)

3.4 Summary

The awk utility is a powerful command line tool, which can handle streams of data: it can take input lines from a pipe. This makes it fit for non−interactive use.

3.5 Answers to self check questions

1. true.

2. true. 3. true.

4. (a) 5. (a) 6. true.

7. true. 8. true.


1. Take the toppers.txt of this chapter. For each year and subject, print

the first name of the topper, marks and then year. 2. Do the same question as listed above but now print the complete name

of the topper followed by marks and then year.

3. See the AWK syntax. We have used only one pattern and its program in our examples. Try using multiple patterns and their corresponding

programs and see the outputs.

IT 102 Unit 3, Lesson 4

LESSON 4. AWK PROGRAMMING

4. AWK PROGRAMMING ................................................................................................ 177

4.0 OBJECTIVES .......................................................................................................... 177

4.1 INTRODUCTION ...................................................................................................... 177

4.2 RELATIONAL AND LOGIC OPERATORS IN AWK ..................................................... 177

4.3 CONTROL STRUCTURES IN AWK ............................................................................ 178

4.3.1 The if-else construct....................................................................................... 178

4.3.2 The for loop ..................................................................................................... 179

4.4 SPECIAL VARIABLES - NF AND NR........................................................................ 180

4.4.1 Using BEGIN and END clauses in awk ...................................................... 180

4.4.2 Using variables in AWK ................................................................................. 181

4.5 RUNNING AWK PROGRAMS KEPT IN FILES ........................................................... 182

4.6 GENERATING REPORTS USING AWK .................................................................... 184

4.6.1 The printf command of AWK ........................................................................ 184

4.6.2 Format specifications in printf....................................................................... 185

4.6.3 Printing the fields in different order than input ........................................... 186

4.6.4 Creating simple reports ................................................................................. 186

4.6.5 Field separator ................................................................................................ 187

4.6.6 Printing heading/heading row and summary/footer .................................. 189

4.7 MISCELLANEOUS FEATURES OF AWK .................................................................. 190

4.7.1 Specifying search patterns in AWK ............................................................. 190

4.7.2 Limiting the lines on which AWK would work............................................. 191

4.7.3 Built-in variables ............................................................................................. 192

4.7.4 Passing arguments to AWK.......................................................................... 193

4.7.5 Arrays and associative arrays in AWK........................................................ 195

4.7.6 String functions in AWK................................................................................. 195

4.7.7 Few interesting, complex examples ............................................................ 196

4.8 SUMMARY .............................................................................................................. 197




177

4. AWK Programming

In the previous chapter we saw how AWK can be used to process the input

data and print in some ways as needed. In this chapter we will see programming features of AWK that make it very powerful.

4.0 Objectives After going through this lesson you will know

How to use AWK programming

Relational and logic operators for conditions

Control structures

Use of variables, BEGIN and END clauses

How to generate reports using AWK

Miscellaneous features of AWK

4.1 Introduction

AWK provides a simple yet powerful programming language. The programming language features are similar to C language constructs.

Note that we will continue to refer to the toppers.txt file from chapter 3 for examples.

4.2 Relational and logic operators in AWK AWK supports comparing fields to create conditions. Relational operators,

that compare two values, are available in awk. For example, a condition like $1 == 2006 can be used. We will see such usage in subsequent examples

below. Relational operators like the following are there

== Compares whether the values specified are

equal != Compares whether the values specified are

not equal > Tells whether a value is greater than the other.

>= Tells whether a value is greater than or equal to other.

< Tells whether a value is less than the other. <= Tells whether a value is less than or equal to other


178

Multiple relational conditions can be combined using logic operators. For

example $1 == 2006 && $2 != 98. This condition will be true only when first field will be 2006 and second will not be equal to 98.

Logic operators like the following are there:

Note an important point here. The relational operators only evaluate to true/false. Unlike C operators they do not return a value which could be

printed or used in an expression. So, for example ($1 == 1) + ($2 == 0) will result in an error during AWK run.

Examples in subsequent sections will show conditions that use relational and logic operators.

