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A shell script is a sequence of commands for which you have a repeated use. This sequence is typically executed by entering the name of the script on the command line. Alternatively, you can use scripts to automate tasks using the cron facility. Another use for scripts is in the UNIX boot and shutdown procedure, where operation of daemons and services are defined in init scripts.

To create a shell script, open a new empty file in your editor. Any text editor will do: vim, emacs, gedit, dtpad et cetera are all valid. You might want to chose a more advanced editor like vim or emacs, however, because these can be configured to recognize shell and Bash syntax and can be a great help in preventing those errors that beginners frequently make, such as forgetting brackets and semi-colons.

Put UNIX commands in the new empty file, like you would enter them on the command line, commands can be shell functions, shell built-ins, UNIX commands and other scripts.

Give your script a sensible name that gives a hint about what the script does. Make sure that your script name does not conflict with existing commands. In order to ensure that no confusion can rise, script names often end in .sh; even so, there might be other scripts on your system with the same name as the one you chose. Check using which, whereis and other commands for finding information about programs and files:

which -a script_name

whereis script_name

locate script_name

In this example we use the echo Bash built-in to inform the user about what is going to happen, before the task that will create the output is executed. It is strongly advised to inform users about what a script is doing, in order to prevent them from becoming nervous because the script is not doing anything or you can be funny and tell them something crazy is happening!

The script should have execute permissions for the correct owners in order to be runnable. When setting permissions, check that you really obtained the permissions that you want. When this is done, the script can run like any other command:

chmod u+x script1.sh

If you did not put the scripts directory in your PATH, and . (the current directory) is not in the PATH either, you can activate the script like this:

./script_name.sh

A script can also explicitly be executed by a given shell, but generally we only do this if we want to obtain special behavior, such as checking if the script works with another shell or printing traces for debugging:

rbash script_name.sh

sh script_name.sh

bash -x script_name.sh

The specified shell will start as a subshell of your current shell and execute the script. This is done when you want the script to start up with specific options or under specific conditions which are not specified in the script.

If you don’t want to start a new shell but execute the script in the current shell, you source it:

source script_name.sh

	source = .
	The Bash source built-in is a synonym for the Bourne shell . (dot) command.

The script does not need execute permission in this case. Commands are executed in the current shell context, so any changes made to your environment will be visible when the script finishes execution: