Aliases

One can define an alias for a command that is used often (and might be more complicated to type).
We will describe how to do it by considering an example.

NOTE: what is said below is correct for the csh and tcsh shells. For others some of the details and files could be different.

As an example, assume we want to remove the various auxiliary files produced by TeX or LaTeX. The name of these files end in .dvi, .aux, .log, etc. Thus, we can do

rm *.dvi *.aux *.log

Since this is somewhat cumbersome, we could define an "abbreviation" for it:

alias clean 'rm *.dvi *.aux *.log'

respectively, for the bash shell

alias clean='rm *.dvi *.aux *.log'

Next time we can type clean to invoke the same command.

If we want to check what the meaning of clean is, we can use

alias clean

To remove this alias, do

unalias clean

To see all the aliases that are defined, use

alias

There are two inconveniences with this approach:

the alias is "known" only in the shell in which we defined it;
the alias will be lost when we exit this shell (e.g., when we log-out).

To have a permanent alias, we should put it in a file devoted to this purpose. For the csh shell, this file is ~/.cshrc. For the tcsh shell, this file is ~/.tcshrc, or ~/.cshrc provided ~/.tcshrc does not exist (see the man page of tcsh, "Startup and shutdown" for more details).
For the bash shell, the file is ~/.bashrc.

This file is read once at the start of each shell.
Hence, changes made after the shell is running are ignored. However, you can source the corresponding file to have the changes take effect.

WARNING: These files contain important information for the shell. Be sure you do not alter that part!

This is a sample ~/.cshrc (or ~/.tcshrc), containing the alias clean as well.
The file has to end with a RETURN character.
Comment lines begin with #.

.... some other stuff

# Aliases 

alias cp	'cp -i'
alias rm	'rm -i'
alias mv	'mv -i'
alias clean     'rm -i *.dvi *.log *.aux *.toc'

(The meaning of -i in these commands is to ask for confirmation before deleting or overwriting a file.)

A few technical details:

Aliases can be nested. That is, an alias definition can contain the name of another alias.
(Thus, if rm is already aliased to rm -i, it is not necessary to use the -i argument in the definition of clean. This argument will take effect only if rm is unaliased.)
Except for the first word, the name of the alias may not appear in its definition, nor in any alias referred to by its definition. Such loops are detected, and cause an error message.
One can pass to an alias arguments, using !*, which stands for "all arguments of the previous command" (note that we have to escape it with the backslash so that it is not executed when the alias is defined). Here are some examples:

alias lm 'ls -l \!* | more
This pipes the output of ls -l into more. All the arguments of lm are passed to ls.
alias cd 'cd \!* ; dirs'
After cd is executed, it echoes the current directory.