A shell script is a computer program which is interpreted by an operating system shell.

Scipts are used to automate procedures that could be manually performed from the command line. They can potentially save a huge amount of time by eliminating repetitive commands. For example, if you're going to compile and test a program 100x, and each compilation and test cycle requires 25 steps (commands), you're looking at performing 2500 steps. It's much more efficient to create a script containing those 25 steps and run it as needed – in fact, you can even set things up so those commands execute automatically as soon as you save a new version of your program.

1. Create a text file containing shell commands. Use any text editor (nano, vi, VS Code, gnome-text-editor, eclipse, …) to create the file.

2. Tell the operating system which shell to use. Add a “shbang” line to the very top of the file, containing the text:

#!/usr/bin/bash

The first two chacters, the sharp (#) and bang (!) give this line its name. They are recognized by the operating system kernel as identifying a script. The remainder of this line is interpreted by the kernel as the name of the shell which is to be used to interpret the script. In this case, /usr/bin/bash is the absolute path of the bash shell. You can substitute other interpreters to write scripts in other shell dialects (such as the Z-shell, /usr/bin/zsh) or languages (such as python, /usr/bin/python).

Note that there must be nothing in font of the #! characters – no space and no blank lines.

3. Ensure that the script has appropriate permissions. The kernel requires execute [x] permission, and the shell requires read [r] permission. Set this up with the chmod command (for example, chmod u+rx scriptname).

Here is a simple example script using two commands, echo and date:

#!/usr/bin/bash
echo "The current date and time is:"
date

Notice the presence of the shbang line.

If this is save into the file named “now”, the permission could be set with this command:

$ chmod u+rx now

The script can then be executed. Normally, the current working directory is not searched, so to run the a script in the current directory, you will need to explicitly specify the directory name like this:

$ ./now
The current date and time is:
Sat Mar  6 12:03:32 EST 2038