4.3 Control structures in awk AWK provides C like control structures as well to facilitate programming.

Control structures in AWK include the following:

4.3.1 The if-else construct

The if-else construct of AWK has the following syntax.

if (condition) statement [ else statement ]

Example 1: To print the first name of the chemistry topper for year 2006, we can use

Note that there is no else in the example above. The else part of if-else is

optional. Example 2: Print whether Maths toppers had more than 97 marks.

&& implies logic and

|| implies logic or ! Implies logic negation

awk ‗/Chemistry/ { if( $3 == 2006 ) print $5 }‘

toppers.txt

will print

Raju


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Note that there is an else part used in this example.

Also note that if there are more than one statements they can be clubbed together with curly braces as we have done here in the example above.

4.3.2 The for loop

The for loop in AWK has the following syntax:

for(initial condition; termination condition; increment) statement; Example 3: To print some text for each of the fields we can use

awk ‗/Maths/ { if( $2 > 98 )

{ print ―In the year ―, $3; print ― ―, $5, ― had more than 98 marks\n‖

} else

{ print ―In the year ―, $3; print ― ―, $5, ― had less than 98 marks\n‖

} }‘ topper.txt

This will print

In the year 2003 Suresh had more than 98 marks.

In the year 2004 Lokesh had less than 98 marks. In the year 2005 Anup had more than 98

marks. In the year 2006 Javed had more than 98

marks.

awk ‗/Maths/ { for(i=1; i<=4 ) print $i, ‖:‖; }‘ toppers.txt

will print

Maths:99:2003:Suresh: Maths:96:2004:Lokesh:

Maths:99:2005:Anup: Maths:98:2006:Javed:


180

Note that $0 contains the entire text input line and $1 onwards contains the fields. Also note that we have used a variable i here. We will see details on

variables in AWK later.


1. AWK programs can compare two fields of the input line. (true/false).

2. Relational operators give true or false but return value cannot be used in expressions (true/false).

3. The if-else construct of AWK mandates that the else part must be there

(true/false). 4. for loop can have a block of statements enclosed in curly brackets (true/false).

4.4 Special variables - NF and NR Awk provides internal special variables called

NF – stands for the number of fields in the currently read line. NR – stands for the total number of records read.

Example 4: Printing only the long lines more than 5 fields:

Example 5: For Maths toppers, if we want to skip printing the year, we can

use the following AWK command:

4.4.1 Using BEGIN and END clauses in awk

awk ‗{if (NF > 5) print}‘ toppers.txt

this will print maths 96 2006 Javed M. K.

Akthar

awk ‗/Maths/ { for(i=1; i<= NR ) if( i != 2) print $i ― ―; print ―\n‖;

}‘ toppers.txt

will print

Maths 99 Suresh Yadav

Maths 96 Lokesh Arora Maths 99 Anup Mathur

Maths 98 Javed M. K. Akhtar


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Usual programming tasks consist of

Initializing some variables

Reading inputs, performing some calculations and outputs

Finally, generating some output based on the complete input set.

The BEGIN clause of AWK lets you specify initializations. And, the END clause lets you perform calculations based on the entire input.

Example 6: Suppose you want to print the total number of toppers.

4.4.2 Using variables in AWK

AWK provides $0, $1, $2, .. etc. as fields. In addition, you can use your own variables as well for any calculations. You need not declare the variable.

Simply using a variable is permitted.

Example 7: Suppose we want to find out the average top marks for physics over the years.

In this example, "marks" is a user defined variable. You can use almost any string of characters as a variable name in AWK, as long as the name doesn't

conflict with some string that has a specific meaning to Awk, such as "print" or "NR" or "END".

There is no need to declare the variable, or to initialize it. A variable handled as a string variable is initialized to the "null string", meaning that if you try to

print it, nothing will be there. A variable hand led as a numeric variable will be initialized to zero.


awk 'END {print ―There are ― NR," toppers"}'

toppers.txt

will print

There are 13 toppers.

awk '/physics/ {marks += $2}

END {print "The average top marks in physics are " marks/NR}' toppers.txt

This will print

The average top marks in physics are 93.375.


