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(update Dickson Chiu, Spr ing 2001)

The UNIX Shell

Learning Objectives:

1. To give detailed description for usingcommands in Unix Shell

2. To introduce more advanced techniques for

handling complicate commands in Unix Shell

3. To understand the the usage of backgroundjobs & shell switching

4. To learn more about pattern matching

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Lecture 5 / Slide 2

The UNIX Shell

Table of Content

Basic Shell Syntax

Command options

How Does the Shell find a Command

Alias Standard Input

Tee

Background Jobs

Jobs

Switching Shells

Combining Commands

More Pattern Matching

More Pattern Matching (grep)

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Lecture 5 / Slide 3

Basic Shell Syntax

command[-[options]] [arg] [arg] The name of the command is first

Options are normally single letters that turn an option

on or off. They can be combined or given separately.

$ ls -dil

$ ls -l -d -i

Options sometimes also take a value. The value can

usually be either given right after the option orseparately:

$ ypcat -d ug.cs.ust.hk passwd

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Lecture 5 / Slide 4

Command Options (1)

Most commands require you to give all options beforefilename arguments

$ cat names -n

Bill Clinton

Bill Gates

Bill GatesBill Clinton

Monica Lewinski

cat: cannot open -n

Spaces separate options. To turn something with spaces intoa single argument, use quotes:

$ grep some pizza letter1

grep: cant open pizzaletter1:It is getting late. Please order some pizza and stop


It is getting late. Please order some pizza and stop

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Lecture 5 / Slide 5

Command Options (2)

Double quotes and single quotes are a bit different (say,when shell variables are involved), but you may use theminterchangeably here.


It is getting late. Please order some pizza and stop To escape a single character (prevent it from being treated

specially) proceed it with a backslash:$ grep We\ll letter1

by my office. We'll tidy up a few more things before

$ echo *

*$ echo **$ echo \**

$ echo *letter1 names secret/

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Lecture 5 / Slide 6

How Does the Shell Find a

Command? (1) The shell searches a list of directories for an executable file

with the same name.

The list of directories is stored in the PATH variable for

Bourne shells and in the path array forcsh/tcsh$ PATH=/usr/local/bin:$PATH sh

% set path=(/usr/local/bin $path) csh, tcsh

If there is a match in more than one directory, the shell uses

the first one it finds. If you want to run a command that is not in one of these

directories, you can give a pathname (relative or absolute)instead.

~horner/bin/csound

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Lecture 5 / Slide 7

How Does the Shell Find a

Command? (2)

A few commands are built into the shell. This varies fromshell to shell. The echo command, for example, is often

builtin, for efficiency. You can find out where the shell is getting a particular

command using the which command in any shell:$ which echo

echo: shell built-in command.

$ which cat

/usr/bin/cat

$ which grep

/usr/bin/grep

$ which ls

ls: aliased to ls -F

Gives / after directories,

* after executables, and

@ after soft links

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Lecture 5 / Slide 8

Alias

The C Shell has the alias command, which allows you tocreate command shortcuts.

$ alias dir "ls -F"

$ alias rm "rm -i"

$ alias + "chmod u+x *"$ alias - "chmod u-x *"

$ alias 111 "cd ~horner/111"

$ pwd

/bin

$ 111$ pwd

/homes/horner/111

If you put the alias commands in your.cshrc file, you canuse them every time you login.

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Lecture 5 / Slide 9

Standard Input (1)

Every time you login, or run a shell, you are connected tothe computer on a particular terminal.

$ who

horner pts/3 Feb 11 10:23 (csnt1.cs.ust.hk)

horner pts/18 Feb 11 11:57 (csz096.cs.ust.hk)

These devices (pts/3) are actually files in the directory

/dev. So, if you are logged in on pts/3, this works just

fine (otherwise Permission denied):$ date > /dev/pts/3

Thu Feb 11 17:08:21 HKT 1999

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Lecture 5 / Slide 10

Standard Input (2)

In fact, you can redirect stdout to a different device (e.g.,pts/18), if you have permission. (The write command

works this way.)

You can find out which terminal a particular shell is

connected to using the tty command:$ tty

/dev/pts/3

$ echo "Hi Andrew!" > /dev/pts/18

$

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Tee

A special command called tee acts like a T-joint inplumbing:

$ who



$ who | sort | tee sortedwho | wc -l

2

$ cat sortedwho



$

In this example, the output of sort is placed in a filesortedwho and piped to wc -l, which counts thenumber of lines.

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Background Jobs (1)

A simple command or pipeline can be put into the background by

following it with the & character:$ sort names > names.sort &

[1] 3236

$

The shell will print the process ID (PID), and ajob number(1, in this

case).

In some shells, you will be notified when the job is done (you may

have to hit return again):$ sort names > names.sort &

[1] 3236

$

[1] Done sort names > names.sort

$

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Background Jobs (2)

Put a job in the background by typing CTRL-Z$ ypcat passwd | sort >passwd.sort

^Z

Suspended

$

The job is suspended - not running - until you either place itin the background using bg:

$ bg

[1] ypcat passwd | sort > passwd.sort &

$or back to the foreground using fg:

$ fg

ypcat passwd | sort >passwd.sort

$

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Jobs (1)

The jobs command tells you what jobs are running:$ ypcat passwd | sort > passwd.sort

^Z

Suspended

$ jobs

[1] + Suspended ypcat passwd | sort > passwd.sort$

You can stop a job with the kill command:$ ypcat passwd | sort > passwd.sort &

[1] 3414 3415

$ kill %1$

[1] Terminated ypcat passwd |

Exit 2 sort > passwd.sort

$

The %1 means job #1. You can also use the PID.

