Comment by homebrewer

It's a waste of time unless you're specifically targeting and testing mac, all of the BSDs, various descendants of Solaris, and other flavors of Unix. I wrote enough "portable shell" to run into so many quirks and slight differences in flags, in how different tools handle e.g. SIGPIPE.

Adding a new feature in a straightforward way often makes it work only on 4/7 of the operating systems you're trying to support. You then rewrite it in a slightly different way (because it's shell — there's always 50 ways to do the same thing). This gets you to 5/7 working systems, but breaks one that previously worked. You rewrite it yet another way, fixing the new breakage, but another one breaks. Repeat this over and over again, trying to find an implementation that works everywhere, or start adding workarounds for each system. Spend an hour on a feature that should have taken two minutes.

If it's anything remotely complicated, and you need portability, then use perl/python/go.