Bun Shell implements a cross-platform bash-like shell, using builtins for popular commands like `rm`, `cd`, `which`, `ls` etc which support the same flags on Linux, macOS, and Windows. A lot of why we made it was for `bun install` to work on Windows without needing polyfill packages like `rimraf` or `cross-env` in package.json scripts.

zx uses the system-provided shell, which means it isn't cross-platform

For the commands Bun Shell implements as builtins, they typically run something like 10x - 20x faster than in zx on mac/linux because spawning processes is expensive (even more expensive on Windows)

One (admittedly obvious) difference is that it runs on Node/V8 instead of Bun/JSC.