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Jobs (2)

The ps command is the main way to find out about jobs:$ ps

PID TTY TIME CMD

1401 pts/0 0:01 csh

$$ ypcat passwd | sort > passwd.sort &

[1] 3476 3477

$ ps

PID TTY TIME CMD

3477 pts/0 0:00 sort1401 pts/0 0:01 csh

3476 pts/0 0:01 ypcat

$

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Jobs (3)

Note that if you put something into the background, youbetter redirect stdout, or the output will appear on yourscreen anyway!

$ ypcat passwd &ma_wmkaa:uq2jXK0sFQ8Jg:36747:5000:Woo Man Kei,,=EXP.2001.05.30=nc99S:/homes/ma_h

ma_chyaa:CS9wq.1zOxnhI:35435:5000:Chu How Yin Agnes,,=EXP.2001.05.30=nc98F:/homs

ee_tkcaa:9LtI7Tipk2Ca6:35651:5000:Tsang Kong Chau,,=EXP.2001.05.30=nc99S:/homesh

eg_cck:yi7XtKxxP5KaQ:43555:10010:Cheung Chi Keung,,ce98_yr1:/homes/eg_cck:/bin/h

cs_wks:dtjvwifI2G7v2:24514:10001:Wong Kin Shing,,cs98_yr1:/homes/cs_wks:/bin/tch

cs_lwk:OWiGoJRXSjn.s:24032:10001:Leung Wai Kei Ricky,,cs98_yr2:/homes/cs_lwk:/bh

ph_chyac:CSJUo9e2KGqKg:35955:5000:Chan Hoi Yan,,=EXP.2001.05.30=nc98F:/homes/phsee_wkkab:dfbi3GqWjvf5U:35644:5000:Wong Ka Keung,,=EXP.2001.05.30=nc99S:/homes/eh

ph_lcy:CSwGgr5IeIvqc:36689:5000:Lam Chi Yin,,=EXP.2001.05.30=nc99S:/homes/ph_lcx

^C

$ ypcat passwd >file &

$

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Switching Shells

You can switch shells by just typing its name:csl3su1.cs.ust.hk> ps

PID TTY TIME CMD

3496 pts/0 0:01 csh

csl3su1.cs.ust.hk> tcsh

csl3su1.cs.ust.hk> ps

PID TTY TIME CMD

3650 pts/0 0:00 tcsh

3496 pts/0 0:01 csh

csl3su1.cs.ust.hk> sh

$ ps

PID TTY TIME CMD

3650 pts/0 0:00 tcsh3496 pts/0 0:01 csh

3659 pts/0 0:00 sh

$ ^D

csl3su1.cs.ust.hk> ps

PID TTY TIME CMD

3650 pts/0 0:00 tcsh

3496 pts/0 0:01 csh

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Combining Commands (1)

Multiple pipelines can be input on one command line byseparating them with semicolons.

When entering a long command, use a backslash (\) tocontinue the command on the next line.

$ date; sort names; \who

Thu Feb 11 19:40:28 HKT 1999

Bill Clinton

Bill Clinton

Bill GatesBill Gates

Monica Lewinski


horner pts/0 Feb 11 19:11 (csz096.cs.ust.hk

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Combining Commands (2)

Commands can be grouped together using parentheses

There are two main reasons to group commands: To create a single command out of a group of commands

(especially useful before a pipe):

$ (cat letter1; head -2 names) | sort >list

To run a set of commands in their own subshell(especially when trying to limit the effect of a cd command):

$ (cd secret; ls | wc -l); ls | wc -l

325

This line has the effect of counting the files in secret, and thencounting the files in the current directory.

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More Pattern Matching (1)

The notation [abcd] matches any single one of the enclosed characters.$ ls [il]*

it it1 ith its@ letter1 letter4

$

The notation [a-z] matches any lowercase letter.

The notation [0-9] matches any digit character. The notation [0-59] matches any the digit characters 0, 1, 2, 3,

4, 5, and 9.$ ls letter*

letter1 letter4

$ ls letter[0-35]

letter1

$ ls letter[0-24]

letter1 letter4

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More Pattern Matching (2)

Most shells allow you to give a list of strings in curlybrackets, comma separated:

$ ls *{1,.sort}

NAMES1 it1 names.sort s1@f1 letter1 passwd.sort

secret1:

secret1: Permission denied

$

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More Pattern Matching grep (1)

Note the patterns for other utilities such as grep, etc., aredifferent. We use grep as an example. The followingsymbols are treated specially:

^ start of line $ end of line. any character \ quote next character

* match zero or more + match one or more

? match zero or one

[aeiou0-9] match a,e,i,o,u, and 0 thru 9

[^aeiou0-9] match anything but a,e,i,o,u, and 0 thru 9

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$ cat names

Bill Gates

Jacky Gs

James Bill

J. Jones

$ grep '^Bill names

Bill Gates$ grep 'Bill$ names

James Bill$ grep J...s names

James Bill

J. Jones

$ grep 'G.*s' namesBill GatesJacky Gs

$ grep 'G.+s' names

$ egrep 'G.+s' namesBill Gates$ egrep 'G.s' names$ egrep 'G.?s' namesJ. Jones

$ grep 'J\.' namesJ. Jones

Some versions of grep

cannot process ?+

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$ cat namesBill Gates

Jacky Gs

James Bill

J. Jones

$ grep 'Ja[cm]' names

Jacky Gs

James Bill

$ grep 'J[^a]' names

J. Jones$ grep [t-y]' names

Bill Gates

Jacky Gs

$ grep -in bill names1:Bill Gates3:James Bill

$ grep -v Bill names

Jacky GsJ. Jones

$ grep -l Bill n*names

$ grep -c Bill names2

-i ignores case

-n shows l ine number

-v means line *not*

con tain ing pattern

-l shows *fi lename*

con tain ing pattern

-c shows match ing count

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