Fundamentals of UNIX

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1

Fundamentals

of

UNIX



2

IndexTopic Page #

UNIX Operating System 3

Unix Commands 25

Files & Directories 34

Unix Utilities 61Process 99

Shell Programming 111

Reference 162Lab 163



3

Chapter 1

UNIX Operating System



4



5



6



7



8

Objectives

In this session, you learn about:• The functions of OS

• The history of Unix

• The features of UNIX• The Unix architecture

• Process management

• CPU scheduling

• Memory management

• File management



9

• OS is a system software

• OS can be defined as an organized collectionof software consisting of procedures for

operating a computer

1.OS provides an environment for execution of

programs

1.OS acts as an interface between the user andthe hardware of the computer system.

Operating System (OS)



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• Operating system interacts with user in two ways1.Operating system commands

Enables user to interact directly with the

operating system.

• Operating system calls

Provides an interface to a running programand the operating system. System calls inUNIX are written in C.

Operating System



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• Ken Thompson of AT&T Bell Laboratories designedUNIX in late 1960s

• Two versions of UNIX that emerged are AT&T Unixand BSD Unix

• In 1989, AT&T and Sun Microsystems joined togetherand developed system V release 4 (SVR4)

• Two of the main standards mainly in use are POSIX

(Portable Operating System Interface) and X/open standard. In 1988, MIT formed Xconsortiumdeveloped vendor-neutral Xwindow System.

History of UNIX



12

What is Linux?

1.An open-source UNIX like operating system

1.Initially created by Linus Torvalds for PCarchitecture

1.Ports exist for Alpha and Sparc processors

1.Developer community world-wide contribute

to its enhancement and growth



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• Multi-user, multitasking, timesharing

• Portability

• Modularity

• File structure

• Security

• Strong networking support & advanced graphics

Features of UNIX



14

hardware

kernel

shell

ld

as

comp

cp

vied

grepwc

date

a.out

who

sh

sort

lsbanner...

Layered Architecture



15

• Unix system follows a layered approach. Ithas four layers

• The innermost layer is the hardware layer

• In the second layer, the kernel is placed

• The utilities and other application programsform the third layer

UNIX System Architecture



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• Kernel is that part of the OS whichdirectly makes interface with thehardware system.

• Actions:1.Provides mechanism for creating and

deleting processes

2.Provides processor scheduling,memory, and I/O management

3.Provides inter-process

communication.

Kernel



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• A utility program that comes with the

UNIX system.

• Features of Shell are:

1.Interactive Processing

2.Background Processing

3.I/O Redirection

4.Pipes5.Shell Scripts

6.Shell Variables

7.Programming Constructs

The Shell



18

• A process is a program in execution

• Several processes can be executed simultaneously

in a UNIX system.

• A process is generally created using the “fork( )”system call.

• The process that invokes the “fork( )” system call isthe parent process, and the newly created process iscalled the child process.

Process Management



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• Unix uses round-robin scheduling tosupport its multi-user and time-sharingfeature.

• Round-robin fashion of scheduling isconsidered to be the oldest, simplest and

widely used algorithm.

• Every process is given a time slice (10-

100 millisec.)

CPU Scheduling



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• UNIX uses a hierarchical file system with“/” as its root.

• Every non-leaf node of the tree is calledas a directory file.

• Every leaf node can either be a file, or anempty directory

File Management



21

dev bin tmp home etc lib usr

console

lp0

shls passwd

inittab bin

user1

user2

var

srcspool

File System



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• File system is the structure in which files arestored on disk

• File in UNIX is sequence of bytes organized in the

form of blocks

• The size of each block is 512 bytes (depends on

architecture)

• Block size can be decided while creating the file

system structure

File System



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Boot Block

Super Block

Inode Block

Data Block

Type of the file

Link counter

Uid, gid, size

Date and time of Creation

Date and time of access

Date and time of modification

:

:

Address of datablock

Address of datablock

::

Address of the addr block



File System Structure



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BSD: Berkeley, BSD

Solaris: Sun Microsystems, Sys 5/BSD

Ultrix: Digital Equipment Corporation, BSD

OSF 1: Digital Equipment Corporation, BSD/sys 5

HPUX: Hewlett-Packard, Sys 5

AIX: IBM, Sys 5 / BSD

IRIX: Silicon Graphics, Sys 5

GNU/Linux: GNU, BSD/Posix

Common UNIX Flavours



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• Broad classification of users

• root (most privileged)

• Non-root (less privileged)

1. Group

1. UNIX allows user IDs to be grouped

2. A single user ID can be member of multiple groups

• Differentiating users w.r.to file access

1. Owner

2. Group

3. Others

Types of UNIX Users



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• User logs in with a valid user ID• User logs out to terminate the login

session

Working With UNIX



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Summary

In this session, you learned about … • The functions of OS

• The History of Unix

• The features of UNIX

• The Unix Architecture

• Process management

• CPU Scheduling

• Memory management

• File management



28

Chapter 2

UNIX Commands



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Objectives

In this session, you will learn to:1.Use the basic Unix commands

1.pwd

2.date

3.who

4.ls5.man

• Use “man” pages



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• pwd

• Displays the current workingdirectory.

