shell script

                                       SHELL SCRIPTING

SHELL: -shell is command line interpreter. It takes commands from user and executes them. it is an interface between user and kernel.  The three most widely used UNIX shells are Bourne shell .kom shell and c shell.

Shell	Developed by	Shell Prompt	Execution Command
Bash shell	Denis Richie	$	sh
Bourne shell	Steven courne	$	Sh
Korn shell	David korn	$	Ksh
C shell	Bill joy, california university student	%	Csh

      Each shell has merits and demerits of its own. Moreover the shell scripts written for one shell may not work with the other shell. This is because different shells use different shells use different mechanism to execute the commands in the shell script. Bourne shell since it is one of the most widely used UNIX shells in existence today.

      Almost all UNIX implementations offer the Bourne shell as part of their standard configuration. It is smaller than the other two shells and therefore more efficient for most shell processing. However it lacks features offered by the c and korn shell.

      All shell programs written for the Boume shell are likely to work with the korn shell, the reverse however may not be true, this is so since the facilities like arrays, command aliasing and history mechanism available in the korn shell are not supported by the bourne shell.

The c shell programming language resembles the c language and is quite different from the language of the bourne shell, only the very basic shell scripts will run under both the c and bourne shell; a vast majority will not shell keeps track of commands as you enter them (history) and allows you to go back and execute them again without typing the command. Or, if you want to, you can recall them, make modifications, and then execute the new command.

chsh

Shell program:-

     A shell program is nothing but a series of such commands, instead of specifying one job at a time, we give the shell a to -do list —a program — that carries out an entire procedure. such programs are known as “shell scripts”.

When to use shell scripts:

1. Customizing your work  environments, for example, every time you log in if                                                  you want to see the current date, a welcome message and the list of users who have logged in you can write a shell script for the same.
2. automating your daily tasks, for example, you a may want to back up all your programs at the end of day, this can be done using a shell script.
3. Automating repetitive tasks. f xth1e, the repetitive task of compiling a c program, linking it with some libraries and executing the executable code can be a shell script.
4. Executing important system procedures like shutting down the system, formatting a disk, creating a file system, mounting the file system, letting the user the floppy and finally unmounting the disk.
5. Performing same operation on many files. Or example, you may want to rep lace a string with a string myprintf in all the c programs present in a directory

Shell Variables:-

           Variable is, a data name and it is used to store value. Variable value can change during execution of the program.

Variables in UNIX are two types.

1. Unix-defined variables or system variables

2. User defined variables

Unix-defined variables:

          These are standard variables which are always accessible the shell provides the values for these variables these variable are usually used by the system itself and govern the environment we work under. If we so desire we can change the values of these variables as per our preferences and customize the system environment.

          The list of all system variables and their values can be displayed by

Saying at the $prompt,

$set

HOME=/usr/sv

HZ=100

IFS=

LOGNAME=sv

MAII=usr/spool/mail/sv

MAILCHECK=600

OPTINd=l

PATH=/bin :Iusr/b in:/usr/sv:/bin:.

PS1=$

SHELL=/bin/sh

TERM=vt100

TZ=IST-5:30

Variable	Meaning
PS1	Primary shell prompt
PS2	The system prompt 2, default value is “>”
PATH	Defines the path which the shell must search in order to execute any command or file
HOME	Stores the de fault working directory of the user
LOGMAN	Stores the login name of  the user
MAIL	Defines the file where the mail of the user is stored
MAILCHECK	Defines the duration after which the shell checks whether the user has received any mail. By default its value is 600(seconds)
IFS	Defines the name of your default working shell
SHELL	Defines the name of your default working shell
TERM	Define the name of the terminal zone in which you are working
TZ	Defines the name of the Zone in which we are working

User Defined Variables

 These are defined by user and are used most extensively in shell programming.

Rules for creating User Defined Shell Variables: -

1.   The first character of a variable name should be alphabet or underscore.

2.   No commas or blanks are allowed within a variable name

3.   Variables names should be of any reasonable length

4.   Variable names are case sensitive, that isName, nAme, name are all different variable names.

5.   Variable name shouldn’t be a reserve word

Shell Keywords:-

Keywords are the Words whose meaning has already been explained to the shell. The keywords are .also called as “Reserve Words”.

The lists of keywords available in Bourne shell are

Echo if Read else Set fi unset while Readonly do Shift Done Export For

Until trap case wait esac eval break exec continue ulimit Exit Umask return

1.Echo

             echo command is used to display the messages on the screen and is used to display the value stored in a shell variable.

Eg 1: $echo “Multics is a training institute”

            Multics is a training institute

Note: double quotes are option in echo statement.

Eg 2: $echo “today date is:`date`”

            Today date is sat mar 4 o4:40:i0 1ST 2005

Note: the UNIX command should be in back quotes in echo statement

Otherwise it treats, as text.

Eg 3: $echo “my file has we —I file lines

               My file has 10 lines

Eg 4: $echo my log name is:`logname`.

             My logname is Sv

Eg 5: $echo “my present working directory is :`pwd` ”

                My present working directory is :/usr/sv/abc

Shell variables

Eg 1: $a=10

Note: there are no predefined data types in UNIX. Each and every thing it treats as character. Each character occupies 1 byte of memory.

Eg 2:$b2000

In above example a occupies 2 bytes and b occupies 4 bytes of memory

Reading of variable

               $ is the operator to read variable value

Eg 1: $n=l00

          $echo $n

             100

Eg 2: $name=”Multics”

          $echo $name

              Multics

$echo welcome to $name

Welcome to Multics

Eg 3: $now=`date`

          $$now

           sat mar 4 o4:40:10 1ST 2005

Eg 4: $mypath=/usr/sv/abc/a 1 /a2

           $cd $mypath

now it changes to a2 directory, to check say at $ promt pwd command $pwd

/usr/sv/abc/a 1 /a2

Null Variables

 A variable which has been defined but has not been given any value is known as a null variable. A null variable can be created in any of the following ways.

1.$n=””

2.$n=”

3.$n=

$echo n

on echoing a null variable, only a blank line appears on the screen.

Constant:

Constant is a fixed value. It doesn’t change during execution of  the  program

$a=20

$readonly  a

When the variable are made readonly, the shell doe not allow us to change their values, so a value can read but can‘t change.

Note: If we want the shell to forget about a variable altogether, we use the unset command.

$unset  a

on issuing the above command the variable a and with it the value assigned to it are erased from the shell’s memory.

SHELL SAMPLE PROGRAMS CLICK HERE