Article 8.5 gives an general overview of what the C shell does as it evaluates a command line. bash does something similar. This article takes a closer look at how you can control one part of those steps: whether bash will choose a shell function, a built-in command, or an external command. (If you're interested in a detailed and humorous look at the way this is handled in the C shell, read article 8.12 .)
Let's say that you want to write shell functions named cd , pushd , and popd . They will run the shell's built-in cd , pushd , or popd command, respectively. Next they execute another shell function named setvars to do some setup in the new directory:
"$@" |
cd() { pushd() { popd() { cd "$@" pushd "$@" popd "$@" setvars setvars setvars } } } |
---|
But which
cd
will
bash
use when you type
cd
: the built-in
cd
or your
cd
function? (Same question for
pushd
and
popd
.) Worse, what if the
cd <">$@<">
command inside the function makes
bash
call your
cd
function again, and that starts an endless loop? Well, that actually
will
start a loop - and you need to know how to prevent it.
Typing command before the name of a command disables shell function lookup. bash will only execute a built-in command or an external command with that name. So, you could keep the functions from re-executing themselves by defining them this way:
cd() { pushd() { popd() { command cd "$@" command pushd "$@" command popd "$@" setvars setvars setvars } } }
In the same way, if you don't want to run your new pushd function for some reason, here's how to use the built-in pushd once:
bash$command pushd
somewhere
The command command still allows bash to run an external command (from your PATH ( 6.4 ) ) with the name you give. To force bash to use a built-in command - but not a shell function or an external command - type builtin before the command name. Although bash will always choose a built-in command before an external command, you can specify the built-in echo unambiguously with:
builtin echo -n 'What next? '
What if you want the external echo command? The easiest way is probably by typing its absolute pathname. For example, when I was revising article 8.20 , I wanted to test the four (!) different external versions of echo on a System V machine - and not get the built-in bash version. So I typed commands like this:
bash$/bin/echo hi \\ there
Finally, you can enable or disable specific built-in bash commands with the enable command. Unlike command and builtin , the effect of enable lasts until you exit the shell. The command enable -n disables one or more built-in commands; give the command names as arguments. For example, in my experiments for article 8.20 , I could have made sure that I'd get an external echo every time by typing this first command once:
bash$enable -n echo
bash$type echo
echo is hashed (/bin/echo)
The
bash
type
command confirms that I'll now be using the external
echo
. You can re-enable a disabled built-in with
enable
command-name
. And
enable -a
lists the status of all
bash
built-ins.
-