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5. Special AWK variable NF stands for

(a) Next field, (b) New Format, (c) Number of fields, (d) Next Line 6. END is used in AWK to

(a) Exit from AWK, (b) To do final calculations 7. You can use any variable in AWK but you need to declare it first

(a) true, (b) false.

4.5 Running AWK programs kept in files As you must have noticed, AWK programs can easily be longer than one line. Typing long programs on command line is quite cumbersome. Moreover,

whenever you create programs, you would want to keep them in files to be able to use them over and over again.

AWK provides a way to run AWK programs. The commands can be written into a file, and then AWK can be told to execute the commands from that file

as follows:

AWK -f <awk program file name> Example 8: Suppose someone has a coin collection with gold and silver coins.

Details of this collection are listed below.

Now we can create an AWK program to print a summary of this coin collection as shown below:

bash>cat coin_collection_details.txt

Coin type weight(gm) year of making

Gold 1 1945 Gold 1 1952 Silver 10 1948

Gold 1 1973 Gold 1 1973

Gold 0.5 1945 Gold 0.1 1933 Silver 1 1943

Gold 0.25 1921


183

Note that AWK programs allow you to put comments as well. See the first two

lines of show_coin_summary fi le listed above.

You can run this AWK program as shown below:

bash>cat show_coin_summary

/gold/ { num_gold++; wt_gold += $2 } # Get weight of gold.

/silver/ { num_silver++; wt_silver += $2 } # Get weight of silver. END { val_gold = 485 * wt_gold; #

Compute value of gold. val_si lver = 16 * wt_silver; # Compute

value of silver. total = val_gold + val_silver; print "Summary data for coin collection:"; #

Print results. printf ("\n");

printf (" Gold pieces: %2d\n", num_gold); printf (" Weight of gold pieces: %5.2f\n",

wt_gold); printf (" Value of gold pieces:

%7.2f\n",val_gold); printf ("\n"); printf (" Silver pieces: %2d\n",

num_silver); printf (" Weight of si lver pieces: %5.2f\n", wt_silver);

printf (" Value of silver pieces: %7.2f\n",val_silver);

printf ("\n"); printf (" Total number of pieces: %2d\n", NR);

printf (" Value of collection: %7.2f\n", total); }

bash>awk –f show_coin_summary coin_summary_details.txt

The Output of this run will be:

Gold pieces: 7 Weight of gold pieces: 4.85 Value of gold pieces: 2352.25

Silver pieces: 2

Weight of silver pieces: 11 Value of silver pieces: 176

Total number of pieces: 9 Value of collection: 2528.25


184

4.6 Generating reports using AWK

AWK programs can be used to quickly process text inputs and create various reports. Because AWK processes each record as fields, AWK is much more

helpful in creating reports, compared to other Unix utilities, like sed.

4.6.1 The printf command of AWK

While print command is available in AWK, print is quite a basic command.

Often more sophisticated formatting is needed, specially while generating reports. For sophisticated output formatting, C like printf command is available in AWK

Printf uses format specifications like %s, %d, etc. for formatting output.

%s prints string %d prints a number in decimal format %f prints a floating point number

In addition, you can use the following as well to control spacing

\t to print a tab \n to print a new line

Note that tabs come in very handy specially to print well aligned columns. The input text fields may vary in lengths. If you separate out fields with spaces, the

fields in output may not align well. Use tabs to get better aligned outputs. Example 1: Printing the topper name and year for Maths, with spaces.

You can see that the output columns are not aligned.

Example 2: Printing the topper name and year for Maths, with tabs.

You can see that the output columns are well aligned now after using tab.

awk ‗/Maths/ {printf(―%s %s\n‖, $4, $3); }‘ toppers.txt

will print Suresh 2003

Lokesh 2004 Anup 2005 Javed 2006

awk ‗/Maths/ {printf(―%s\t%s\n‖, $4, $3); }‘

toppers.txt will print

Suresh 2003 Lokesh 2004 Anup 2005

Javed 2006


185

4.6.2 Format specifications in printf

The printf command of AWK accepts many format specifiers. Moreover, for

each of the format specifier, you can control how the output will be printed. This control specially helps further in making the reports better readable.

