+1 to the proposal as written! Yes please!
I have had this issue myself when I wanted to run a second copy of my own program as a subprocess.
I think the placement as a static property on CommandLine seems right. I’m glad it’s proposed as a String rather than a C string.
The only question I have is about the justification re. not choosing FilePath - there’s ongoing discussion about sinking System into the toolchain and while everyone seems to disagree about that, the one point of near universal agreement in that thread is that we should get FilePath into the toolchain. It’s true that System isn’t meant to be cross platform, but it is meant to “avoid unnecessary differences” (from the README) and as far as I can tell it is unconditionally available on every platform System builds for. And I’m hard pressed to think of a platform where “executable path” makes sense as a concept but “file path” does not. Especially since FilePath is already useful for both UNIXy paths and Windowsy paths. Does it really make sense to avoid FilePath for cross platform abstractions which are dealing in the concept of a path?