&gt;What&#x27;s a shell? It&#x27;s a program designed to be a layer around the OS, exposing all of the capabilities of the OS to the user in a convenient form.<p>I think that&#x27;s what they were getting at when they said:<p>&gt;optimized around the need to scale down to a convenient line-by-line way to work with the system in a REPL<p>The only real difference is optimization for the REPL. Many of the things you might consider part of the experience aren&#x27;t even shell built-ins, they&#x27;re utilities maintained separately from your shell. Powershell is a good example of a shell scripting language that has a lot more going on than your traditional shell. Native object pipelines, .net libraries, etc. but the most important thing about it is that it&#x27;s optimized for the REPL.