Question

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osx/linux: pipes into two processes?

Is there a way to pipe the output from one command into the input of two other commands, running them simultaneously?

Something like this:

$ echo 'test' |(cat) |(cat)
test
test

The reason I want to do this is that I have a program which receives an FM radio signal from a USB SDR device, and outputs the audio as raw PCM data (like a .wav file but with no header.) Since the signal is not music but POCSAG pager data, I need to pipe it to a decoder program to recover the pager text. However I also want to listen to the signal so I know whether any data is coming in or not. (Otherwise I can’t tell if the decoder is broken or there’s just no data being broadcast.) So as well as piping the data to the pager decoder, I also need to pipe the same data to the play command.

Currently I only know how to do one – either pipe it to the decoder and read the data in silence, or pipe it to play and hear it without seeing any decoded text.

How can I pipe the same data to both commands, so I can read the text and hear the audio?

I can’t use tee as it only writes the duplicated data to a file, but I need to process the data in real-time.

Answer 1

It should be ok if you use both tee and mkfifo.

mkfifo pipe
cat pipe | (command 1) &
echo 'test' | tee pipe | (command 2)