UNIX COMP-145

PARTIAL NOTES FOR TEST 1 REVIEW

SOURCE: S. DAS, “YOUR UNIX: THE ULTIMATE GUIDE”, 2ND EDITION, MCGRAW HILL, 2006

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UNIX OPERATING SYSTEM

• SOFTWARE THAT MANAGES THE COMPUTER HARDWARE• DEVELOPED IN HIGH LEVEL LANGUAGE - “C”

– Not assembler, thus INTERPRETED into Assembler language– Similarities to Windows are superficial– Provides hardware portability– Written by programmers for own use, – Command function not often obvious from Command Name

• COMMAND INTERPRETER – SHELL– User to system interactions

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HOW IT WORKS

• POWER OF UNIX – IN COMBINATION OF SHELL AND COMMANDS– System comes with library of SHELLs– Supports creation of personal SHELLs– Korn SHELL (ksh) library, Bourne SHELL (sh), Bash SHELL (bash),

• EXAMPLE OF SHELL: LOGON AUTHENTICATION

Login: foobarPassword: ********

• PASSWORD NOT SHOWN FOR SECURITY/PRIVACY REASONS

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BASIC UNIX COMMANDS

• DIRECTORY SYNTAX: <ROOT>/<SUB-DIR>[/<SUB-DIR2>[/<SUB-DIR3>[…]]

• COMMAND LINE PROMPT: “$”• SAMPLE COMMANDS: - ALL COMMANDS USE LOWER CASE

SPELLINGlogin & passworddate – Provides date and timeexit or logout – Terminate the user sessionwho – Lists all current users of the systemps – Reveals the process id (PID) of the current process kill – permits a user to terminate a process by PID echo – Lists all current echo $SHELL – Reveals the shell type (ksh vs bash)

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UNIX COMMANDS

• GENERALLY, COMMANDS ARE EXECUTABLE DISK FILES (C PROGRAMS)– Commands execute like any other program (remember chp 1)– UNIX supports commands written in any language (e.g., Java)– Files do not need extension– Shell (exception) automatically invoked once you log in

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TYPES OF COMMANDS

• INTERNAL COMMAND OF THE SHELL WHICH COULD BE

• a built-in (like cd, pwd, etc.)• an alias DEFINED BY THE USER THAT INVOKES THE

DISK

• EXTERNAL PROGRAM ON DISK WHICH COULD BE• A BINARY EXECUTABLE (WRITTEN IN C, C++).• A SCRIPT FILE (LIKE A SHELL OR perl SCRIPT).• INTERNAL VERSION IN A SPECIFIC MANNER.

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THE PATH

• A SHELL VARIABLE (OR ENVIRONMENT VARIABLE) o SPECIFIES A LIST OF DIRECTORIES TO SEARCH.

• SHELL LOOKS AT PATH ONLY WHEN o COMMAND IS NOT USED WITH A PATHNAME o ALSO NOT A SHELL BUILT-IN.

• COMMAND CAN STILL BE EXECUTED IF NOT IN PATH BYo USING A PATHNAME.o MODIFYING PATH TO INCLUDE THE DIRECTORY CONTAINING

THE COMMAND.

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FLEXIBILITY OF COMMAND USAGE (Cont’d)

• USE OUTPUT OF ONE COMMAND AS INPUT OF ANOTHER:

date | cut -d” “ -f2

• REDIRECT OUTPUT INTO A FILE VIA > (A CHEVRON) OR >>:

ls -lRa / > $HOME/ls-lRa.txt

• RUN A COMMAND IN THE BACKGROUND BY APPENDING &:

ls -lRa / > $HOME/ls-lRa.txt &

USING man

• DISPLAYS DOCUMENTATION OF COMMANDS, CONFIGURATION FILES, SYSTEM CALLS AND LIBRARY FUNCTIONS.

• ORGANIZED IN A NUMBER OF SECTIONS. COMMANDS ARE FOUND IN SECTION 1.

• MAY NEED TO USE SECTION NUMBER WHEN ENTRY EXISTS IN MULTIPLE SECTIONS

(e.g. man passwd and man -s 5 passwd).

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UNDERSTANDING A man PAGE

EXAMPLE: WC SYNTAX/SYNOPSIS wc [ -c | -m | -C ] [ -lw ] [ file ... ]

• MOST USEFUL INFORMATION AVAILABLE IN SYNOPSIS AND DESCRIPTION.

• WHEN OPTIONS GROUPED IN [ ] WITHOUT A |, ONE OR MORE OF THEM CAN BE USED.

(-l, -w and -lw are valid.)

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UNDERSTANDING A man PAGE

• THE | SIGNIFIES AN OR CONDITION. (E.G., ONLY ONE OF -C, -M OR -C CAN BE USED.)

• THE ... MEANS THAT MULTIPLE OCCURRENCES OF THE PRECEDING ITEM ARE POSSIBLE.

(WC CAN BE USED WITH MULTIPLE FILES.)

