I've been trying to stick with bash for at least 20 years (I did have a brief tcsh phase before that) since a time when sticking with it meant having to install it on every unix system I touched. However, now that MacOS defaults to zsh, I'm starting to wonder if I should summon up the effort to change again.
I think the most noticeable difference is that `$foo` doesn't word-split unless you do `${=foo}`. Beyond that, I don't think one would notice they were using zsh instead of bash. In most ways, zsh is pretty much a superset of bash.
personally, I don't have interest in zsh because bash is the default shell in Linux, which is the UNIX system I use (I find mac's unix to be really a weird variant). Mac switching to zsh is just more evidence they're diverting from the mainstream.
I know what the reason is. I don't care. I use GPLv3 and have no interest in an OS that doesn't. You can run bash on windows or compile it yourself on any platform. microsoft even distributes it with windows.
IIRC, MacOS had to ship with a fairly old version of bash by default because newer versions have a more copyleft license; I always assumed that their swap to zsh was because they wanted something both up to date and compatible in terms of the license (and the fact that most bash "just works" when you run it in zsh is a plus).
