Unix in a Nutshell, 4th Edition

Command Execution

When you type a command to Bash or ksh93, they look in the following places until they find a match:

1. Keywords such as if and for.

2. Aliases. You can’t define an alias whose name is a shell keyword, but you can define an alias that expands to a keyword, e.g., alias aslongas=while. (Bash, when not in POSIX mode, does allow you to define an alias for a shell keyword.)

3. Special built-ins like break and continue. The list of POSIX special built-ins is . (dot), :, break, continue, eval, exec, exit, export, readonly, return, set, shift, times, trap, and unset. The Korn shell adds alias, login, typeset, and unalias, while Bash adds source.

4. Functions. When not in POSIX mode, Bash finds functions before built-in commands.

5. Nonspecial built-ins like cd and test.

6. Scripts and executable programs, for which the shell searches in the directories listed in the PATH environment variable.

The distinction between “special” built-in commands and nonspecial ones comes from POSIX. This distinction, combined with the command command, makes it possible to write functions that override shell built-ins, such as cd. For example:

    cd () {                       Shell function; found before built-in cd
        command cd "$@"           Use real cd to change directory
        echo now in $PWD          Other stuff we want to do
    }

In ksh88, the search order is different, all built-ins are found before shell functions. Thus you have to do more work to override a built-in command with a function. You do so using a combination of functions and aliases:

    _cd () {                      Shell function; note leading underscore
        cd "$@"                   Use real cd to change directory
        echo now in $PWD          Other stuff we want to do
    }
    alias cd=_cdAlias found first