Re: How to detect bash?

[mwoehlke]

Stephane Chazelas wrote:
> On Tue, Oct 10, 2006 at 05:12:07PM -0500, mwoehlke wrote:
> > Dave Rutherford wrote:
> > > On 10/10/06, mwoehlke <mwoehlke@tibco.com> wrote:
> > > > Anyone have any clever, VERY reliable tricks for detecting if the
> > > > current shell is bash?
> > > Well, I don't know if it's clever, but how about:
> > Oh, my... Where do I *start*?
> >
> > > $ if [ "${SHELL//*/bash}" = "bash" ]; then echo y; fi
> > $ echo $SHELL
> > /bin/sh
> > $ echo $BASH
> > /bin/bash
> > $ foo
> > bash: foo: command not found
> >
> > There is *ABSOLUTELY* no guarantee that $SHELL correctly points to bash, 
> > or that $SHELL is even remotely correct for that matter. This is /worse/ 
> > than relying on $BASH.
> >
> > But it does bring up an interesting possibility:
> > [ "`/dev/null 2>&1`" = "bash: /dev/null: Permission denied" ]
> [...]
>
> You quest looks a bit pointless to me. What prevents the user to
> edit a copy of your script to remove the check anyway?
>
> $ zsh -c 'echo "`/dev/null 2>&1`"' bash
> bash: /dev/null: Permission denied
>
> $ zsh
> $ ARGV0=bash ash -c 'echo "`/dev/null 2>&1`"; echo $BASH'
> bash: /dev/null: Permission denied

Eh? I get:

$ zsh -c 'echo "`/dev/null 2>&1`"' bash
zsh:1: permission denied: /dev/null

$ ARGV0=bash ash -c 'echo "`/dev/null 2>&1`"; echo $BASH'
/dev/null: permission denied

So neither of your counter-examples is working for me (although both 
look like they *should*; go figure). But since you didn't counter 
BASH_SUBSHELL (and since I'm too lazy to change it now) I guess I'll 
stick with that. :-)

Clearly the exact output depends on the version/flavor of the shell, 
meaning that relying on verbatim output like this might not work (I 
didn't test against any 2.x bash's). Whereas BASH_SUBSHELL ought to be 
safer w.r.t. false negatives.

This is a fascinating discussion though; thanks (to Dave R., also) for 
the input!

> $ echo '/dev/null(){echo "bash: /dev/null: Permission denied"}' \
> >   >> ~/.zshenv
> $ zsh -c 'echo "`/dev/null 2>&1`"'
> bash: /dev/null: Permission denied
>
> And whatever check you do can be worked around in a way or
> another.

True, but the main point of the exercise is to go with a check that's 
unlikely to be worked around "by accident". If someone intentionally 
circumvents the check (and you're right, editing the script would be 
easy), well then they deserve whatever happens. But I *am* paranoid 
enough to not trust that $BASH is never set - other than by bash - for 
some reason. Or that it hasn't been *unset* (since that seems to kill it 
forever), because we have 'clean environment' scripts that would do this 
sort of thing.

--
Matthew
KDE: Desktop Excellence