bash Cookbook

Using or Replacing Built-ins and External Commands

Problem

You want to replace a built-in command with your own function or external command, and you need to know exactly what your script is executing (e.g., /bin/echo or the built-in echo). Or you’ve created a new command and it may be conflicting with an existing external or built-in command.

Solution

Use the type and which commands to see if a given command exists and whether it is built-in or external.

# type cd
cd is a shell builtin

# type awk
awk is /bin/awk

# which cd
/usr/bin/which: no cd in (/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/
sbin:/usr/bin/X11:/usr/X11R6/bin:/root/bin)

# which awk
/bin/awk

Discussion

A built-in command is just that; it is built into the shell itself, while an external command is an external file launched by the shell. The external file may be a binary, or it may be a shell script itself, and its important to understand the difference for a couple of reasons. First, when you are using a given version of a particular shell, built-ins will always be available but external programs may or may not be installed on a particular system. Second, if you give one of your own programs the same name as a built-in, you will be very confused about the results since the built-in will always take precedence (see Naming Your Script Test). It is possible to use the enable command to turn built-in commands off and on, though we strongly recommend against doing so unless you are absolutely sure you understand ...