1.date

1.Displays the current date and time

Simple Commands



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• who

• Displays the names of all the userswho have currently logged in

1.who am i

1.Displays the name of the current

user.

Simple Commands



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• lsSyntax :ls [options] [file….]

options: -l list in long format

-a list all files including those

beginning with a dot-i list inode no of file in first column

-s reports disk blocks occupied by file

-R recursively list all sub directories

-F mark type of each file

-C display files in columns

Listing the Directory Contents



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Meta Characters

MetaCharacters Purpose Example

* Match with one or more characters or none $ ls – l *.c file*? Match with any single character $ ls – l file?[…] Match with any single character within the

brackets$ ls – l file[abc]

; Command separator $ cat file1; cat file2

| Pipe two commands $ cat abc | wc( ) Group commandsUseful when the output of thecommand grouphas to be redirected

$ (echo “==== x.c ====”; cat x.c) > out

`command` Execute the command enclosed within back quotes. Useful when the output of a commandinto a variable in a shell script

count=`expr $count + 1`

assuming count has value3, this

increments the value of count „string‟ Quote all characters with no substitution(ex. no special meaning for $)

echo „expr $count + 1‟ displays expr $count + 1

“string” Quote all characters with substitution.The characters $, \ (back slash) and back quotehave special meaning.

echo “expr $count + 1” displays expr 3 + 1assuming the variable count has value 3



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$ ls –l

total 6

-rwxr-xr-x 1 user1 projA 12373 Dec 15 14:45a.out

drwxr-xr-x 2 user2 projD 4096 Dec 22 14:00awkpro

-rw-r--r-- 1 user1 projA 12831 Dec 12 13:59

c-rw------- 1 user1 projA 61440 Dec 15 11:16

core

-rw-r--r-- 1 user3 projC 255 Dec 20 14:29cs

File type

File accesspermissions

Link countGroup id

Date & time ofmodification

Listing the Directory Contents

User id File sizein bytes

File name



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• The Unix manual, usually called man pages, is available on-line

to explain the usage of the Unix system and commands.

Syntax:

• man [options] command_name

Common Options

-k keyword list command synopsis line for all keywordmatches

-M path path to man pages

-a show all matching man pages (SVR4)

• info command_name - help for commands

• help - –command_name – gives command synatx

Getting Help on Commands



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Summary

In this session, you have learned to …

1.use the basic Unix commands like

1.pwd2.date

3.who

4.ls5.man

• use “man” pages



37

Chapter 3

Files & Directories



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• In this session, you will learn to:1.set file permissions using the chmod

command

2.use directory-related commands namelymkdir, rmdir, cd commands

3.use file-related commands namely cp, mv,rm commands

4.access advanced file permissions usingcommands umask, suid, sgid, linking files,stickybit

5.create and edit files using the vi editor

Objectives



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• Refers to the permissions associated with a file with

respect to the following

• Permission Levels

• User (owner) (u)

• Group (wheel, staff, daemon, etc.) (g)

• World (guest, anonymous and all other users) (o)

1. Permission Settings

1. Read (r)

2. Write (w)

3. Execute (x)

File Access Permissions

il A i i



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• No read permission does not allow the user to:

• List the contents of directory

• Remove the directory

1. No Write permission does not allow the user to :

1. copy files to the directory2. remove files from the directory

3. rename files in the directory

4. make a subdirectory

5. remove a subdirectory from the directory6. move files to, and from the directory




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• No execute permission does not allowthe user to:

– display the contents of a directory filefrom within the directory

– change to the directory

– display a file in the directory


Ch i P i i h d



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• chmod u+x file_name

Syntax:chmod <category> <operation> <permission> <filename(s)>

or

chmod <octal number> filename

Octal Number

4 - for read

2 - for write

1 - for execution

$ chmod 744 xyz

this sets read, write and execute permissions for owner,read permission for group and others

