Writing shell scripts - Lesson 1: Writing your first script and getting it to work

Writing shell scripts - Lesson 1: 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-like 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:

#!/bin/bash
# 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
hello_world?



The first line of the script is important. This
is a special clue, called a shebang, 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 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 its arguments 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 hello_world

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]$ ./hello_world

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 only
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 or .profile file (depending on your distribution)
to include the above
command. That way, it would be done automatically
every time you log in.



Most 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]$ hello_world

and your script will run. On some distributions, most notably
Ubuntu, you will need to open a new terminal session before your
newly created bin directory will be recognised.