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 what you are doing. enable -a will list all built-ins and their enabled or disabled status.

One problem with built-in commands is that you generally can’t use a -h or --help option to get usage reminders, and if a manpage exists it’s often just a pointer to the large bash manpage. That’s where the help command, which is itself a built-in, comes in handy. help displays help about shell built-ins.

# help help
help: help [-s] [pattern ...]
    Display helpful information about builtin commands. If PATTERN is
    specified, gives detailed help on all commands matching PATTERN,
    otherwise a list of the builtins is printed. The -s option
    restricts the output for each builtin command matching PATTERN to
    a short usage synopsis.

When you need to redefine a built-in you use the builtin command to avoid loops. For example:

cd () {
    builtin cd "$@"
    echo "$OLDPWD --> $PWD"
}

To force the use of an external command instead of any function or built-in that would otherwise have precedence, use enable -n, which turns off shell built-ins, or command, which ignores shell functions. For example, to use the test found in $PATH instead of the shell built-in version, type enable -n test and then run test. Or, use command ls to use the native ls command rather than any ls function you may have created.

See Also

• man which

• help help

• help builtin

• help command

• help enable

• help type

• Naming Your Script Test

• “Built-in Shell Variables” in Appendix A