I'm running a zsh shell:

$SystemShell = "/bin/zsh" 

My problem is that some basic system-level commands don't work in RunProcess. For example, echo works but exec, source, and many other build it shell commands don't:

It gives an error:

RunProcess::pnfd: Program source not found. Check Environment["PATH"]. >> 

The docs say that by default, the subprocess will inherit its process directory and environment variables from the Wolfram Engine, so I tried setting the ProcessEnvironment with an association to match my environment vars in the terminal. But that didn't work.

What am I doing wrong here, shouldn't all system level shell commands be accessible?