The table below lists how values will be printed when certain format specifiers are used:


8. If you use tabs in printf, the output will not be aligned (true/false) 9. Tab is printed by putting (a) \T, (b) \tab, (c) \t, (d) tab

10. For printing a string using print, a format specification is needed (true/false)

Format Value Results

%s ―Hello‖ ―Hello‖ %10s ―Hello‖ ―Hello ― %-10s ―Hello‖ ― Hello‖

----------------------------------------- %c 100 "d"

%10c 100 " d" %010c 100 "000000000d" --------------------------------------------

%d 10 "10" %10d 10 " 10"

%10.4d 10.123456789 " 0010" %10.8d 10.123456789 " 00000010" %.8d 10.123456789 "00000010"

%010d 10.123456789 "0000000010" --------------------------------------------

%e 987.1234567890 "9.871235e+02" %10.4e 987.1234567890 "9.8712e+02" %10.8e 987.1234567890 "9.87123457e+02"

%f 987.1234567890 "987.123457"

%10.4f 987.1234567890 " 987.1235"

%010.4f 987.1234567890 "00987.1235"

%10.8f 987.1234567890 "987.12345679" --------------------------------------------

%g 987.1234567890 "987.123" %10g 987.1234567890 " 987.123" %10.4g 987.1234567890 " 987.1"

%010.4g 987.1234567890 "00000987.1" %.8g 987.1234567890 "987.12346"


186

11. If you use a printf with %10s and give ―worlds‖ as argument to the printf, the output will come as ―10worlds‖ (true/false).

4.6.3 Printing the fields in different order than input

If you want to print some of the fields in a order that is different from the input,

you can simply change the order of the $ variables in the print commands. This powerful feature is often useful when creating reports as well.

Example 3:

AWK features make it very useful to process data and print reports, especially when the data is arranged in columns like our toppers.txt example. Let‘s see a

few examples before looking at more AWK features.

4.6.4 Creating simple reports

Creation of simple reports is straightforward using AWK.

Example 4: If you want to print the physics toppers for years prior to 2005, you

can use the following command: (note year is the 3‘rd field in input text):

Example 5: If you want to print a simple yes/no answer whether the topper

had more than 92 marks or not, you can use the following:

awk ‗{if ($3 == 2006) print $3,‖ ―, $1); }‘ toppers.txt

will print the following

2006 95 2006 88 2006 89

2006 96

awk '/Physics/ {if ($3 < 2005) printf(―%s %s %s %s\n‖, $3,$5,$6,$7,$8}' toppers.txt >

phy_toppers_before_2005.txt bash>cat phy_toppers_before_2005.txt

2003 Abhay Malhotra 2004 Shriesh Jadhav


187

You can see how quickly awk can be used to generate reports like this. Example 7: For Maths toppers, if we want to put a colon between fields except

in the names, we can use the following AWK command:

Note that the special variable NF has been used to define the terminating condition. With the use of NF you can work with data having variable number of columns as well like we are able to print names that fit in 2 fields (e.g.,

Lokesh Arora) and names that need 4 fields (e.g., Javed M. K. Akhtar).

Also note that we have used if-else inside a for loop. The if-else part is ensuring that there are no colons in the names.

4.6.5 Field separator

awk ‗{if ($2>92)

printf(―%s\t%s\types\n‖, $3, $1) else printf(%s\t%s\tno\n‖, $3, $1); }‘ toppers.txt

> more_than_92.txt

bash>cat more_than_92.txt 2003 Physics no 2003 Chemistry yes

2003 Maths yes 2004 Physics yes

2004 Chemistry yes 2004 Maths yes 2005 Physics no

2005 Chemistry no 2005 Maths yes

and so on.

awk ‗/Maths/ { for(i=1; i<= NF ) {

if( i < 4) printf(‖%s:‖ , $i);

else printf(―$s ―, $i); }

printf(―\n‖); }‘ toppers.txt‘

will print Maths:99:2003:Suresh Yadav

Maths:96:2004:Lokesh Arora Maths:99:2005:Anup Mathur Maths:98:2006:Javed M. K. Akhtar


188

AWK works by reading one input record (one line) and breaking it up into fields. By default, AWK uses white-spaces (space and tabs) as the field

separator. However, you may encounter tabular data that uses some other characters as separator. For example, your input data may look like the output

of example 8.

