Writing Shell Scripts

Shell scripts are a convenient way of doing all sorts of tasks: collecting data, searching for a word or phrase in a text and many other useful things. The following example shows a small shell script that prints a text:

Example 20.1. A Shell Script Printing a Text

#!/bin/sh 
# Output the following line: 
echo "Hello World"

	The first line begins with the Shebang characters (`#!`) which is an indicator that this file is a script. The script is executed with the specified interpreter after the Shebang, in this case /bin/sh.
	The second line is a comment beginning with the hash sign. It is recommended to comment difficult lines to remember what they do.
	The third line uses the built-in command echo to print the corresponding text.

Before you can run this script you need some prerequisites:

Every script should contain a Shebang line (this is already the case with our example above.) If a script does not have this line, you have to call the interpreter manually.
You can save the script wherever you want. However, it is a good idea to save it in a directory where the shell can find it. The search path is determined by the environment variable PATH. Usually a normal user does not have write access to /usr/bin. Therefore it is recommended to save your scripts in the directory ~/bin/. The above example gets the name hello.sh.
The script needs executable permissions. Set the permissions with the following command:
```
chmod +x ~/bin/hello.sh
```

If you have fullfilled all of the above prerequisites, you can execute the script with either ~/bin/hello.sh or just hello.sh. The first call uses an absolute path whereas the second one searches for the command in each directory given by the PATH environment variable.