Command vs Expression Mode

Mash Shell Quick Reference

Mish is a sublanguage within Mash for launching processes, with a similar syntax to a traditional shell.

By default, Mash is in "regular Mash" mode. If a command line starts or ends with !, the entire line is interpreted as a Mish command. If ! is issued by itself as a command, then Mash toggles into Mish by default, and the prompt changes from a $ to a !.