Here colon (‗:‘) is the separator.

In such cases, you can tell AWK what character to use as field separator. The field separator is an optional argument to the awk command. awk -F<ch>

Example 8: If the input line is Maths:99:2005:Anup Mathur

Note that $4 here will contain the entire name itself because the separator has been set as colon.

Example 9: You can pipe the output of one awk into another awk as well. So we can pipe the output of the example 7 above into another AWK.

Maths:99:2003:Suresh Yadav Maths:96:2004:Lokesh Arora

Maths:99:2005:Anup Mathur Maths:98:2006:Javed M. K. Akhtar

e.g., awk -F: tells AWK to use colon as a separator

awk -F'|' tells AWK to use bar as a separator

awk -F'\"' tells AWK to use double quote as a separator

And AWK is run with –F: as an argument, the $1 will contain Maths

$2 will contain 99 $3 will contain 2005 $4 will contain ―Anup Mathur‖


189

4.6.6 Printing heading/heading row and summary/footer

The BEGIN and END clauses can be used even to print headings and summary for reports, thus making the report more readable and attractive.

Example 10: Here we will print the physics toppers with headers and will print

a summary at the end.

This will print

awk ‗ { for(i=1; i<= NR )

{ if( i < 4) printf(‖%s:‖ , $i); else printf(―$s ―, $i);

} printf(―\n‖);

}‘ toppers.txt‘ | awk –F: ‗{printf(―%-18s\t%d\n‖, $4, $3); }‘

will print Suresh Yadav 2003

Lokesh Arora 2004 Anup Mathur 2005 Javed M. K. Akhtar 2006

awk ‗BEGIN { printf(―Physics toppers details:\n‖)

printf(―-----------------------------------------\n‖); printf(―Year\tMarks\tName of the topper\n‖); printf(―-----------------------------------------\n‖);

} /Physics/ {

printf(―%d\t%d\t%s\n‖, $3, $2, $4); } sum += $2 }

END { printf(―-----------------------------------------\n‖);

printf(―Avg top marks in physics were %f \n‖, sum/NR) printf(―-----------------------------------------\n‖);

}‘ topper.txt

---------------------------------------------

Year Marks Name of the topper ---------------------------------------------

2003 92 Abhay 2004 94.5 Shiesh 2005 89 Vandana

2006 98 Ramakant ---------------------------------------------

Avg top marks in physics were 93.375


190

Self-Check Questions 12. AWK always prints the fields in the same order as they appear in the input

(true/false). 13. AWK can generate reports containing only the input fields. No other items can

be added. (true/false).

14. Filed separator in AWK is fixed and cannot be changed (true/false).

4.7 Miscellaneous features of AWK

4.7.1 Specifying search patterns in AWK

As we have seen in several examples and in AWK syntax, search patterns, along with their respective programs can be used in AWK. So far we have used simple search patterns like the example below:

awk „/Physics/ {print}‟ toppers.txt

However, AWK supports much more sophisticated patterns also, as listed below.

/The/ matches any lines containing The So this will match lines containing There, These, Them too.

But this will not match lines containing the, these, them, etc. because AWK uses case sensitive matching.

/^The/ matches any lines beginning with The. So this will match lines which contain The, These, Them in the beginning only.

/The$/ matches any lines ending with The

/The\$$/ matches any lines ending with The$

/[Tt][Hh][Ee]/ matches any lines with THE, The, tHe, thE, etc.

/^[a-zA-Z][a-zA-Z0-9_]*$/ matches lines containing only identifiers. /(Îndia)|(^Pakistan)/ matches lines beginning with India or Pakistan

You can even use complex regular expressions in AWK. The regular

expressions can be created by using the following characters:


191

? matches zero or one occurrence of

character before it

+ matches one or more occurrences of character before it

* matches zero or more occurrences of character before it

. The dot matches any character

For example, the following expression will match any line containing only a signed integer. The matched line cannot contain any other characters.

