What is a shell script in Unix ?


A shell script is a computer program designed to be run by the Unix shell, a command line interpreter. The various dialects of shell scripts are considered to be scripting languages. Typical operations performed by shell scripts include file manipulation, program execution, and printing text.



Writing your first script and getting it to work

To successfully write a shell script, you have to do three things:

  1. Write a script
  2. Give the shell permission to execute it
  3. Put it somewhere the shell can find it

Writing a script

A shell script is a file that contains ASCII text. To create a shell script, you use a text editor. A text editor is a program, like a word processor, that reads and writes ASCII text files. There are many, many text editors available for your Linux system, both for the command line environment and the GUI environment. Here is a list of some common ones:

Name Description Interface
vi, vim The granddaddy of Unix text editors, vi, is infamous for its difficult, non-intuitive command structure. On the bright side, vi is powerful, lightweight, and fast. Learning vi is a Unix rite of passage, since it is universally available on Unix/Linux systems. On most Linux distributions, an enhanced version of the traditional vi editor called vim is used. command line
emacs The true giant in the world of text editors is emacs by Richard Stallman. emacs contains (or can be made to contain) every feature ever conceived for a text editor. It should be noted that vi and emacs fans fight bitter religious wars over which is better. command line
nano nano is a free clone of the text editor supplied with the pine email program. nano is very easy to use but is very short on features. I recommend nano for first-time users who need a command line editor. command line
gedit gedit is the editor supplied with the Gnome desktop environment. graphical
kwrite kwrite is the “advanced editor” supplied with KDE. It has syntax highlighting, a helpful feature for programmers and script writers. graphical

Now, fire up your text editor and type in your first script as follows:

# My first script

echo "Hello World!"

The clever among you will have figured out how to copy and paste the text into your text editor 😉

If you have ever opened a book on programming, you would immediately recognize this as the traditional “Hello World” program. Save your file with some descriptive name. How about my_script?

The first line of the script is important. This is a special clue given to the shell indicating what program is used to interpret the script. In this case, it is /bin/bash. Other scripting languages such as perl, awk, tcl, Tk, and python can also use this mechanism.

The second line is a comment. Everything that appears after a “#” symbol is ignored by bash. As your scripts become bigger and more complicated, comments become vital. They are used by programmers to explain what is going on so that others can figure it out. The last line is the echo command. This command simply prints what it is given on the display.

Setting permissions

The next thing we have to do is give the shell permission to execute your script. This is done with the chmod command as follows:

[me@linuxbox me]$ chmod 755 my_script

The “755” will give you read, write, and execute permission. Everybody else will get only read and execute permission. If you want your script to be private (i.e., only you can read and execute), use “700” instead.

Putting it in your path

At this point, your script will run. Try this:

[me@linuxbox me]$ ./my_script

You should see “Hello World!” displayed. If you do not, see what directory you really saved your script in, go there and try again.

Before we go any further, I have to stop and talk a while about paths. When you type in the name of a command, the system does not search the entire computer to find where the program is located. That would take a long time. You have noticed that you don’t usually have to specify a complete path name to the program you want to run, the shell just seems to know.

Well, you are right. The shell does know. Here’s how: the shell maintains a list of directories where executable files (programs) are kept, and just searches the directories in that list. If it does not find the program after searching each directory in the list, it will issue the famous command not found error message.

This list of directories is called your path. You can view the list of directories with the following command:

[me@linuxbox me]$ echo $PATH

This will return a colon separated list of directories that will be searched if a specific path name is not given when a command is attempted. In our first attempt to execute your new script, we specified a pathname (“./”) to the file.

You can add directories to your path with the following command, where directory is the name of the directory you want to add:

[me@linuxbox me]$ export PATH=$PATH:directory

A better way would be to edit your .bash_profile file to include the above command. That way, it would be done automatically every time you log in.

Most modern Linux distributions encourage a practice in which each user has a specific directory for the programs he/she personally uses. This directory is called bin and is a subdirectory of your home directory. If you do not already have one, create it with the following command:

[me@linuxbox me]$ mkdir bin

Move your script into your new bin directory and you’re all set. Now you just have to type:

[me@linuxbox me]$ my_script





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