When you want to save yourself from typing an unwieldy command over and over again you can create and use an alias for it. It will then act as a shortcut to the larger command, which you can type and run instead.
Creating aliases in UNIX (and Linux) is done with a simple alias command which follows this format: alias name=’command you want to run’.
Replace the “name” with your shortcut command, and “command you want to run” with the larger command you want to create an alias of. Here’s a simple example:
alias accesslog='tail -f /var/log/lighttpd/access.log'
In this example I’ve effectively created a new accesslog command which is an alias of the tail -f /var/log/lighttpd/access.log command. What it does is follow the access.log file and display new entries in it as they happen. Now instead of having to write the whole tail -f command every time I want to look at what’s happening in the access.log file I can simply run the accesslog alias command instead, which is pretty nifty.
What if I want to unset the alias once I no longer need it or wish to set a new better alias? Well, simply run:
unalias accesslog
Quite logical. Now the accesslog alias no longer exists.
One thing to keep in mind though is that aliases that are set this way get lost the moment you close the command line session, or in other words, they are temporary. If you want to save aliases permanently you will have to edit the bash configuration file, which is usually .bashrc or .bash_profile residing in your user home directory. You can edit whichever you prefer, or whichever exists on your system.
To edit .bashrc just open it in a command line text editor such as nano, or any other you might prefer, and add the same exact alias command as in the above example at the bottom of it, or find where other aliases are already set and add yours after them.
nano .bashrc
Once you add your aliases save the file, which in the nano editor is done by pressing the Сtrl-x keyboard shortcut, answering “y” when asked to save, and hitting enter.
Now your alias is saved permanently, and it will therefore work even after you close the session and come back. Of course, to remove the permanent alias just edit the file again and remove the line you’ve just added. If it’s still set run the unalias command as shown above and it will be gone.
Note that aliases are set for the currently active user. So you have to edit the .bashrc file in the home directory of that user. If you’re logged in as root that would be /root/.bashrc, and if you’re logged in as joe, for example, it will be in /home/joe/.bashrc. If you try to run root’s alias while acting as joe or vice versa you’ll get a “command not found” error.
Also note that aliases added to .bashrc aren’t active immediately after you save the file since that file is read on user’s login. If you log out and log back in then it will work.
Finally, once you have a bunch of aliases set up you might want to check up on which aliases are available. To do that just run the alias command by itself:
alias
And it will list something like this:
alias accesslog='tail -f /var/log/lighttpd/access.log'
alias ls='ls --color=auto'
The list represents all of the aliases that have been set in .bashrc, or on the command line during the current session. In the above example we see my accesslog alias, and another one for the ls command associating it with the ls –color=auto command, which simply adds some coloring to our ls lists.
That brings us to the final point worth a mention, as demonstrated by the above ls alias, and that is that you can alias an already existing real command. For example if we have a nmon command installed, which shows various system activity information, we can actually turn it into an alias for the top command, which also shows system activity.
You probably don’t want to do this, or at least, you don’t want to keep this alias, but for the sake of demonstration:
alias nmon='top'
And now when you run nmon, instead of opening the actual nmon program it will open top. In other words the alias is masking the original command.
This serves as a word of caution when it comes to setting names of aliases; try to avoid setting names that match existing commands. Chances are you’ll want those commands doing what they’re supposed to do, except in special cases like the above ls alias, which simply aliases to its own coloring options.
And that’s how aliases work in UNIX (and Linux).