/^[+-]?[0-9]+$/ matches signed integers.

Example 1: A data file contains some text and some integer numbers. Here is the data file:

4.7.2 Limiting the lines on which AWK would work

By default, awk works on each of the lines of input. We have already seen that we can use search patterns to limit the lines on which AWK would work.

In addition, you can limit AWK to work only on some block of input lines. /Îndia/,/^Pakistan/ will operate on lines starting with India and will end

operation with the line starting at Pakistan.

NR == 15 will operate only on the 15'th line! NR==10,NR==25 will operate on lines 10 to 25. $1 == "India" will operate on lines where the first field is "India"

$1 ~ /India/ will operate on lines where the first field contains India.

bash>cat data_file.txt

The number of loans given 12399

The number of loans fully repaid by now 2893 The number of defaulters

129 Defaulted amount (loss)

-8929972 Loss after adjusting procedural expenses -9288990.72

awk ‗/^[+-]?[0-9]+$/ {print }‘ data_file.txt

will print

12399 2893

129 -8929972


192

You can even create complex conditions using &&, || operators

e.g., ((NR >= 30) && ($1 == "India")) || ($1 == "Pakistan")

Example 2: If you know that your input data has some header text and some footer text and the data of your interest lies in between, then you should use

such patterns to limit AWK to work only on data and not on the header and footer.


15. AWK search patterns are case-insensitive. (true/false) 16. /NASA/ will match only lines containing NASA. (true/false).

17. AWK will work on each line of input. There is no way to limit the scope. (true/false)

4.7.3 Built-in variables

We have used many of the built-in variables of AWK, such as $0, $1, $2,.. etc.

and NF, NR. In addition, AWK has few other bui lt in variables as listed below.

Note that these variables are not read-only. That means, during a AWK program‟s run, the program itself can change the value of the variable!

FS : Field separator. By default AWK uses spaces as field separator and we have seen the –F option that can be used on the command line to specify the

bash>cat data.txt

------------------------------------------------- The weather report for 24.05.2007

------------------------------------------------- City Humidity Max Temp Agra 92 38

Delhi 93 39 Mumbai 98 34

Copyright CNN world Data from 2pm IST

awk ‗NR > 3 && NR < 8 {printf (―%s \tTemp=%d\n‖, $1, $3); }‘ data.txt

will print Agra Temp=38 Delhi Temp=39

Mumbai Temp=34


193

field separator to be used by AWK. In addition, AWK has a built in variable FS that specifies the field separator.

RS : Record separator. By default AWK reads each line as an input line which means the default record separator is the new line. However, you can use RS

to change the record separator.

OFS: Stores the "output field separator", which separates the fields when Awk

prints them. The default is a "space" character.

ORS: Stores the "output record separator", which separates the output lines when Awk prints them. The default is a "newline" character.

FILENAME: Contains the name of the current input file.

4.7.4 Passing arguments to AWK

So far we have seen AWK programs and commands where the values were

fixed. For example, consider example from chapter 4 where a fixed value is being used:

Example 3: Print whether Maths toppers had more than 98 marks.

This will print

Now, you may be asked to print the same report but for 94 marks. In which

case, you will need to copy and modify the same script to replace 98 by 94. Such copying must be avoided because (a) it creates multiple scripts doing

nearly the same things, (b) if you fix some error in one fi le you will need to fix it in all the files of same type, (c) the operation of copying and modifying is very error prone (what if the change from 98 to 94 is done in all places but

gets accidentally left out at one place). Therefore, it is safer to make your

awk ‗/Maths/ { if( $2 > 98 )

{ print ―In the year ―, $3; print ― ―, $4, ― had more than 98

marks\n‖ }

else {

print ―In the year ―, $3;

print ― ―, $4, ― had less than 98 marks\n‖ } }‘ topper.txt

In the year 2003 Suresh had more than 98 marks. In the year 2004 Lokesh had less than 98

marks. In the year 2005 Anup had more than 98

marks. In the year 2006 Javed had more than 98 marks.


