What is an alias? It isn't the ability to call yourself Thaddeus Jones when
your real name is Jedediah Curry. Instead, in a Linux-context it is the
ability to use a different
name for a command. In principle, personal aliases can be anything you want.
They are special names that you define to accomplish tasks. They aren't
shell scripts, as a shell script is external to your shell.
To start up a shell script, type in its name. The system then starts a shell
as a child process of your current shell to run the script.
Aliases, too, are
started by typing them in. However, they are internal to the shell
uses aliases). That is, they are internal to your shell process.
Instead of starting a sub-shell,
executes the alias
has the obvious advantage of being quicker, as there is no overhead of starting
the new shell
or searching the hard disk.
Another major advantage is the ability to create new commands. You
can do this with shell scripts (which we will get into later), but the overhead
of creating a new process does not make it worthwhile for simple tasks. Aliases
can be created with multiple commands strung together. For example, I created an
alias, t, that shows me the time. Although the date command does that, all I
want to see is the time. So, I created an alias,
t, like this:
alias t=`date | cut -c12-16`
When I type in t, I get the hours and minutes, just
exactly the way I want.
Aliases can be defined in either the .profile, .login or
the .cshrc, depending on your shell.
However, as I described above, if you want them for all sub-shells,
they need to go in .cshrc. If you are running a Bourne Shell, aliasing may be
the first good reason to switch to another shell.
Be careful when
creating aliases or functions so that you don't redefine existing commands.
Either you end up forgetting the alias,
or some other program uses the original
program and fails because the alias
gets called first. I once had a call from a
customer with a system in which he could no longer install software. We tried
replacing several programs on his system, but to no avail. Fortunately, he had
another copy of the same product, but it, too, died with the same error. It
didn't seem likely that it was bad media. At this point, I had been with him for
almost an hour, so I decided to hand it off to someone else (often, a fresh
perspective is all that is needed).
About an hour later, one of the other
engineers came into my cubicle with the same problem. He couldn't come up with
anything either, which relieved me, so he decided that he needed to research the
issue. Well, he found the exact same message in the source code and it turned
out that this message appeared when a command could not run the sort command.
Ah, a corrupt sort binary.
Nope! Not that easy. What else was there? As it
turned out, the customer had created an alias
called sort that he used to sort
directories in a particular fashion. Because the Linux command couldn't work with
this version of sort, it died.
Why use one over the other? Well, if
there is something that can be done with a short shell script, then it can be
done with a function. However, there are things that are difficult to do with an
alias. One thing is making long, relatively complicated commands. Although you
can do this with an alias,
it is much simpler and easier to read if you do it
with a function. I will go into some more detail about shell
functions later in
the section on shell
scripting. You can also find more details in the bash
On some systems, you will find that they have already provide a number of
aliases for you. To see what alias are currently configured, just run
alias with no options and you might get something
alias +='pushd .'
alias ..='cd ..'
alias ...='cd ../..'
alias beep='echo -en "\007"'
alias dir='ls -l'
alias l='ls -alF'
alias la='ls -la'
alias ll='ls -l'
alias ls='ls $LS_OPTIONS'
alias ls-l='ls -l'
alias md='mkdir -p'
alias rehash='hash -r'
alias unmount='echo "Error: Try the command: umount" 1>&2; false'
alias which='type -p'
alias you='yast2 online_update'
As you can see there are many different ways you can use aliases.