What Are Shell Scripts?
In the simplest terms, a shell script is a file
containing a series of commands. The shell reads this file and
carries out the commands as though they have been entered directly on
the command line.
The shell is somewhat unique, in that it is both a
powerful command line interface to the system and a scripting
language interpreter. As we will see, most of the things that can be
done on the command line can be done in scripts, and most of the
things that can be done in scripts can be done on the command line.
Writing
Your First Script And Getting It To Work
To successfully write a shell script, you have to do three things:- Write a script
- Give the shell permission to execute it
- 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:
vi
nano
gedit
emacs
kwrite
Now, fire up your text editor and type
in your first script as follows:
#!/bin/bash
# My first script
echo "Hello World!"
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. Eg: 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:
$ 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:$ ./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.
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:
$
echo $PATH
You can add directories
to your path with the following command, where directory is
the name of the directory you want to add:
$
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:
$
mkdir bin
Move your script into
your new bin directory and you're all set.
Now you just have to type:
$
hello_world
Note:You
can also simply type the script as sequence of commands and save it
with some name hello_world
#my
first script
echo
“Hello World”
Then
execute the script with command $bash hello_world
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