Configure Command Line Aliases in Bash

(11.24.09 – This post was edited to expand the discussion on the various files read by Bash for login and sub-shells — iceflatline)

This post will describe how to set up command line aliases in Free Software Foundation’s “Bourne Again Shell” (BASH) for reducing common input mistakes and improving efficiency at the terminal.

Let’s take the simple case of a command like ls –a, which prints a list of the current directory’s files, including the hidden ones. Let’s change that command to something perhaps more efficient:

Nice. Now we need only to type lsa to get the same results. Not enough of an improvement over simply typing ls –a? Let’s take another example. Say we use something like the following command to routinely download a specific directory from a web site: wget -nH -r -l inf ftp://ftp.somewebsite.com/directory1/directory2/. Sure we could try to remember a string like this each time we need it and debug if mistakes are made, but let’s make it easier:

You get the idea; I’m sure you can think of other examples. The point here is that each of these aliases will start to add up after awhile to help save you time and reduce mistakes.

Now, let’s say we open up a terminal and enter a few aliases like ones above. What happens when we close and reopen the terminal? Unfortunately aliases entered like this won’t carry over to the next bash shell session. To accomplish that we need to set up something more permanent.

There are three files in your home directory that hold a special meaning to Bash, allowing you to set up your environment automatically when you log in, when you start another Bash shell, and when you log out. These files may or may not exist depending on the Linux distribution you’re using. If they’re missing, Bash defaults to /etc/profile. These files are:

.bash_profile: read by Bash when you log into the system

.bashrc: read by Bash when you start a sub-shell

.bash_logout: read by Bash when a login shell exits

Bash allows two replacements for .bash_profile: .bash_login and .profile. However, only one of these files is read when you log in. If .bash_profile isn’t there, Bash will look for .bash_login. If that file is missing, it will look for .profile. If you start a sub-shell (e,g, a new terminal or X windows), Bash will read commands from .bashrc. Most users, however, want to have the same commands run regardless of whether it is a login shell or a sub-shell. This is typically done by including a small script (similar to the one for .myaliases below) or the line source .bashrc within .bash_profile to execute .bashrc. All the commands, including aliases, are then placed in .bashrc.

Using a text editor you can add your own aliases directly to .bashrc or, in some cases, simply uncomment ones that the distribution may have added for you (Ubuntu for example does this). Another approach is to create your own file containing your aliases and then simply point to that file from within .bashrc. This might be handy if you want to easily carry your aliases from one system to another. To do this, first create the file:

Now, open the file in your favorite editor and enter your aliases and save it. Here’s a list of the ones I often use:

Now, open .bashrc and add or uncomment the following script, making sure to modify the file name to match the file you created for your aliases; then save the file.

Now reboot the system or simply issue the following command to have bash recognize the aliases:

That’s it. Your new aliases should be ready to go. You can get a list of your aliases at anytime using the command alias without any arguments, and you can temporarily “unalias” any alias you’ve set up with the command unalias aliasname. However, this will only last for as long as the shell session does. To permanently eliminate the alias you’ll need to delete it from your alias file.

But what if we want to have these aliases available when we log in or run a session as root, or another user? To do that we need to add our list of aliases to a file that is accessible to bash on a system-wide bases. In the case of a Fedora or a Fedora-based distribution, that file is most likely going to be /etc/bashrc, and the case of Ubuntu or other debian based distribution that file is likely to be /etc/bash.bashrc. Place your aliases at the end of those files and restart the system or use the command exec bash and they will be available regardless of which user you are.

References

Newham, C., and Bill Rosenblatt. Learning the bash Shell. 2nd ed. Sebastopol, CA, USA: O’Reilly, 1998. Print.