I know about https://github.com/rroart/freevms/tree/master/dcl/src but my impression was FreeVMS is too incomplete to use in anger.

> I've seen things like mpsh

You mean https://cca.org/mpsh/ ? I am wondering how close it is to DCL, in terms of the specific aspect of it I’m talking about here

> and the ITS debugger/shell (DDT) ported to Unix...

Do you have a link?

One aspect of ITS I find fascinating is a debugged program can execute debugger commands. I actually implemented that before using a GDB Python script - have a do-nothing function passed a memory buffer containing a GDB command go run, the Python script sets a breakpoint on the function, and when that breakpoint is hit, it runs the GDB command, it can even write the results back into the buffer, and then it continues execution