Monday, April 11, 2011

Basic Shell Scripting-step by step

Basic Shell Scripting-step by step

If you are not a beginner go to summery page

What is a script?

Group of commands in one file for a specific or a number of actions to achieve a result

Shell script structure

1. Select your editor

You must have a text editor to write shell programs

Select an editor in which you are comfortable

Most common editors are Vi and Emacs

2. Structure of a script

a)First line

“#!/path/to/shell”

b) Comments

Purpose of script, author, created date, modifications, bug fix notes etc

c) Body of script

Commands to be executed

This line will decide which shell to be used for executing the commands in script

If the line is not present, system will use default shell

E.g.:- 1

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

#!/bin/bash

echo “Hello world”

E.g. Explained

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

The above script will execute

echo “Hello world”

in bash shell

3. Execute a script

If you logged in as root, the home directory will be /root/

We will create a file in /root using vi editor

[root@localhost]# vi hello.sh

The files have an extension of “.sh” so that others can easily identify the file as script

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Note: - The “file” is a command used to identify, what kind of file it is.

Suppose we have file with name “abcd” How can I say it is a shell script or an executable binary or else another type..?

Here we will use “file” command

Eg:-

[root@localhost]# file abcd

ASCII Long line text

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We are in vi now (I’m going ahead with the assumption that you have basic knowledge of VI editor)

Type the text mentioned in e.g.: 1

#!/bin/bash

echo “Hello world”

Save the file

Now we have created a” bash shell script” named as “file.sh”

To execute a file that need execute permission

So give execute permission to “file.sh” using chmod command

[root@localhost]# chmod 755 file.sh


Now we can execute the script as

[root@localhost#]./file.sh

Hello world

[root@localhost#]

Summary

1. Create a script that will print “Hello world”

[root@localhost#]

[root@localhost#]

[root@localhost#]vi file.sh

#!/bin/bash

echo “Hello world”

[root@localhost#]chmod 755 file.sh

[root@localhost#]./file.sh

Hello world

[root@localhost#]

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