3.5.2. Creating and removing aliases

Aliases are created using the alias shell built-in. For permanent use, enter the alias in one of your shell initialization files; if you just enter the alias on the command line, it is only recognized within the current shell.

franky ~> alias dh='df -h'

franky ~> dh
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda7             1.3G  272M 1018M  22% /
/dev/hda1             121M  9.4M  105M   9% /boot
/dev/hda2              13G  8.7G  3.7G  70% /home
/dev/hda3              13G  5.3G  7.1G  43% /opt
none                  243M     0  243M   0% /dev/shm
/dev/hda6             3.9G  3.2G  572M  85% /usr
/dev/hda5             5.2G  4.3G  725M  86% /var

franky ~> unalias dh

franky ~> dh
bash: dh: command not found

franky ~>

Bash always reads at least one complete line of input before executing any of the commands on that line. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The commands following the alias definition on that line are not affected by the new alias. This behavior is also an issue when functions are executed. Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a compound command. As a consequence, aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a separate line, and do not use alias in compound commands.

Aliases are not inherited by child processes. Bourne shell (sh) does not recognize aliases.

More about functions is in Chapter 11.

	Functions are faster
	Aliases are looked up after functions and thus resolving is slower. While aliases are easier to understand, shell functions are preferred over aliases for almost every purpose.

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