Re: Check existence of a program

["Benjamin R. Haskell" <zsh@xxxxxxxxxx>]

On Tue, 1 Feb 2011, Anonymous bin Ich wrote:

> Hello!
>
> I am having trouble checking for existence of a program.
>
> This works:
>
> % cat working.zsh
> #!/bin/zsh
> set -x
> prog="identify"
> path=$(which ${prog})
> %
> % ./working.zsh
> +./working.zsh:3> prog=identify
> +./working.zsh:4> path=+./working.zsh:4> which identify
> +./working.zsh:4> path=/usr/bin/identify
> %
>
> But this doesn't:
>
> % cat notworking.zsh
> #!/bin/zsh
> set -x
> prog="exiftime"
> path=$(which ${prog})
> if [[ ${?} -ne 0 ]]; then
>    prog="identify"
>    path=$(which ${prog})
> fi
> %
> % ./notworking.zsh
> +./notworking.zsh:3> prog=exiftime
> +./notworking.zsh:4> path=+./notworking.zsh:4> which exiftime
> +./notworking.zsh:4> path='exiftime not found'
> +./notworking.zsh:5> [[ 1 -ne 0 ]]
> +./notworking.zsh:6> prog=identify
> +./notworking.zsh:7> path=+./notworking.zsh:7> which identify
> +./notworking.zsh:7> path='identify not found'
> %
>
> Any idea?

You shouldn't use a variable named 'path', is perhaps what's going on 
here.  That one bites me occasionally: $path is an array whose contents 
are specially tied to $PATH.  So, by setting path=$(which $prog), you're 
setting your $PATH parameter, which is probably undesirable.

I tend to use this idiom for checking that a command exists:

if (( $+commands[command-name] )) ; then
	# it exists
fi

In general, (( $+param )) returns true if param is set.  $commands is an 
associative array that lists commands.  (Provided by the zsh/param 
module, IIRC.)  (( $+commands[command-name] )) returns true if the key 
'command-name' exists.

So, e.g.:

#!/bin/zsh
set -x
unset touse
for prog in exiftime identify ; do
	(( $+commands[$prog] )) || continue
	touse=$prog
	break
done
(( $+touse )) || { echo "Couldn't find a usable EXIF info program" ; return 1 }
$touse /path/to/image.jpg

--
Best,
Ben