Linux in a Nutshell, Fifth Edition

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Chapter 6: The Bash Shell and Korn Shell

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

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 directo-

ries 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 ...