Alias substitution


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Alias substitution
The shell maintains a list of aliases which can be set, unset and printed by
the alias and unalias commands. After a command line is parsed
into simple commands (see Commands) the first word of each command,
left-to-right, is checked to see if it has an alias. If so, the first word is
replaced by the alias. If the alias contains a history reference, it undergoes
History substitution (q.v.) as though the original command were the
previous input line. If the alias does not contain a history reference, the
argument list is left untouched.

Thus if the alias for `ls' were `ls -l' the command `ls /usr' would become `ls
-l /usr', the argument list here being undisturbed. If the alias for `lookup'
were `grep !^ /etc/passwd' then `lookup bill' would become `grep bill
/etc/passwd'. Aliases can be used to introduce parser metasyntax. For
example, `alias print 'pr \!* | lpr'' defines a ``command'' (`print') which
pr(1)s its arguments to the line printer.

Alias substitution is repeated until the first word of the command has no
alias. If an alias substitution does not change the first word (as in the
previous example) it is flagged to prevent a loop. Other loops are detected and
cause an error.

Some aliases are referred to by the shell; see Special aliases.

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