• EXIT STATUS INDICATES VALUES RETURNED ON ERROR.

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WHO IS LOGGED ON & WHAT ARE THEY DOING?

• users– SHOWS A LIST OF ALL CURRENTLY LOGGED ON USERS

• who and whoami– DISPLAY A LIST OF ALL CURRENTLY LOGGED ON USERS

AND WHERE THEY ARE LOGGED ON FROM• w– SHOWS A LIST OF ALL CURRENTLY LOGGED ON USERS

AND WHAT THEY ARE RUNNING• finger– TELLS A LITTLE MORE INFORMATION ABOUT USERS

• touch– CREATE AN EMPTY FILE

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THE UNIX FILE & NAMING

• A CONTAINER FOR STORING INFORMATION AND

DATA.• FILENAME - 1. LIMITED TO 255 CHARACTERS.

2. CAN’T CONTAIN / OR NULL.• FILENAMES ARE CASE-SENSITIVE, I.E.,

CHAP AND CHAP ARE TWO DIFFERENT FILENAMES.

o UPPER CASE “A” NOT SAME ASCII VALUE AS “A”.

• GROUP OF FILENAMES HELD TOGETHER IN A DIRECTORY.

• DIRECTORY LISTING CONTAINS NAME OF THE FILE.• BOTH FILES AND DIRECTORIES ARE SUBJECT TO

ACCESS CONTROL.

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THE UNIX FILE (Cont’d)

• FILE CONTENT CAN BE ANY SEQUENCE OF ASCII CHARACTERS OR BINARY ENCODED CHARACTERS

• NEITHER FILE NAME OR SIZE IS STORED IN FILE

• FILENAMES CAN BE: UP TO 255 CHAR IN LENGTH, FILE EXTENSIONS ARE

OPTIONAL. CAN’T CONTAIN “/” OR NULL (ASCII VALUE = 0 [I.E., ZERO] AVOID USING $ ` ? * & IN NAMES

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FILE TYPES

• ORDINARY OR REGULAR FILE: CONTAINS DATA AS A STREAM OF CHARACTERS. THIS FILE CAN BE A TEXT FILE (PROGRAM SOURCES, CONFIGURATION FILES) –

CONTAINS ONLY PRINTABLE AND EOL CHARACTERS VISIBLE WITH “OD” COMMAND.

BINARY FILE (EXECUTABLES, GRAPHIC AND MULTIMEDIA FILES).• DIRECTORY: CONTAINS THE FILENAME AND A

NUMBER (INODE NUMBER).• DEVICE FILE: CONTAINS NO DATA

WHATSOEVER.• SYMBOLIC LINK: CONTAINS THE LOCATION OF

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PROPERTIES OF FILES

• ALL FILES HAVE THESE PROPERTIES– PERMISSIONS– LINKS

• FOR DIRECTORIES, THIS LISTS THE NUMBER OF SUBDIRECTORIES

– OWNER– OWNER GROUP– SIZE– TIMESTAMP– Name

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THE HIERARCHICAL STRUCTURE OF THE FILE SYSTEM

• A SINGLE HIERARCHICAL STRUCTURE THAT CONTAINS ALL FILES.

• TOP SIGNIFIED BY ROOT (/).• EXISTENCE OF A PARENT-CHILD RELATIONSHIP.• PARENT OF ANY FILE MUST BE A DIRECTORY.• FILES ACCESSED WITH PATHNAMES (E.G.,

/etc/passwd).

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HIERARCHICAL FILE STRUCTURE ILLUSTRATED

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SPECIAL NOTATION FOR TRAVERSING DIRECTORIES

. (PERIOD)– SIGNIFIES THE CURRENT DIRECTORY

.. ( DOUBLE PERIOD)– SIGNIFIES THE DIRECTORY DIRECTLY ABOVE THE CURRENT

DIRECTORY

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SPECIAL NOTATION FOR TRAVERSING DIRECTORIES

~ (TILDE CHARACTER)– YOUR HOME DIRECTORY

THESE DIRECTORIES CAN BE “STACKED” FOR EASY TREE TRAVERSAL“../..” STANDS FOR TWO DIRECTORIES DIRECTLY ABOVE

THE CURRENT DIRECTORY

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PATHNAMES: TWO TYPES

• ABSOLUTE PATHNAME: SPECIFIES LOCATION WITH REFERENCE TO THE FILE SYSTEM TOP

(e.g, cat /etc/passwd).• RELATIVE PATHNAME: SPECIFIES LOCATION

WITH REFERENCE TO THE USER’S CURRENT LOCATION

(e.g., cd ../include).• BOTH COMMANDS AND FILENAME

ARGUMENTS CAN BE REPRESENTED IN EITHER FORM.

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USEFUL PATHNAMES

• /bin & /usr/bin DIRECTORIES:o User accessible - commonly used command repositoryo bin indicates binary file store

• /sbin & /usr/sbin DIRECTORIES.o SuperUser or Adminsitrator accessible command repository

• /etc DIRECTORY o Configuration file repository: e.g., /etc/passwd & /etc/shadow.