Changing Permissions - chmod

Di C i



43

Command Syntax

mkdir [OPTION] DIRECTORY$ mkdir <path>/<directory>

$ mkdir – m <directory>

$ mkdir – p <directory1>/<directory2>/<directory3>

Example:

$ mkdir project1

This creates a directory project1 under current directory

Note: Write and execute permissions are needed for the

directory in which user wants to create a directory

Directory Creation

Di t R l



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rmdir command removes directory

Syntax

– rmdir <directory name>

Example

Removes project1 directory in the current directory

– rmdir project1Remove multiple directories

rmdir pos1 pos2

Remove the directory recursively

rmdir –p dir1/dir2/dir3

rmdir removes a directory if it is empty and is not thecurrent directory

Directory Removal



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Command - cd

cd command is used to change thedirectory

• cd - take to the home directory

• cd .. - takes to the parent directory

• cd / - takes to the root directory

Fil R l t d C d



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File Operation

Copying a file

Moving a file

Removing a file

Displaying a fileand concatenating files

Command

cp

mv

rm

cat

File-Related Commands

C d



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Used to copy files across directories

Syntax

cp <source file> <new file name>

Example

cp file1 file2

Command - cp

C d



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Options to cp

1. -p• Copies the file and preserves the following attributes

1.owner id

2.group id

3.permissions4.last modification time

• -r

• recursive copy; copy subdirectories under the

directory if any1. -i

1.interactive; prompts for confirmation beforeoverwriting the target file, if it already exists

Command - cp

C d m



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Used to move a file, or rename a file

Preserves the following details

• owner id

• group id• permissions

• Last modification time

-f suppresses all prompting (forces overwriting of target)

-i prompts before overwriting destination file

Command - mv

Command rm



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Used to remove a file

1.Syntax : rm file(s)

-f suppresses all prompting

-i prompts before deleting destinationfile

-r will recursively remove the file from adirectory (can be used to delete adirectory along with the content )

Command - rm



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$ ls –l-rwxr-xr-x 1 user1 training 12373 Dec 15 14:45 a.out-rwxr-xr-x 3 user1 faculty 4096 Dec 24 11:56 awkpro

$chown user2 a.out

$ls –l-rwxr-xr-x 1 user2 training 12373 Dec 15 14:45 a.out-rwxr-xr-x 3 user1 faculty 4096 Dec 24 11:56 awkpro

$ chgrp training awkpro

$ls –l-rwxr-xr-x 1 user2 training 12373 Dec 15 14:45 a.out-rwxr-xr-x 3 user1 training 4096 Dec 24 11:56 awkpro

Command – chown & chgrp



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umask value is used to set the default permission of a file and

directory while creating

umask command is used to see the default mask for the filepermission

Default umask value will be set in the system environment filelike /etc/profile

umask 022 will set a mask of 022 for the current session

– The file permission after setting this umask value will be644

– And the directory permission will be 755

Command - umask



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Linking files

1.Hard Link (in the same filesystem)

1.$ ln /usr/bin/clear /usr/bin/cls

1.Hard link uses the same inodenumber

• Soft Link (in different filesystems alsoused to link directories)

• $ ln –s /usr/bin/clear /home/user1/cls

Command - ln

S i l P i i Bit



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• Set user ID (SUID)1.This means that if the SUID bit is set for any application then

your user ID would be set as that of the owner ofapplication/file rather than the current user, while running thatapplication

1.“set user ID” bit can be set in one of the two ways:

• chmod u+s <filename>

1.chmod 4755 <filename>

1.The leftmost octal number 4 indicates “set user ID” bitto be set, other octal digits indicate regular filepermissions. This is meaningful for executable files

only.

Special Permission Bits



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• Set group id (SGID)1.Just like SUID, setting the SGID bit for a file sets your group

ID to the file's group while the file is executing

1.“set group ID” bit can be set in one of the two ways: • chmod g+s <filename>

1.chmod 2755 <filename>

1.The leftmost octal number 2 indicates “set group ID”bit to be set, other octal digits indicate regular filepermissions. This is meaningful for executable filesonly.




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• Sticky bit (SVTX)

1.Typically set to a directory that is shareable

2.Any user can create a file in such sharable directory

3.Only owner of the file or super user (root) can remove a filefrom the directory

1.“sticky” bit can be set in one of the two ways:

• chmod +t <directoryname>

1.chmod 1555 <directoryname>• The leftmost octal number 1 indicates “sticky” bit to be

set, other octal digits indicate regular file permissions.


