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From: ZyX <zyx.vim@gmail.com>
Reply-To: zyx.vim@gmail.com
To: zsh-workers@zsh.org
Subject: Re: Check existence of a program
Date: Tue, 1 Feb 2011 23:21:02 +0300
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References: <4D4850F7.6060205@gmail.com> <201102012212.10494@-zyx>
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Reply to message =C2=ABRe: Check existence of a program=C2=BB,=20
sent 22:12:09 01 February 2011, Tuesday
by ZyX:

Sorry, I should have read your question more correctly (and test $(cmd) var=
iant=20
better). $(cmd) actually propagates exit code, so the reason is overriding =
of=20
$path as others have mentioned.

Original message:
> Reply to message =C2=ABCheck existence of a program=C2=BB,
> sent 21:29:11 01 February 2011, Tuesday
> by Anonymous bin Ich:
>=20
>     prog=3Dexiftime
>     path=3D=3D$prog
>     if [[ $? -ne 0 ]] ; then
>         prog=3Didentify
>         path=3D=3D$prog
>     endif
>=20
> It works because zsh takes first =3D as assignment operator and expands
> second `=3D$prog' construct into a full path. If `=3D$prog' expansion fai=
ls,
> it throws an exception, exception prevents variable from being set and
> thus last expression fails what is indicated in $?.
>=20
> If you use $(cmd) construct, then though `cmd' fails, $(cmd) that does not
> care about exit code just expands into output of `cmd', so last expression
> (which is variable assignment, NOT `cmd') does not fail. Also note that
> `=3D$prog' does not produce new fork.
>=20
> Original message:
> > Hello!
> >=20
> > I am having trouble checking for existence of a program.
> >=20
> > This works:
> >=20
> > % cat working.zsh
> > #!/bin/zsh
> > set -x
> > prog=3D"identify"
> > path=3D$(which ${prog})
> > %
> > % ./working.zsh
> > +./working.zsh:3> prog=3Didentify
> > +./working.zsh:4> path=3D+./working.zsh:4> which identify
> > +./working.zsh:4> path=3D/usr/bin/identify
> > %
> >=20
> > But this doesn't:
> >=20
> > % cat notworking.zsh
> > #!/bin/zsh
> > set -x
> > prog=3D"exiftime"
> > path=3D$(which ${prog})
> > if [[ ${?} -ne 0 ]]; then
> >=20
> >      prog=3D"identify"
> >      path=3D$(which ${prog})
> >=20
> > fi
> > %
> > % ./notworking.zsh
> > +./notworking.zsh:3> prog=3Dexiftime
> > +./notworking.zsh:4> path=3D+./notworking.zsh:4> which exiftime
> > +./notworking.zsh:4> path=3D'exiftime not found'
> > +./notworking.zsh:5> [[ 1 -ne 0 ]]
> > +./notworking.zsh:6> prog=3Didentify
> > +./notworking.zsh:7> path=3D+./notworking.zsh:7> which identify
> > +./notworking.zsh:7> path=3D'identify not found'
> > %
> >=20
> > Any idea?

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