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LinuxCommand

Learning the shell

Writing shell scripts

Script library

SuperMan pages

Who, What, Where, Why

Editing the scripts you already have

by William Shotts. Jr.

Before we get to writing new scripts, I want to point out that you have some scripts of your own already. These scripts were put into your home directory when your account was created and are used to configure the behavior of your sessions on the computer. You can edit these scripts to change things.

In this lesson, we will look at a couple of these scripts and learn a few important new concepts about the shell.

Commands, commands everywhere

Up to now, we really have not discussed exactly what commands are. Commands can be several different things. Some commands are built into the shell itself. That is, the shell automatically understands a few commands on its own. The commands cd and pwd are in this group. Commands implemented in the shell itself are called shell builtins. To see a list of the commands built into bash, use the help command.

The second type of commands are the executable programs. Most commands are in this group. Executable programs are all the files in the directories included in your path.

The last two groups of commands are contained in your runtime environment. During your session, the system is holding a number of facts about the world in its memory. This information is called the environment. The environment contains such things as your path, your user name, the name of the file where your mail is delivered and much more. You can see a complete list of what is in your environment with the set command.

The two types of comamnds contained in the environment are aliases and shell functions.

Aliases

Now, before you become too confused about what I just said, let's make an alias. Make sure you are in your home directory. Now, using your favorite text editor, open the file .bash_profile and add this line to the end of the file:

alias l='ls -l'

The .bash_profile file is a shell script that is executed each time you log in. By adding the alias command to the file, we have created a new command called "l" which will perform "ls -l". To try out your new command, log out and log back in. Using this technique, you can create any number of custom commands for yourself. Here is another one for you to try:

alias today='date +"%A, %B %-d, %Y"'

This alias creates a new command called "today" that will display today's date with nice formatting.

By the way, the alias command is just another shell builtin. You can create your aliases directly at the command prompt, however they will only remain in effect during your current shell session. For example:

[me@linuxbox me]$ alias l='ls -l'

Shell functions

Aliases are good for very simple commands, but if you want to create something more complex, you should try shell functions. Shell functions can be thought of as "scripts within scripts" or little sub-scripts. Let's try one. Open .bash_profile with your text editor again and replace the alias for "today" with the following:

function today {
	echo "Today's date is:"
	date +"%A, %B %-d, %Y"
}

Believe it or not, function is a shell builtin too, and as with alias, you can enter shell functions directly at the command prompt.

[me@linuxbox me]$ function today {
> echo "Today's date is:"
> date +"%A, %B %-d, %Y"
> }
[me@linuxbox me]$

type

Since there are many types of commands, it can become confusing to tell what is an alias, a shell function or executable file. To determine what a command is, use the type command. type will display what type of command it is. It can be used as follows:

[me@linuxbox me]$ type command

.bashrc

Though placing your aliases and shell functions in your .bash_profile will work, it is not considered good form. There is a separate file named .bashrc that is intended to be used for such things. You may notice a piece of code near the beginning of your .bash_profile that looks something like this:

if [ -f ~/.bashrc ]; then
	. ~/.bashrc
fi

This script fragment checks to see if there is a .bashrc file in your home directory. If one is found, then the script will read its contents. If this code is in your .bash_profile, you should edit the .bashrc file and put your aliases and shell functions there.

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