Vi Editor



57

• vi is a visual editor used to create and edit text files.

• A screen-oriented text editor

• Included with most UNIX system distributions

• Command driven

1. Categories of commands include

1.Cursor movement2.Editing commands

3.Search and replace commands

1. The vi editor is invoked by the following command:

$ vi filename

Vi Editor

Navigation



58

Backspace

h

Space

l j k

the quick brown fox

ww w

the quick brown fox

2 w

the quick brown fox

bb b

the quick brown fox

$

the quick brown fox

^

Navigation



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• Text insertion / replacement

1.i - inserts text to the left of the cursor

2.a - inserts text to the right of the cursor

3.I - inserts text at the beginning of the line

4.A - appends text at end of the line5.o - opens line below

6.O - opens line above

7.R - replaces text from cursor to right

8.s - replaces a single character with any number ofcharacters

• S - replaces entire line

Editing Commands



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• Deletion

1.x - to delete character at cursor position

2.3x - to delete 3 characters at cursor position3.dw - to delete word

4.2dw - to delete 2 word

5.dd - to delete a line

6.2dd - to delete 2 lines

Editing Commands



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• Yanking

• Y - copy line into buffer

• 3Y - copy 3 lines into buffer

• p - copy buffer below cursor

• P - copy buffer above cursor

1. Save and quit

1. :w - to save

2.:w! - to name a file (:w! filename -> save as)3.:x - save and quit

4.:q - cancel changes

5.:q! - cancel and quit

Editing Commands



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The following commands are applicable for vi

editor in Linux

/pat searches for the pattern pat

and places cursorwhere pattern occurs.

/ repeat last search

:%s/old/new/g to change every

occurrence in the whole file.

Search & Replace Commands



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Summary

• In this session, you have learned how to … 1.use file permissions using the chmod

command

2.use directory-related commands namelymkdir, rmdir, cd commands

3.use file-related commands namely cp, mv,rm commands

4.access advanced file permissions usingcommands umask, suid, sgid, linking thefiles, stickybit

5.create and edit files using the vi editor



64

Chapter 4

UNIX Utilities

Obj ti



65

In this session, you will learn how to:

1.use the Unix utilities such as

1.cat, echo, touch, more, file, wc, cmp, comm, find

• employ redirection operators• use filters such as

• sort, grep, cut, head, tail, tr, and paste

1.use communication commands

1.telnet, ftp

2.use backup commands

1.zip/gzip and tar

Objectives

cat



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• cat command takes the input from the keyboard, and

sends the output to the monitor

1. We can redirect the input and output using theredirection operators

$ cat > file1

Type the content here

press <ctrl d>

$ cat file1Displays the content of the file

$cat >> file1

This will append standard input to the content of file1

cat



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• touch is used to change the time stamp of the file

Syntax:

touch [options] file

• Options:

• -a to change the access time

• -m to change the modification time

• -c no create if not exists

• touch <file> will change the time of change of the file if thefile exists

• If the file does not exist, it will create a file of zero byte size.

touch



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• echo command is used to print output to the screenecho “This is an example”

This is an example

x=10

echo $x

10

• read command allows to read input from user and assign it

to the variable specified.

read x

echo & read

General Purpose Utilities



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• more• Allows user to view one page-full of

information at a time.

1. file

• Used to display the type of the file

2. tty

1.Prints the terminal‟s name





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• wc1.A filter used to count the number of lines,

words, and characters in a disk file or fromthe standard input.

2.-l - displays the number of lines3.-w - displays the number of words

4.-c - displays the number of characters




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• Lets user to search set of files and directories based on various

criteria• Syntax: find [path...] [expression]

• [path]

• where to search

• [expression]

– What type of file to search (specified with –type option)

– What action to be applied ( –exec, –print, etc.)

– Name of the files (specified as part of –name option,enclosed in “ “)

1. Example

find . –name “*.c” -print

lists all files with .c extension from the current dir & itssubdirectories

find



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• Finding files on the basis of file size

1. – size [+ –]n[bc]

n represents size in bytes (c) or blocks (b) of 512

bytes

find . –size 1000c lists all files that are exactly

1000 bytes in size

find . –size +1000c lists all files that are more than1000 bytes in size

find . –size –1000c lists all files that are less than

1000 bytes in size

find



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• Finding files on the basis of access time (atime) or modified time

(mtime)

1. – atime [+-]n

2. – mtime [+-]n

n represents number of days ( actually 24 * n hours)

find . –atime 2 lists files accessed exactly 2 days ago

find . –atime +2 lists files accessed more than

2 days ago

find / –mtime –2 lists files modified less than 2

days ago

find

find



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• Applying a command on files matching the criteria with –exec and –okoptions

