Fish shell 2.1

I thought too that when I first started using fish, and in fact I was going back to zsh daily at first, then once or twice a week, and then I realised it had been months!

You can still write your scripts in POSIX shells - the main pain point in existing scripts is those that expect to be sourced in. Popular tools such as virtualenv do now have fish equivalents, though.

One useful tip is that many programs expect $SHELL to be POSIX-compliant (or even bash compliant), so I generally leave my login shell as zsh and then exec fish from my .zshrc.