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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 22:12:09 +0300
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Reply to message =ABCheck existence of a program=BB,=20
sent 21:29:11 01 February 2011, Tuesday
by Anonymous bin Ich:

    prog=3Dexiftime
    path=3D=3D$prog
    if [[ $? -ne 0 ]] ; then
        prog=3Didentify
        path=3D=3D$prog
    endif

It works because zsh takes first =3D as assignment operator and expands sec=
ond=20
`=3D$prog' construct into a full path. If `=3D$prog' expansion fails, it th=
rows an=20
exception, exception prevents variable from being set and thus last express=
ion=20
fails what is indicated in $?.

If you use $(cmd) construct, then though `cmd' fails, $(cmd) that does not =
care=20
about exit code just expands into output of `cmd', so last expression (whic=
h is=20
variable assignment, NOT `cmd') does not fail. Also note that `=3D$prog' do=
es not=20
produce new fork.

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 [[ ${?} -ne 0 ]]; then
>      prog=3D"identify"
>      path=3D$(which ${prog})
> 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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