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The shell maintains a list of aliases
that may be set and unset with the alias and
unalias builtin commands.
The first word of each command, if unquoted,
is checked to see if it has an
alias. If so, that word is replaced by the text of the alias.
The alias name and the replacement text may contain any valid
shell input, including shell metacharacters, with the exception
that the alias name may not contain =.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time. This means that one may alias
ls to "ls -F",
for instance, and Bash does not try to recursively expand the
replacement text. If the last character of the alias value is a
space or tab character, then the next command word following the
alias is also checked for alias expansion.
Aliases are created and listed with the alias
command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text,
as in csh.
If arguments are needed, a shell function should be used.
Aliases are not expanded when the shell is not interactive.
The rules concerning the definition and use of aliases are
somewhat confusing. 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.
This means that 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 the 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.
Note that for almost every purpose, aliases are superseded by shell functions.
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