If the first word of a command is a defined alias, it is replaced by the text of the alias. An alias name consists of any number of characters excluding metacharacters, quoting characters, file expansion characters, parameter and command substitution characters, and the equal sign =. The replacement string can contain any valid POSIX shell script including the metacharacters listed above. The first word of each command in the replaced text will in turn be tested for aliases. If the last character of the alias value is a blank, the word following the alias will also be checked for alias substitution.
Aliases can be used to redefine built-in commands but cannot be used to redefine the reserved words listed above. Aliases can be created, exported and listed on standard output with the alias command and can be removed with the unalias command. Exported aliases remain in effect for scripts invoked by name, but must be reinitialized for separate invocations of the POSIX shell.
Aliasing is performed when scripts are read, not while they are being executed. Thus for an alias to take effect the alias definition command has to be executed before the command which references the alias is read.
Aliases are frequently used as a short hand for full path names. An option to the built-in alias command allows the value of the alias to be automatically set to the full path name of the corresponding command. These aliases are called tracked aliases. The value of a tracked alias is defined the first time the corresponding command is looked up and becomes undefined each time the PATH variable is reset. These aliases remain tracked so that the next reference will redefine the value. Several tracked aliases are compiled into the POSIX shell. The -h option of the built-in set command makes each referenced command name into a tracked alias.
The following exported aliases are compiled into the POSIX shell but can be unset or redefined with unalias:
autoload='typeset -fu' false='let 0' functions='typeset -f' hash='alias -t' history='fc -l' integer='typeset -i' nohup='nohup ' r='fc -e -' true=':' type='whence -v'
Example
The following aliases could be defined to construct l, ll and lf:
alias l='/bin/ls -m' alias ll='/bin/ls -l' alias lf='/bin/ls -CF'