1. – exec command {} \;

command is command to be applied on the matching files (does

not prompt user)

find . -name “*.dat” –exec ls –l {} \;

Long listing of all files with .dat extension in the current and its

subdirectories

– -ok command {} \;

Functionality is similar to –exec, but prompts user before applying

the command on the file matching the criteria.

find



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• Standard Input file

• Keyboard, file descriptor is 0

1.Standard Output file

1.Monitor, file descriptor is 1

• Standard Error file

–Monitor, file descriptor is 2

Standard Files

I/O Redirection



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< file redirect standard input from

file> file redirect standard output to file

2> file redirect standard error to file

2>&1 merge standard error with

standard output

$ cat > abc

$ ls –l > outfile

I/O Redirection

Filters



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• Filters are programs that takes its input from thestandard input file, process it, and sends it to thestandard output file.

• Commonly used filter commands1.sort

2.grep

3.cut

4.head5.tail

6.paste

Filters

sort



78

Sorts the contents of the given file based on the first char of each line.

-n numeric sort (comparison made

according to strings numeric value)

-r reverse sort

-t specify delimiter for fields

+num specify sorting field numbers

+num [-num] to specify the range

sort



79

• grep -Global Regular Expression Printeris used for searching regular expressions

• Syntax

1.grep <options> <pattern><filename(s)>

grep

grep options



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-c displays count of the number of occurrences

-n displays line numbers along with the lines

-v displays all lines except lines matching pattern

-i Ignores case for matching

grep options

P



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* - matches 0 or more characters

[^pqr] - Matches a single character which is not p ,q or r

^pqr -Matches pqr at the beginning of the line

pqr$ -Matches pqr at the end of the line

“.” - Matches any one character

\ - ignores the special meaning. grep “New \[abc\ ]” filename

Patterns

Filt C d h d



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Displays the first n lines of the file

$ head -3 file1

Filter Command - head

Filt C d t il



83

Displays the last n lines of a file

$ tail -3 file1

Can also specify the line number from which

the data has to be displayed till the end of file

$ tail +5 file1

Filter Command - tail

Filt d t



84

tr - translate filter used to translate a given set of characters

Example :

tr [a-z] [A-Z] < filename

This converts standard input read from lower case to uppercase.

option -s can be used to squeeze the repeated characters.

Filter command - tr

Filt d t



85

Useful options for tr

• -s char

Squeeze multiple contiguous occurrences of thecharacter into single char

• -d charRemove the character

Filter command - tr

C d Pi i



86

• Allows the output (only the standardoutput) of a command to be sent as inputto another command.

• Multiple pipes may appear in onecommand line.

Example:

$ cat * | wc

Command Piping

Filt C d t



87

• tee command allows the normal output tothe standard output, as well as to a file

• Useful to capture intermediate output of a

long command pipeline for furtherprocessing, or debugging purpose.

• Example• who | tee userlist

• cat - | tee file1 | wc -l

Filter Command – tee

Filt C d t



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Used to extract specified columns of a text

Option remark

-c used to extract characters

-d Delimiter for fields

-f Field no.

Examples

$ cut -c2-5 file1

$ cut -d “|” -f2,3 file1

Filter Command – cut

ftp



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• Ftp is a file transfer program

• Provides necessary user interface to the standard File Transfer

Protocol

• Allows users to transfer files to and from a remote host

• Syntax

$ ftp hostname

p

ftp - commands



90

• Ftp program supports the following commands

• get receive file from host

• mget receive multiple files from host

•

put send file to host• mput send multiple files from host

p

ftp - commands



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• Ftp program supports the following commands as well

• ls list directory of host

• cd change directory on the host

•

lcd change directory on the local machine

• To set transfer format

• ascii set to ascii mode

• binary set to binary mode

p

ftp - commands



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• Progress indication of transfer

• hash command

• Quitting ftp session

• byecommand

Tape Archive - tar



93

• Tar is an archiving utility to store and retrieve files from anarchive, known as tarfile.

• Though archives are created on a tape, it is common to havethem as disk files as well.

1. tar c|t|x [vf destination] source...

Tape Archive - tar



94

Examples:Create a new tar file containing all .dat files (assuming a.dat, b.dat and c.dat

exist)

$ tar – cf mytar *.dat

Compression Utilities



95

gzip,Usage is very similar to compress and pack utilities in Unix:

gzip [-vc] filename

where -v displays the compression ratio.

