Re: m4 newbie q: ifelse not operating based on the value of sysval

[Eric Blake]

On Sat, Nov 19, 2022 at 09:42:07PM -0500, David Niklas wrote:
> Hello,
> 
> I know I could do this with bash, awk, and probably several other
> languages, I wanted to use m4.
> 
> I was trying to do a basic "If the file is there then include it,"
> directive with m4.
> 
> echo 'define(`mkv'\'', `ogg'\'')ifelse(`1'\'', syscmd(`test -f
> f'\'')sysval, `-i'\'' f `-map 1'\'')dnl' | m4 -D f="$i" -

You are testing whether 'f' exists, rather than whether the filename
contained in macro f exists.  You probably want your m4 script to say
something like:

syscmd(`test -f 'defn(`f'))

plus the extra quoting needed to write ' in shell.

> 
> As a bit of a bonus question, can I somehow tack a period onto the front
> of the mkv macro expansion? That way, instead of matching "mkv", it would
> match ".mkv".

If you are asking "can m4 treat '.mkv' as a macro name", the answer is
no for m4 1.4 unless it was built with --enable-changeword (which
penalizes speed performance).  But if you are asking if you can write
an m4 macro that inspects whether its argument includes the substring
'.mkv' and if so does something, then yes.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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