[aeadio]

Bash and Zsh are both just about as old as one another. Zsh came out less than a year after Bash.

Both derive most of their improvements from ksh, which is why they look superficially similar and many scripts are cross-compatible. Zsh arguably took the language improvements much further than Bash, ie look at [1] and [2] in https://news.ycombinator.com/item?id=39642678.

Bash made the understandable but very regrettable decision of being only a superset of POSIX sh, which means it carries along all the footguns, like word splitting by default.

Zsh is also eminently flexible and scriptable, with many core features (like most line editor functionality) exposed as hooks that custom scripts can extend or change.

Zsh is also notably faster than Bash.

It's one of the great disappointments of open source history that Bash largely took off as the default shell everywhere and the default shell scripting language, despite Zsh being superior in just about every way.