If you care to, I believe this could be simplified to echo ${conf} > config.h directly instead of creating an intermediate.

I thought so too but somehow a lot of other packages are doing it the same way. Take abduco or st for example. Any good reason for this?

Could be that toFile handles large text chunks better. Other than that I fail to see the benefit of an intermediate here. Could be lack of imagination on my part, ofc :)

Also, I guess toFile is guaranteed to be "dumber" and so may be preferred.

I've played around with echo but -- regardless of all the different kinds of quoting I tried -- somehow it didn't work out. Shouldn't the expression just be as follows?

preBuild = optionalString (conf!=null) "echo ${conf} > config.def.h";

conf itself was originally read by readFile in my overlays which should be a string. I guess there is a reason why toFile is preferred.

What about something like

optionalString (conf != null) ''echo "${conf}" > config.h''

I suppose a heredoc would also work, in cases where echo wouldn't. Anyway, if you prefer an intermediate, that's fine.

I've tried that and did some debugging. Looks like both echo ${conf} and echo "${conf}" are not correctly escaping the given config which contains characters like ">\. Only a heredoc solves this. Given these problems I'll keep it as it is.