I'd like something in the form of (fill in the blanks):
cmd1 | ____ cmd2 ____ That was equivalent to this, but without writing any files:
cmd1 > temp cmd2 < temp | diff temp - cmd1 isn't actually a command, it's a stdin stream from a text editor, so it cannot be run more than once, so this won't work:
diff <(cmd1) <(cmd1 | cmd2) But that is essentially what I want.
I'm using macOS, but I'd prefer it linux compatible as well.
Generally when you do not want to write a file, you can replace it by a (named) pipe, but a pipe cannot be read by two processes simultaneously. In your case you can use tee to split your input and use a named pipe for one of its ends:
mkfifo p cmd1 | tee p | cmd2 | diff p - rm p This, however, has one major drawback: the pipes have a limited buffer (64k on Linux), so when diff would read one of them excessively comparing to the other, the whole pipeline will be deadlocked. The problem can be circumvented using the buffer command for at least one pipe:
mkfifo p cmd1 | tee p | cmd2 | buffer | diff p - rm p See also here for other buffering options.
Another solution for deadlocks is to use a variable to buffer the whole cmd1 output in it (loading it to the memory):
cmd1 | ( b=$(cat); cmd2 <<<"$b" | diff <(echo "$b") - ) 
cmd1 | to be outside of the mkfifo? I don't know how to pipe direct stdin to the 2nd of a semi colon delimited chain of commands, in other words. buffer is not standard (at least not on osx) so I probably can't use that. It's starting to feel like I should just use temporary emacs buffers for this instead, but then that begs the question on how I might send 2 emacs buffers at once to diff... mkfifo does not read stdin, you may just write mkfifo p; tee p ... or create the fifo once, and not to bother with it here.As for the buffer, did you try other options under the link? buffer, macOS lacks pv, buffer and mbuffer all. pv is available through HomeBrew.
cmd1needs to be stored somewhere for it to be able to be processed by bothcmd2anddiff.