It should be simpler than the way you have it....

all that "|" means is give the write end to stdout of the left hand process and the read end to stdin for the right hand process.

but apart from > and <, the stdin/stdout/stderr should just be left alone.

To do ">" you just open a file and use it for stdout, end of story.

I personally would parse the line into a tree where each node is what I want to run in one process, each node would have pointers to where they get their stdin/stdout/stderr from.