How can I send the stdout of one process to multiple processes using (preferably unnamed) pipes in Unix (or Windows)?

Question

I'd like to redirect the stdout of process proc1 to two processes proc2 and proc3:

         proc2 -> stdout
       /
 proc1
       \ 
         proc3 -> stdout

I tried

 proc1 | (proc2 & proc3)

but it doesn't seem to work, i.e.

 echo 123 | (tr 1 a & tr 1 b)

writes

 b23

to stdout instead of

 a23
 b23

Answer 1

<sup>Editor's note:
- >(…) is a process substitution that is a nonstandard shell feature of some POSIX-compatible shells: bash, ksh, zsh.
- This answer accidentally sends the output process substitution's output through the pipeline too: echo 123 | tee >(tr 1 a) | tr 1 b.
- Output from the process substitutions will be unpredictably interleaved, and, except in zsh, the pipeline may terminate before the commands inside >(…) do.</sup>

In unix (or on a mac), use the tee command:

$ echo 123 | tee >(tr 1 a) >(tr 1 b) >/dev/null
b23
a23

Usually you would use tee to redirect output to multiple files, but using >(...) you can redirect to another process. So, in general,

$ proc1 | tee >(proc2) ... >(procN-1) >(procN) >/dev/null

will do what you want.

Under windows, I don't think the built-in shell has an equivalent. Microsoft's Windows PowerShell has a tee command though.