alias new=original

(Notice that there are no spaces on either side of the equal sign (=); this is required syntax.) The alias command defines new to be an alias for original; whenever you type new, the Korn shell substitutes original internally. (You cannot use any of the shell's special characters, such as *, $, =, and so on, in alias names.)

There are a few basic ways to use an alias. The first, and simplest, is as a more mnemonic name for an existing command. Many commonly used Unix commands have names that are poor mnemonics and therefore are excellent candidates for aliasing; the classic example is:

alias search=grep

search Fred phonelist

Another popular alias eschews exit in favor of a more widely used command for ending a login session:

alias logout=exit

alias logout='. ~/.ksh_logout; exit'

You might want the file .ksh_logout to "clean up" your history files, as we discussed in the last chapter. Recall that we created history files with names like .hist.42, which guarantees a unique name for every serial line or window. To remove these files when the shells exit, just put this line in your .ksh_logout file:

rm ~/.hist.*

alias emcas=emacs
alias mali=mail
alias gerp=grep

This can be handy, but we feel you're probably better off suffering with the error message and getting the correct spelling under your fingers. Another common way to use an alias is as a shorthand for a longer command string. For example, you may have a directory to which you need to go often. It's buried deeply in your directory hierarchy, so you want to set up an alias that will allow you to cd there without typing (or even remembering) the entire pathname:

alias cdcm='cd ~/work/projects/devtools/windows/confman'

As before, the quotes around the full cd command are needed, because the string being aliased has more than one word.

alias lf='ls -F'

A few things about aliases are important to remember. First, the Korn shell makes a textual substitution of the alias for that which it is aliasing; it may help to imagine ksh passing your command through a text editor or word processor and issuing a "change" or "substitute" command before interpreting and executing it.

alias printall='pr * | lp'

alias pa=printall

Recursive aliasing makes it possible to set up an "infinite loop" of definitions, wherein an alias ends up (perhaps after several lookups) being defined as itself. For example, the command:

alias ls='ls -l'

sets up a possible infinite loop. Luckily, the shell has a mechanism to guard against such dangers. The above command works as expected (typing ls produces a long list with permissions, sizes, owners, etc.). Even more pathological situations work, such as these:

alias listfile=ls
alias ls=listfile

Aliases can only be used for the beginning of a command string -- albeit with certain exceptions. In the cd example above, you might want to define an alias for the directory name alone, not for the entire command. But if you define:

alias cm=work/projects/devtools/windows/confman

and then type cd cm, the Korn shell will probably print a message like ksh: cd: cm: [No such file or directory].

An obscure, rather ugly feature of the Korn shell's alias facility -- one not present in the analogous C shell feature -- provides a way around this problem. If the value of an alias (the right side of the equal sign) ends in a space or a tab, then the Korn shell tries to do alias substitution on the next word on the command line. To make the value of an alias end in a space, you need to surround it with quotes.

This feature exists so that it is possible to have aliases for commands that themselves run other commands, such as nohup and nice. For example, nohup is aliased to 'nohup '. That way, when you type:

nohup my_favorite_alias somefile