-c sends the compressed output to standard output and leaves

the original file intact.

gunzip

gunzip can uncompress files originally compressed with compress.

Summary



96

Summary• In this session, you have learned to:

1.use the Unix Utilities like1.cat, echo, touch, more, file, wc,

find

• employ redirection operators• use filters like

•sort, grep, cut, head, tail, tr, ftp

1.backup commands1.tar and zip/gzip



97

Chapter 5

Process

Objectives



98

In this session, you will learn to:

1.Use process-related commands like

1.ps, kill, sleep

• Start a background process

• Use background and foreground-relatedcommands like

• bg, fg, jobs , nice , nohup

j

Processes



99

• Process - a program in execution

• When program is executed, a new process is

created

• The process is alive till the execution of theprogram is complete

• Each process is identified by a number calledpid

Login shell



100

As soon as the user logs in, a process is createdwhich executes the login shell.

Login shell is set for each login in /etc/passwd

file.

ps



101

• The ps command is used to display thecharacteristics of a process

• It fetches the pid, tty, time, and the command

which has started the process.1.-f lists the pid of the parent process also.

2.-u lists the processes of a given user

3.-a lists the processes of all the users4.-e lists all the processes including the

system

processes

Background Process



102

• Enables the user to do more than one task at atime.

• If the command terminates with an ampersand

(&), UNIX executes the command in the

background

• Shell returns by displaying the process ID

(PID) and job id of the process

Background Process

Controlling Background



103

• jobs

• List the background process

1. fg % <job id>

• Runs a process in the foreground

2. bg %<job id>

1.Runs a process in the background

Processes

The kill Command



104

• kill: Kills or terminates a process

• kill command send a signal to the process

• The default signal is 15 ( SIGTERM)

1. kill -9 (SIGKILL)

1. Terminates the process abruptly

The kill Command

Summary



105

Summary• In this session, you learned to:

1.Define a process

2.Use process-related commands like

1.ps, kill, sleep

3.Start a background process

4.Use background and foreground-related commands like

1.bg, fg, jobs



106

Chapter 6

UNIX Shell Programming

Objectives



107

• In this session, you will learn to:

1.Use Shell variables2.Write scripts to process positional parameters

3.Use “test” command

4.Use “if” construct 5.Use “for” loop

6.Use “while” loop

7.Use “case” construct8.Define and use functions

9.Debug shell scripts

Flavours of the Unix shell



108

• Bourne shell sh

• C shell csh

• Korn shell ksh

• Bourne again shell bash

(shell distributed with linux)

Command processing



109

1.Displays the shell prompt and reads the commandtyped by the user.

1.Interprets the command and classifies it as aninternal (built-in), or an external command.

1.If it is NOT a built-in command, searches for thecommand in the PATH-specified directories, andexecutes that command if it is found.

Shell Features



110

Parent shell process

(bash)

User

Mode

Kernel

Mode

$ vi test.c

command typed by

user

Child shell process(bash)

fork

Exec of “vi test.c”

Additional Shell Features



111

1.Each shell, apart from the basic features,provides additional features such as:

–Maintaining command history (C, kornand bash)

–Renaming (aliasing) a command (C,korn, bash)

History



112

• some UNIX shells support command history

• facility to keeps track of commands that were executed

• facility to rerun previously executed commands

• bash shell supports the following

!! recall the last command and execute it.

!num execute nth command where n is the

the num specified after !

alias



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• alias can be used to give new name to an existing command• A better name that represents a single command or a sequence of

commands to be executed, often with appropriate options

• alias is an internal command

alias newname=command

$ alias l=„ls –l‟

The unalias command cancels previously defined alias.

Shell Programming



114

• Allows

1.Defining and referencing variables2.Logic control structures such as if,

for, while, case

3.Input and output

Shell Programming

Shell Variables



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• A variable is a name associated with a data value,

and it offers a symbolic way to represent andmanipulate data variables in the shell. They areclassified as follows

1.user-defined variables2.environment variables

3.predefined variables

• value assigned to the variable can then be referredto by preceding the variable name with a $ sign.

Shell Variables



116

• The shell provides the facility to definenormal, and environment variables.

• A normal variable can be only used in theshell where it is defined.

• An environment variable can be used inthe shell where it is defined, plus any childshells invoked from that shell.

