Writing and Executing a Shell Script

Why should you write and use shell scripts? Shell scripts can save you time and typing, especially if you routinely use the same command lines multiple times every day. Although you could also use the history function (press the Up or Down keys while using bash or use the history command), a shell script can add flexibility with command-line argument substitution and built-in help.

Although a shell script won't execute faster than a program written in a computer language such as C, a shell program can be smaller in size than a compiled program. The shell program does not require any additional library support other than the shell or, if used, existing commands installed on your system. The process of creating and testing shell scripts is also generally simpler and faster than the development process for equivalent C language commands.

Note

Hundreds of commands included with Ubuntu are actually shell scripts, and many other good shell script examples are available over the Interneta quick search will yield numerous links to online tutorials and scripting guides from fellow Linux users and developers. For example, the startx command, used to start an X Window session from the text console, is a shell script used every day by most users. To learn more about shell scripting with bash, see the Advanced Bash-Scripting Guide, listed in the "Reference" section at the end of this chapter. You'll also find Sams Teach Yourself Shell Programming in 24 Hours a helpful guide to learning more about using the shell to build your own commands.

When you are learning to write and execute your first shell scripts, start with scripts for simple, but useful tasks. Begin with short examples, and then expand the scripts as you build on your experience and knowledge. Make liberal use of comments (lines preceded with a pound # sign) to document each section of your script. Include an author statement and overview of the script as additional help, along with a creation date or version number. Write shell scripts using a text editor such as vi because it does not automatically wrap lines of text. Line wrapping can break script syntax and cause problems. If you use the nano editor, include its -w flag to disable line wrap.

In this section, you learn how to write a simple shell script to set up a number of aliases (command synonyms) whenever you log on. Instead of typing all the aliases every time you log on, you can put them in a file by using a text editor, such as vi, and then execute the file. Normally these changes are saved in systemwide shell configuration files under the /etc directory to make the changes active for all users or in your .bashrc, .cshrc (if you use tcsh), or .bash_profile files in your home directory.

Here is what is contained in myenv, a sample shell script created for this purpose (for bash):

#!/bin/sh alias ll='ls -l' alias ldir='ls -aF' alias copy='cp'

This simple script creates command aliases, or convenient shorthand forms of commands, for the ls and cp commands. The ll alias provides a long directory listing: The ldir alias is the ls command, but prints indicators (for directories or executable files) in listings. The copy alias is the same as the cp command. You can experiment and add your own options or create aliases of other commands with options you frequently use.

You can execute myenv in a variety of ways under Linux. As shown in this example, you can make myenv executable by using the chmod command and then execute it as you would any other native Linux command:

This line turns on the executable permission of myenv, which can be checked with the ls command and its -l option like this:

-rwxrwxr-x 1 winky winky 11 Aug 26 17:38 myenv

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