Command Aliases in Unix shells

One of the really useful features almost every Unix shell has is support for command aliases – a way to run a command or a series of Unix commands using a shorter name you get associated with such commands.

An example of a command alias in Unix shell

Here’s one of the most useful aliases I have for Solaris systems:

solaris$ alias ls='/usr/local/gnu/bin/ls --color -F'

What is allows me to do is to simply type “ls” instead of the really long command line it refers to: /usr/local/gnu/bin/ls –color -F.

You see, the ls command which is shipped with Solaris, doesn’t have many options of the more up-to-date GNU ls command, and working with many Linux systems I quite like some of them like color highlighting of different directory objects – files, directories and executables.

Try typing the longer command a few times in a row, and compare it to the “ls” to get the idea of how much of a productivity gain one command alias can be!

Why you should use command aliases

Like many other things in Unix, aliases are a way to become more productive. The general rule of thumb is this: if you have to run some command something more than once every day – consider creating an alias for it. These are just a few cases where it makes sense to employ them:

if you repeatedly check whether some files exist or get updated
if you’re monitoring a certain aspect of your OS and you get the values using grep command
if you’re connecting to the same hosts using rsh or ssh

All of these and many more examples are greatly simplified if you alias them to some shorter commands.

Creating new aliases in bash

Setting up a new alias is quite easy, the syntax for alias command is very straightforward. Let’s say I want to automate the confirmation of swap usage based on a free command in Linux:

redhat$ free
total       used       free     shared    buffers     cached
Mem:       2075156     945712    1129444          0     177292     503416
-/+ buffers/cache:     265004    1810152
Swap:      2040244          0    2040244

The result I’m after is this command:

redhat$ free | grep Swap
Swap:      2040244          0    2040244

And here’s how I can create an alias called “swp” which refers to this series of commands:

redhat$ alias swp='free | grep Swap'

Once you execute this command, you can start using swp as a Unix command:

redhat$ swp
Swap:      2040244          0    2040244

Important: such a creation of new aliases is going to be only active for your current Unix shell and sub-shells you may spawn. To make your alias permanent, you’ll have to update one of your initialization scripts like. For Linux and bash, you should add the same alias command to your .bashrc file.

Removing aliases in Unix

In very much the same way, you can use the unalias command to get rid of a certain alias. The nature of this command is such that you’ll most likely use it when creating and debugging new aliases. It’s unlikely that you’ll need to use it in your initialization scripts.

Following the example above, here’s how to get rid of the swp alias and verify that it’s gone:

redhat$ unalias swp
redhat$ swp
bash: swp: command not found

How to list your current aliases

If you run the alias command without any parameters, you’ll be shown a full list of aliases currently configured for your user account, here’s an example from one of my systems:

l.      ls -d .* --color=tty
ll      ls -l --color=tty
ls      ls --color=tty
vi      vim