NATIONAL INSTITUTE OF TECHNOLOGY

JAMSHEDPUR

Computer Science and Engineering Dept.

SHELL SCRIPTING

by

KUMAR SOURABH

c.s.e

241/10

INDEX:

• What is Shell

• How Shell Works

• How Shell Better

• Type of Shell

• What is Shell Script

• Why We Write Shell Script

• How We Run Shell Script

• Variable in Shell

• Problem Statement

Shell is a agency that sits between the user and the UNIX

system

Shell is an command language interpreter that read

commands from the standard input device (keyboard) or

from a file and execute them

Shell is not part of system kernel, but uses the system kernel to execute programs, create files etc.

WHAT IS SHELL?

WHAT IS SHELL ?

USER2

HOW SHELL WORKS?

Shell is a agency that sits between the user and the UNIX system

Computer understand the language of 0's and 1's called binary language. which is difficult for all of us, to read and write. So In OS there is special program called Shell. Shell issues the prompt and waits for us to enter a command

After a command is entered , the shell scans the command line for meta- character and expands abbreviation (like * in rm *)to recreate a simplified command line

It then passes command line to kernel for execution

Now the shell waits for the command to complete and normally it can’t do any work while the command is running

After command execution is complete , shell returns to its waiting role to start the next cycle . Now we are free to enter another command

However we can change this behavior and instruct not to wait

HOW SHELL WORKS?

Our

command

or

Shell script

Linux

Shell

Converted to

Binary language

by Shell

Now Linux

Kernal

Understad

Our

Request

ls

date

time BASH 00101000

10101011

01010010

LINUX

KERNEL

Example:

Linux Shell is similar to Windows command prompt

but are much more powerful.

For example:

On Linux it’s quite feasible to have multiple shells

installed, with different users able to pick the one they

prefer. :input and output can be redirected using <and >

$ ls -l > lsoutput.txt

HOW SHELL IS BETTER

Sh – simple shell

BASH – Bourne Again Shell

KSH – Korne Shell

CSH – C Shell

SSH – Secure Shell

To use a particular shell type the shell name at the

command prompt.

Eg $csh – will switch the current shell to c shell

To view the available shells in the system, type cat

/etc/shells at the command prompt

To view the current shell that is being used, type echo

$SHELL at the command prompt

TYPE OF SHELL-1

TYPE OF SHELL-2

Shell Name Developed

by Where Remark

BASH ( Bourne-

Again SHell )

Brian Fox

and Chet

Ramey

Free

Software

Foundation

Most common shell in

Linux.

CSH (C SHell) Bill Joy University of

California

The C shell's syntax and

usage are very similar to

the C programming

language.

KSH (Korn SHell) David Korn AT & T Bell

Labs

Used in Commercial

Application

Shell Script is series of command written in plain

text file.

Shell script is just like batch file in MS-DOS but

have more power than the MS-DOS batch file.

shell script is also known as shell program

WHAT IS SHELL SCRIPT

Shell script can take input from user, file and output

them on screen.

Useful to create our own commands.

Minimizes typing of repetitive command

Save lots of time.

Can schedule jobs to run in the system

To automate some task of day today life.

System Administration part can be also automated.

WHY TO WRITE SHELL SCRIPT

HOW TO WRITE SHELL SCRIPT

$ vi first

#!/bin.sh

#

# My first shell script

#

clear

echo “Time nd Tide wait 4 no one"

Explanation:

Script Command(s) Meaning

$ vi first

vi editor

#

# My first program

#

# followed by any text is considered as

comment

echo “Time nd Tide wait 4 no one” To print message or value of variables

on screen, we use echo command

Syntax:

echo "Message"

HOW TO RUN SHELL SCRIPT

Run our script as

Syntax: ./your-shell-program-name

For e.g.

$ ./first

In Linux (Shell), there are two types of variable:

System variables-: Created and maintained by Linux

itself and defined in CAPITAL LETTERS.

User defined variables (UDV)-: Created and

maintained by user. This type of variable defined in

lower letters.

VARIABLE IN SHELLS

SYSTEM VARIABLES :

System Variable Meaning

1. BASH=/bin/bash Our shell name

2. HOME=/home/kumar Our home directory

3. PWD=/home/students/Common Our current working directory

4. USERNAME=kumar User name who is currently login to PC

5. SHELL=/bin/bash Our shell name

TEST OPERATORS

Test Works in 3 Ways:

o Numeric comparison

o String comparison

o File tests

EXAMPLE OF NUMERIC COMPARISON

$ a=5; b=7; c=7.2

$test $a –eq $b ; echo $?

1

$test $a -lt $b ; echo $?

0

$test $c –gt $b ; echo $?

1

$test $c –eq $b ; echo $?

0

EXAMPLE OF FILE TESTS

$ ls –l emp.txt

-rw-rw-rw- …

$ [ -f emp.txt ] ; echo $?

0

$ [ -x emp.txt ] ; echo $?

1

$ [ -d emp.txt ] || echo “file is not a directory”

file is not a directory

STRING TEST

Test

True if

S1=s2 String s1=s2

S1!=s2 String s1 is not equal

to s2

-n stg String stg is not null

string

-z stg String stg is null

string

Conditions (Case)

case expr in Option1) stmt ;; Option2) stmt ;; *) stmt ;; esac Every option should be terminated with a double semicolon. Denotes default case Case should be termniated with esac

ARRAY

Initialising an array A[0] = 10 A[1] = Hi

Using an array ${A[0]} : retrieves 10

Here arrays can contain data belonging to different data types Uninitialised index in arrays will have null value by default

FUNCTION

Local Function Written at the command prompt Lasts for the current session alone

Global Function Written in .bashrc file Available as long as the definition is there in .bashrc file

Function in shell script Available with in the file alone

ALIAS

Alias – Alternate name for an entity Entity here refers to command We can give another name or alias name for a command either at the command prompt or in the .bashrc file. The former will be temporary and will vanish if the session ends and the latter will be permanent as long as the definition exists in the .bashrc file

Alias and Unalias

alias newname=oldname

Eg alias copy=cp

Then we can use copy in the same way we use cp

command

Eg copy file1 file2 //copies content of file1 to file2

To remove alias use unalias command

unalias copy

After this we cannot use copy to perform copying

function

export : EXPORTING SHELL VARIABLES

Reference:

1. A Practical Guide to Ubuntu Linux

by MARK G SOBEL

2. Linux Foundation Organisation

Thank you!!