Using Normal Variables



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To define a normal variable, use the following syntax:

• variable_name=value

Examples:

x=10

textline_1=„This line was entered by $USER‟

textline_2=“This line was entered by $USER”

allusers=`who`

usercount=`who | wc –l`

Using Normal Variables



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• Once variables are defined, one canuse the echo command to display thevalue of each variable:

$ echo $x$ echo $textline_1

$ echo $textline_2

$ echo $allusers$ echo $usercount

Using Environment Variables



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• To define an environment variable, usefollowing syntax:

variable_name=valueexport variable_name

• Examples:1.$ x=10; export x

2.$ allusers=`who` ; export allusers

Built-in environment variables



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•PATH

1.BASH_E

NV

2.HOME

3.PWD

4.SHELL5.TERM

• MAIL

1.USER

2.LOGNAME

3.PS1

4.PS2

Sample Shell Script



121

#! /bin/bash# The above line has a special meaning. It

must be the

# first line of the script. It says that thecommands in

# this shell script should be executed by the

bash# shell (/bin/bash).

# ---------------------------------------------------------

------

Executing Shell Scripts



122

There are two ways of executing a shellscript:

By passing the shell script name as an

argument to the shell. For example:

sh script1.sh

If the shell script is assigned executepermission, it can be executed using it‟sname. For example:

Passing Parameters to Scripts



123

• parameter can be passed to a shell script

• parameters are specified after the name of the

shell script when invoking the script.

• Within the shell script, parameters arereferenced using the predefined variables $1through $9.

• In case of more than 9 parameters, otherparameters can be accessed by shifting.

Built-in variables



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• Following are built-in variables supported

1.$0, $1…$9 - positional arguments

2.$* - all arguments

3.$@ - all arguments

4.$? - exit status of previous command

executed

5.$$ - PID of the current process

6.$! - PID of the last background

process




125

• Consider following shell script:

----------------------script2.sh--------------------------

echo “Total parameters entered: $#”

echo “First parameter is : $1”

echo “The parameters are: $*”

shift

echo “First parameter is : $1”

------------------------------------------------------------

• Execute the above script using the “script2.shthese are the parameters” command.

ass g a a ete s to Sc pts




126

• The shell parameters are passed as strings.

• to pass a string containing multiple words as asingle parameter, it must be enclosed within

quotes.

• For example,

$ ./script2.sh “this string is a singleparameter”

g p

Doing Arithmetic Operations



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• Arithmetic operations within a shell script can

be performed using expr command.

• Example,

x=10

y=5

number_1 = èxpr $x + $y`

number_2 = èxpr $x - $y`

number_3 = èxpr $x / $y`

number_4 = èxpr $x \* $y`

number_5 = èxpr $x % $y`

g p

Using the test Command



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The general syntax of test command is:

test expression

• The expression can be formed using acombination of shell variables and theoperators supported by the test command.These operators provide facility to comparenumbers, string and logical values, file typesand file access modes.




129

To compare two integers using test following operators are available:

-eq (equal to)

-ne (not equal to)

-lt (less than)

-le (less than or equal to)-gt (greater than)

-ge (greater than or equal to)




130

• General syntax

test expression

or[ expression ]

test integer1 operator integer2OR

[ integer1 operator integer2 ]




131

To compare two strings using the test command,following operators are available:

1.string1 = string2 (equal to, please note it is asingle =)

2.string1 != string2 (not equal to)

3.string1 (string is not NULL)4.-n string1 (string is not NULL and exists)

5.-z string1 (string is NULL and exists)




132

The syntax for this string comparison is:

test string1 operator string2

OR

[ string1 operator string2 ]

OR

test operator string

OR

[ operator string ]




133

To check a file type/access permissions

using the test command, followingoperators are available:

–-s file (file is not empty and exists) –-f file (Ordinary file and exists)

–-d file (file is a directory and exists)

–-r file (file is readable and exists)

–-w file (file is write-able and exists)

–-x file (file is executable and exists)




134

To check a file type/access permissions using

the test command, following operators areavailable:

– -b file (file is a block device and exists) – -c file (file is a character device and exists)

– -p file (file is a named pipe and exists)

– -g file (file has sticky bit set)

– -u file (file has setuid bit set)

– -t file_des (file descriptor is standardoutput)

Combining Conditions



135

• It is possible to combine conditions byusing following operators:

1.-a (logical AND operator)2.-o (logical OR operator)

3.! (logical NOT operator)

Combining Conditions



136

The syntax for this is:

test expression_1 –a expression _2,OR

[ expression _1 –a expression _2 ]

test expression_1 –o expression _2,OR

[ expression_1 –o expression_2 ]

test ! expression _1OR

[ ! expression _1 ]

Condition Checking in Scripts



137

Bash shell provides the if command to test if a condition istrue. The general format of this command is:

if condition

thencommand

fi

The condition is typically formed using the test command.

