Pipe output to two different commands [duplicate]

Question

Possible Duplicate:
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

as well:

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

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 somefile.ext /dev/fd/5 |
    md5sum >/dev/fd/4
  ) 5>&1 |
  sha1sum
) 4>&1

or

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

or event more usefull:

url='http://ftp.uio.no/debian-cd/current/amd64/iso-cd'
read -r file < <(
    wget -qO - "$url" |
        sed -ne 's/.*href="\(debian-[1-9]\+\..*\.iso\)".*/\1/p')
wget -O - "$url/$file" | (
    ( tee /dev/fd/5 "$file" |
        sha256sum |
            sed -ne "h;s/-/$file/;w ${file%.*}.sha256" \
                 -e 'x;s/-/SHA256/;w /dev/fd/4'
    ) 5>&1 |
        sha512sum |
            sed -e "h;s/-/$file/;w ${file%.*}.sha512" \
                -e 'x;s/-/SHA512/'
) 4>&1

Will produce 3 files and output two lines:

64d727dd5785ae5fcfd3ae8ffbede5f40cca96f1580aaa2820e8b99dae989d94  SHA256
0262488ce2cec6d95a6c9002cfba8b81ac0d1c29fe7993aa5af30f81cecad3eb66558b9d8689a86b57bf12b8cbeab1e11d128a53356b288d48e339bb003dace5  SHA512

Which could be compared with:

for i in 256 512;do wget -qO - "$url/SHA${i}SUMS" | grep $file;done

64d727dd5785ae5fcfd3ae8ffbede5f40cca96f1580aaa2820e8b99dae989d94  debian-12.4.0-amd64-netinst.iso
0262488ce2cec6d95a6c9002cfba8b81ac0d1c29fe7993aa5af30f81cecad3eb66558b9d8689a86b57bf12b8cbeab1e11d128a53356b288d48e339bb003dace5  debian-12.4.0-amd64-netinst.iso

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.