194

scripts in a generic way. Consider the example 3 again but made generalized as example 4 below:

Example 4: Print whether Maths toppers had more than N marks.


195

It is invoked as awk –f report_script N=94 toppers.txt

Note that we are passing N=94 in the command line. So if another report is needed to find with N=55, we need not copy/modify the file but we can simply pass N=55 on the command line itself.

4.7.5 Arrays and associative arrays in AWK

Any user defined variable can work as an array in AWK. You can simply assign values with indexing. For example,

Field[1] = $1

Field[3] = $3 AWK also supports associative arrays.

For example, if $i contains the name of city and $j contains the city‘s

temperature, you can store this information in an associative array. Temperature[ $i ] = $j;

4.7.6 String functions in AWK

If you place multiple strings side by side, they will be joined.

length() function returns the length of a given string.

substring(str, startIndex, length) function takes out the substring. substring("DTU", 5, 3) wi ll return "bag".

bash>cat report_script if( $2 > N )

{ print ―In the year ―, $3;

printf( ― %s had more than %d marks\n‖, $5, N); } else

{ print ―In the year ―, $3;

printf( ― %s had less that %d marks \n‖, $5, N); }

a = "DTU" "Delhi" # a will become "DTUDelhi".


196

Note that index starts from 1, not 0.

index(str, searchStr) gives the index of the searchStr or 0. index("DTU", "bag") will return 5.

index("DTU", "DEI") will return 0. split(str, array [,separator]) splits an string by separator and fills them into an

array. split("mera bharat mahan", slogan) will put

slogan[1] as "mera" slogan[2] = "bharat", etc.

Self-Check Questions 18. AWK provides a built in variable for field separator (true/false).

19. Built in variables are read only (true/false). 20. Variables passed to AWK are accessed as $1, $2, etc. (true/false)

21. AWK does not support complex structures but supports associative arrays (true/false).

4.7.7 Few interesting, complex examples

Few interesting examples are listed below. These exemplify the power of

AWK.

Example 5: Counting non blank lines in a fi le: awk 'NF != 0 {++count} END {print count}' input_file.txt

Example 6: Computing avg size of files in a directory

ls -l | awk 'NR!=1 {s+=$5} END {print "Average: " s/(NR-1)}' Example 7: Print Fibonacci numbers:

awk 'BEGIN {a=1;b=1; while(++x<=10){print a; t=a;a=a+b;b=t}; exit}'

Example 8: Sometimes we may repeat words unintentionally like: "When I was going there". Detecting these manually is difficult, But we can write an

AWK program to do this!!

BEGIN { dups=0; w="xy-zzy" } { for( n=1; n<=NF; n++)

{ if ( w == $n ) { print w, "::", $0 ; dups = 1 } ; w = $n }

}

END { if (dups == 0) print "No duplicates found."}


197

4.8 Summary

Awk is a very powerful utility in Unix. It helps in scripting and report generation.


1 true

2 true 3 false 4 true

5 (c) 6 (b)

7 (b) 8 false 9 (c)

10 false 11 false

12 false 13 false 14 false

15 false 16 false 17 false

18 true 19 false

20 false 21 true


1. Take the toppers.txt of this chapter. For each year and subject, print the first name of the topper, marks and then year.

2. Do the same question as listed above but now print the complete name of the

topper followed by marks and then year.

3. Print the chemistry toppers marks, year and names for even years. 4. Print the years whenever the toppers scored >= 97 marks.

5. Input contains name and phone number records. To simplify, assume there is only one name (first name) and only one phone number per name. Use associative arrays to store numbers and names and at the end print them.

6. Upgrade example 8 to print the line number too where the repeated word is there.


198

7. See the AWK syntax. We have used only one pattern and its program in our examples. Try using multiple patterns and their corresponding programs and

see the outputs. 8. Generalize the coins example of chapter 4 by passing the values of per gram

of gold and solver in place of hard coded values used in that example.

Date post:	06-May-2015
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