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 } } } |
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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.
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