[SOLVED] Howto split into bash pipelines or merge 2 pipelines into one

Howto split into bash pipelines or merge 2 pipelines into one

If I want to create a signal flow graph using small processing blocks connected via pipes, how could I most nicely implement a flow where I i.e. split the 2 channels of a stereo stream into 2 seperate pipes? In pseudocode I'd

genpcm-stereo | split -c 2 .. 
[ ( lowpass | reverb ),
  ( highpass | reverb )
] 
.. merge -c 2

Where split should create 2 output streams where one channel would be processed with lowpass pipe, while the other with highpass pipe. After that merge would interleave the 2 streams again.

Is there some elegant simple way to write this in bash?

Solution

One option is to use named pipes. This Shellcheck-clean Bash code demonstrates one way to do it:

#! /bin/bash -p

mkfifo pipe{1..4}

genpm-stereo | splitchannels pipe1 pipe2 &
lowpass <pipe1  | reverb >pipe3 &
highpass <pipe2 | reverb >pipe4 &
mergechannels pipe3 pipe4 &
wait

rm pipe{1..4}

All processing is done in background processes (... &) to prevent deadlocks due to pipe buffers being full or empty.
The wait command waits until all of the background processes have exited.
splitchannels is assumed to split its standard input into channels and write them to its two filename arguments.
mergechannels is assumed to read its two filename arguments, interleave the contents, and write the result to standard output.
This demonstration code blindly creates and deletes pipes using fixed names. Real code would need to ensure that pipe names don't clash with filenames used by other programs (e.g. by using mktemp).