> I think the OpenVMS's DCL interface has a libre implementation.

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

Well, on OpenVMS:

https://github.com/johnsonjh/PC-DCL

https://0x0.st/XoDG.patch

DDT, I think Lars Brinkoff had a port:

https://github.com/larsbrinkhoff/portable-ddt

Just reading about DDT reminds me of the "WizBiz"[0] series of fantasy novels, it's funny how just the mention of something from so long ago can evoke memories.

[0] https://tvtropes.org/pmwiki/pmwiki.php/Literature/WizBiz

DDT in the 60's was a bug killing spray which soon was banned because it was toxic.