I meant screen, sorry. Probably doesn't change the answer though. But where would the output go instead? Sure, if given access to them. Just something to keep in mind. External commands have to be in a seperate context. There's just no other way to run them. Builtins could be run in threads, so wouldn't need a separate context.

Even if you fork(), it's possible to share memory by other means. Memory you've created with mmap() can be shared between such processes. So just being in a subshell doesn't mean you'd have to lose access to variables. Environment variables though come preallocated so can't be shared that way without torturing your process environment in cruel and unusual ways.

Some builtins, like echo, might not need any context. When you know the amount of text is smaller than the size of your pipe, you know it'll never block -- so just cram the text in the write-end and close the write-end in advance. You don't need a new thread just to do that. Why not let the mutexes do the blocking? Reference, please? Things I've seen suggest quite differently. Then again, that's something I saw inside the linux pthreads library, so that might be things they had to do to prevent threads being cloned rather than things I'd have to do...

If true, that would make it a ton easier.