Sunday, April 13, 2014

How to write shell script

How to write shell script



Following steps are required to write shell script:
(1) Use any editor like vi or mcedit to write shell script.
(2) After writing shell script set execute permission for your script as follows
syntax: 
chmod permission your-script-name
Examples:
$ chmod +x your-script-name
$ chmod 755 your-script-name

Note: This will set read write execute(7) permission for owner, for group and other permission is read and execute only(5).
(3) Execute your script as
syntax: 
bash your-script-name
sh your-script-name
./your-script-name
Examples:
$ bash bar
$ sh bar
$ ./bar

NOTE In the last syntax ./ means current directory, But only . (dot) means execute given command file in current shell without starting the new copy of shell, The syntax for . (dot) command is as follows
Syntax:
. command-name
Example:
$ . foo
Now you are ready to write first shell script that will print "Knowledge is Power" on screen. See the common vi command list , if you are new to vi.
$ vi first
#
# My first shell script
#
clear
echo "Knowledge is Power"
After saving the above script, you can run the script as follows:
$ ./first
This will not run script since we have not set execute permission for our script first; to do this type command
$ chmod 755 first
$ ./first

First screen will be clear, then Knowledge is Power is printed on screen.
Script Command(s)
Meaning
$ vi firstStart vi editor
#
# My first shell script
#
# followed by any text is considered as comment. Comment gives more information about script, logical explanation about shell script.
Syntax:
# comment-text
clearclear the screen
echo "Knowledge is Power"To print message or value of variables on screen, we use echo command, general form of echo command is as follows
syntax: 
echo "Message"
 How Shell Locates the file (My own bin directory to execute script)
Tip: For shell script file try to give file extension such as .sh, which can be easily identified by you as shell script.
Exercise:
1)Write following shell script, save it, execute it and note down the it's output.
$ vi ginfo
#
#
# Script to print user information who currently login , current date & time
#
clear
echo "Hello $USER"
echo "Today is \c ";date
echo "Number of user login : \c" ; who | wc -l
echo "Calendar"
cal
exit 0
Future Point: At the end why statement exit 0 is used? See exit status for more information.

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