bash Pocket Reference

Command Execution

When you type a command, Bash looks in the following places until it finds 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. When not in POSIX mode, Bash 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. Bash adds source.

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

5. Nonspecial built-ins such as cd and test.

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

7. When a command is not found, if a function named command_not_found_handle exists, the shell will call it, passing the command words as the function arguments.

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
}

If Bash exits due to receiving SIGHUP, or if the huponexit shell option is set, Bash will send a SIGHUP to all running child jobs. Use disown -h to prevent ...