zsh was the default sh until OS X 10.2 or 10.3. However, the zsh version was quite out of date (3.x when 4.x was already mature/stable, IIRC). A horse race be

zsh was the default sh until OS X 10.2 or 10.3. However, the zsh version was quite out of date (3.x when 4.x was already mature/stable, IIRC). A horse race between the old zsh and the latest bash was jiggered to show bash as much faster, and therefore worth switching to, never mind that the then-current zsh demonstrated performance parity with bash. Source: I was tasked with the performance comparison and (despite my objections) the conversion to bash. On the bright side, I learned a lot about zsh in the process and have used it as my shell rather than bash ever since.