Example



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# to check if the current directory is the same as

your home directorycurdir=`pwd`

if test “$curdir” != “$HOME”

thenecho your home dir is not the same as yourpesent working directory

else

echo $HOME is your current directory

fi

Checking Multiple Conditions



139

The complex form of if statement is as follows:

if condition_1

then

commandelif condition_2

then

commandelse

command

fi

Using for Loop



140

Example:

for i in 1 2 3 4 5

doecho -n $i \* $i

= " "

echo `expr $i \* $i `

done

The Bash shell provides a for loop.

The syntax of this loop is:

for variable in listdo

command

…

commanddone

Example



141

----------------------script.sh--------------------------

#! /bin/sh

usernames=`who | cut –d “ “ –f1`

#

for user in ${usernames}do

echo $user

done-----------------------------------------------------------

-

Using while Loop



142

The Bash shell provides a while loop. The syntaxof this loop is:

while conditiondo

command

… command

done

Example



143

Shell script checks for a blank/non blankstring

eg: read nam

while [ -z “$nam” ]

do

read nam

done

echo the string is $nam

Shell script to compute factorial of a given number

Example



144

Shell script to compute factorial of a given number

#!/bin/bashn=$1if [ $n -eq 0 ]; thenfact=0

elsefact=1while [ $n –ne 0 ]do

fact=`expr $fact \* $n`

n=`expr $n – 1`done

fiecho $fact

The case Statement



145

The structure of case statement

case value in

pattern1)

command

command;;

pattern2)

command

command;;

patternn)command;;

esac

Example



146

#!/bin/bash

echo enter 2 nos

read num1

read num2

echo “enter 1 (for

addition) or 2 (for

subtraction)

read choice

(contd.)

case $choice in

1) res=`expr $num1 + $num2`

echo result is $res;;

1) res=`expr $num1 - $num2`

echo result is $res;;

*) echo invalid input;;

esac

#!/bi /b h

Example



147

#!/bin/bash

read numbercase $number

1) echo 1st

break;;

2) echo 2ndbreak;;

3) echo 3rd

break;;

*) echo ${number}th

break;;

esac

Functions



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Shell functions are a way to groupcommands for later execution using a singlename for the group. They are executed just

like a "regular" command.

Shell functions are executed in the currentshell context; no new process is created tointerpret them. Functions are declared usingthis syntax:

Functions



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• Shell functions can accept arguments

• Arguments are passed in the same way asgiven to commands

• Functions refer to arguments using $1, $2etc., similar to the way shell scripts refer tocommand line arguments

Functions



150

Function to convert standard input intoupper case

toupper(){

tr a-z A-Z

}

This function can be used as

Debugging Shell Scripts



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• Two options help in debugging shell scripts

– “-v” (verbose) option:

causes shell to print the lines of the script as theyare read.

$ bash –v script-file

– “-x” (verbose) option:

prints commands and their arguments as they areexecuted.

$ bash –x script-file

Programming in C vs Shell



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Comparison between

• A solution in C

• A shell solution written like a Cprogram

• A proper “shell/unix” solution

e.g:

The problem is to count the no of lines in afile ( the file is called the_file)

A solution in C A shell solution



153

#include <stdio.h>

void main(void){

int lcount=0;

FILE *infile;

char line[500];

infile=fopen("the_file","r");

while(!feof(infile))

{

fgets(line,500,infile);

lcount ++;}

printf("no of lines is%d\n",lcount);

}

count=0

while read linedo

count=`expr $count + 1`

done < the_file

echo Number of lines is $count

Solution using existing

commands

$ wc – l the_file

Summary



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• In this session, you have learned to:

1.Use Shell variables

2.Write scripts to process positional parameters

3.Use “test” command

4.Use “if” construct

5.Use “for” loop

6.Use “while” loop

7.Use “case” construct

8.Define and use functions

9.Debug shell scripts

UNIX Bibliography



155

UNIX in a Nutshell for BSD 4.3: A Desktop QuickReference For Berkeley (O'Reilly & Associates,Inc., 1990, ISBN 0-937175-20-X).

UNIX in a Nutshell: A Desktop Quick Reference forSystem V & Solaris 2.0 (O'Reilly & Associates,Inc., 1992, ISBN 0-56592-001-5).

The UNIX Programming Environment, Brian W.Kernighan & Rob Pike (Prentice Hall, 1984).



Thank You

Fundamentals of UNIX

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