Pipe output to two different commands [duplicate]

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Closed 8 years ago.

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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)

Answer 2

Recent bash present >(command) syntax:

echo "Hello world." | tee >(sed 's/^/1st: /')  >(sed 's/^/2nd cmd: /') >/dev/null

May return:

2nd cmd: Hello world.
1st: Hello world.

download somefile.ext, save them, compute md5sum and sha1sum:

wget -O - http://somewhere.someland/somepath/somefile.ext |
    tee somefile.ext >(md5sum >somefile.md5) | sha1sum >somefile.sha1

or

wget -O - http://somewhere.someland/somepath/somefile.ext |
    tee >(md5sum >somefile.md5) >(sha1sum >somefile.sha1) >somefile.ext

Old answer

There is a way to do that via unnamed pipe (tested under linux):

 (( echo "hello" |
         tee /dev/fd/5 |
             sed 's/^/1st occure: /' >/dev/fd/4
    ) 5>&1 |
    sed 's/^/2nd command: /'
 ) 4>&1

give:

2nd command: hello
1st occure: hello

This sample will let you download somefile.ext, save them, compute his md5sum and compute his sha1sum:

(( wget -O - http://somewhere.someland/somepath/somefile.ext |
    tee /dev/fd/5 |
    md5sum >/dev/fd/4
  ) 5>&1 |
  tee somefile.ext |
  sha1sum
) 4>&1

Answer 3

Maybe take a look at tee command. What it does is simply print its input to a file, but it also prints its input to the standard output. So something like:

echo "Hello" | tee try.txt | <some_command>

Will create a file with content "Hello" AND also let "Hello" (flow through the pipeline) end up as <some_command>'s STDIN.