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From: Greg Wooledge <wooledg@eeg.ccf.org>
Newsgroups: gnu.bash.bug
Subject: Re: Worth mentioning in documentation
Date: Mon, 10 Aug 2015 08:19:25 -0400
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On Mon, Aug 10, 2015 at 10:18:52AM +0200, Juanma wrote:
> > [ is an ordinary command (a "shell builtin")
> 
> Here is another point I find confusing: I thought a "shell builtin" didn't
> have a separate binary executable file, like 'cd' (which cd => fail), but
> some of them do have such form (which [ => /usr/bin/[ ; which pwd =>
> /bin/pwd). I also fail to see how 'test' modifies the state of the shell
> itself (like 'cd' does), or why it is "impossible or inconvenient to obtain
> [its functionality] with separate utilities".

Don't use which(1).  Which is an external program, so it has no knowledge
of the shell's builtins, aliases, functions and keywords.  Instead, use
type.

imadev:~$ type cd
cd is a shell builtin
imadev:~$ type [[
[[ is a shell keyword
imadev:~$ type -a test
test is a shell builtin
test is /usr/bin/test
test is /bin/test

Bash implements test as a builtin not because it's necessary, but simply
for efficiency.  Forking a whole process to test whether two strings are
equal would be horribly wasteful.