Fix stray stderr output on machines with GNU which

[phinze]

On a NixOS machine I was getting a message like the following on every Rails boot:

which: no bun in (... [my path] ...)

I chased it down to this library's usage of which to find the mjml binary. Using backticks to call it meant stderr was passing through to the terminal.

Ubuntu and MacOS use different versions of which that do not print to stderr, so it's likely not many people have run into this issue.

[phinze]

Here's a fun history lesson I stumbled on while researching this issue:

3BSD first implemented 'which': https://minnie.tuhs.org/cgi-bin/utree.pl?file=3BSD/usr/man/man1/which.1 (1979). This was csh-specific and I think it later became built-into csh.

FreeBSD 2.1 introduced a new non-built-in implementation: https://www.freebsd.org/cgi/man.cgi?query=which&apropos=0&sektion=0&manpath=FreeBSD+2.1.0-RELEASE&arch=default&format=html (1995). This was written in Perl and then rewritten in C a few years later.

Debian's implementation started out in 1995 or 1996 as a shell script calling 'type', and remains a shell script.

Red Hat has yet another implementation: https://www.freebsd.org/cgi/man.cgi?query=which&apropos=0&sektion=0&manpath=CentOS+7.1&arch=default&format=html, https://carlowood.github.io/which/ (1999 or earlier). This is written in C.

So 'which' has a very long history in csh, which was the default interactive shell in many versions of Unix. Its availability to other shells dates back to the 90s (at least) but is not portable due to there being multiple very different implementations.

Via https://groups.google.com/g/linux.debian.devel/c/1h7vtg5ty0U/m/ZX-fU2daAQAJ

[sighmon]

@phinze Thanks so much Paul! Really loved the history of which which is which.

p.s. I'd love to hear more about how your transition to climate-related work is going.