command, builtin, and enable

Before moving on to the last part of the command-line processing cycle, we'll take a look at the command lookup order that we touched on in Chapter 4 and how it can be altered with several shell built-ins.

The default order for command lookup is functions, followed by built-ins, with scripts and executables last. There are three built-ins that you can use to override this order: command, builtin, and enable.

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command removes alias and function lookup.[9] Only built-ins and commands found in the search path are executed. This is useful if you want to create functions that have the same name as a shell built-in or a command in the search path and you need to call the original command from the function. For instance, we might want to create a function called cd that replaces the standard cd command with one that does some fancy things and then executes the built-in cd:

cd ( )
{
    #Some fancy things
    command cd
}

In this case we avoid plunging the function into a recursive loop by placing command in front of cd. This ensures that the built-in cd is called and not the function.

command has some options, listed in Table 7-8.

Table 7-8. command options

Option

Description

-p

Uses a default value for PATH

-v

Prints the command or pathname used to invoke the command

-V

A more verbose description than with -v

-

Turns off further option checking

The -p option is a default path which guarantees that the command lookup will find all of the standard UNIX utilities. In this case, command will ignore the directories in your PATH.[10]

builtin is very similar to command but is more restrictive. It looks up only built-in commands, ignoring functions and commands found in PATH. We could have replaced command with builtin in the cd example above.

The last command enables and disables shell built-ins—it is called enable. Disabling a built-in allows a shell script or executable of the same name to be run without giving a full pathname. Consider the problem many beginning UNIX shell programmers have when they name a script test. Much to their surprise, executing test usually results in nothing, because the shell is executing the built-in test, rather than the shell script. Disabling the built-in with enable overcomes this.[11]

Table 7-9 lists the options available with enable.[12] Some options are for working with dynamically loadable built-ins. See Appendix C for details on these options, and how to create and load your own built-in commands.

Table 7-9. enable options

Option

Description

-a

Displays every built-in and whether it is enabled or not

-d

Deletes a built-in loaded with -f

-f filename

Loads a new built-in from the shared-object filename

-n

Disables a built-in or displays a list of disabled built-ins

-p

Displays a list of all of the built-ins

-s

Restricts the output to POSIX "special" built-ins

Of these options, -n is the most useful; it is used to disable a built-in. enable without an option enables a built-in. More than one built-in can be given as arguments to enable, so enable -n pushd popd dirs would disable the pushd, popd, and dirs built-ins.[13]

You can find out what built-ins are currently enabled and disabled by using the command on its own, or with the -p option; enable or enable -p will list all enabled built-ins, and enable -n will list all disabled built-ins. To get a complete list with their current status, you can use enable -a.

The -s option restricts the output to POSIX `special' built-ins. These are :, ., source, break, continue, eval, exec, exit, export, readonly, return, set, shift, trap, and unset.