• /dev DIRECTORYo All device fileso May have sub-dirctories: pts, dsk, rdsk

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USEFUL PATHNAMES (Cont’d)

• /lib & /usr/lib DIRECTORIES:o USER ACCESSIBLE – ALL REUSABLE FILES IN BINARY FORM.

• /usr/include DIRECTORY:o USER ACCESSIBLE – ALL STANDARD HEADER FILES USED IN C

PROGRAMSo NAMING: USUALLY HAVE A SUFFIX “.h”o C-code #include stdio.h WHICH INSTRUCTS

COMPILER TO INCLUDE THE FILE stdio.h WHEN IT CREATES THE EXECUTABLE FOR C-PROG.

• /usr/share/man DIRECTORY o man PAGE REPOSITORY.

• /tmp DIRECTORY• Users create temporary files.

THE HOME DIRECTORY

• DIRECTORY WHERE USER IS PLACED ON LOGIN.• DETERMINED BY SIXTH FIELD IN /etc/passwd:

romeo:x:500:100:romeo vincent:/home/romeo:/bin/bash

• CAN ALSO BE REFERRED TO BY• THE SHELL VARIABLE $HOME (e.g. cat $HOME/foo).• TILDE (~) EXPANSION IN MOST SHELLS: (e.g. cat ~/foo).

• cd COMMAND USED WITHOUT ARGUMENTS RETURNS USER TO HOME DIRECTORY.

• USER CAN CREATE AND REMOVE FILES IN THEIR HOME DIRECTORY BUT NOT IN OTHER DIRECTORIES.

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SECURITY STARTS WITH USERSWHO GETS TO SEE AND DO WHAT

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DIFFERENT SECURITY LEVELS

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WHO CAN LOG-ON TO THE SYSTEM?

• TYPICAL ENTRYJMENSING:*:17791:32:JOHN

MENSING:/HOME/FAC/JMENSING:/BIN/KSH

• FIELDS DELIMITED BY COLONS (:) • ENTRY SYNTAX:

Login name:password:user id:group id: full name:Home directory:shell to run

• LOOK AT /etc/group FOR AVAILABLE GROUPS

DETERMINED BY AN ENTRY IN FILE passwd IN THE /etc DIRECTORY

Each directory and file has permissionsOnce in, who sees which files

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HOW TO SEE WHO CAN DO WHAT USE ls –l TO DISPLAY PERMISSIONS

• FOR DIRECTORIESr – read list of filesw – add/delete to the list of filesx – able to change (cd) to the directory

(this is needed for anyone to see the contents of the directory)• FOR FILES

r – read the filew – modify, change, or delete the file x – run the file

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EXAMPLE OF ls -l

$ ls -ltotal 1562-rw------- 1 jmensing fac 96 Aug 5 12:41 ch10-rw-r--r-- 1 jmensing fac 382 Sep 15 10:00

dateoutdrwxr-xr-x 2 jmensing fac 512 Sep 16 10:07 la

FORMAT OF RESULTING LISTING IS PERMISSIONS, LINKS, OWNER, GROUP, FILE SIZE, LAST MODIFIED, FILENAME

POSSIBLE TO SEE THAT A FILE IS IN A DIRECTORY BUT UNABLE TO READ THE CONTENTS OF THE FILE

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TO CHANGE PERMISSIONS USE chmod

chmod comes in two flavors symbolic and octal

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SYMBOLIC chmod CODES

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chmod EXAMPLE

$ ls –l file1-r-------- 1 maryann 48 Sep 12 12:02 file1

$ chmod a=r,u+w file1$ ls –l file1-rw-r--r-- 1 maryann 48 Sep 12 12:02 file1

NOTE: FILES ALWAYS START WITH A – WHILE DIRECTORIES ALWAYS START WITH d

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OCTAL MODES

• 0 NONE• 1 EXECUTE• 2 WRITE• 3 WRITE AND EXECUTE• 4 READ• 5 READ AND EXECUTE• 6 READ AND WRITE• 7 READ WRITE AND EXECUTE

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OCTAL chmod OPTIONS

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USER MASKWHAT PERMISSIONS TO REMOVE WHEN CREATING A FILE OR

DIRECTORY

UMASK OF 022DIRECTORIES ARE SET TO 777

IF THE UMASK IS 022THEN THE RESULT IS 755

FILES ARE SET TO 666 IF THE UMASK IS 022 THEN THE RESULT IS 644

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TO CHANGE THE OWNER OF A FILE USE chown COMMAND

NOTE: Once you change the owner you can not get ownership back, the new owner needs to do that

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TO CHANGE THE GROUP OF A FILE USE chgrp

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USE groups COMMAND TO DISPLAY WHICH GROUPS YOU

BELONG TO

A user can belong to more than one group newgrp

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Figure 4-7

CHANGING PERMISSIONS

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USE THE umask COMMAND TO ASSESS DEFAULT SETTINGS (FOR FILE

CREATION)

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