View Source: NamedPipesInBash

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Here's something that somebody just recently told me.  This is a
useful and completely non-obvious tip, so I'm documenting it here in
hopes that others will find it useful.

My specific problem was that I wanted to diff the output of two
processes.  You can't do this with normal shell reidrection, the best
you can do is:

* send the output of one process to a tempfile
* diff the tempfile and the output of the second process, i.e.:

<verbatim>
# /sbin/lsmod >/tmp/lsmod.tmp
# ssh sys1 /sbin/lsmod | diff /tmp/lsmod.tmp -
</verbatim>

that works, but it requires a temporary file.

The answer to this problem is process substitution.  Bash has some
process substitution operators (<(foo) and >(foo)) which are very
poorly explained in the bash man page (and trust me, I've read that
man page __a lot__).  =foo= can be any command that produces output on
stdout.  Bash execs the command, creates a named pipe from the output,
and replaces the operator with the name of that pipe.  You can then
read stdout from that pipe as you would from a regular file.  Thus in
this case the output of foo might be fed through the file /dev/fd/64.
Now our diff example can be written like this:

<verbatim>
# diff <(/sbin/lsmod) <(ssh sys1 /sbin/lsmod)
</verbatim>

Note you can stuff stdout __into__ named pipes with the >(foo)
operator too, thus the output of a process can be sent into another
process which you invoke on the command line.  This is more flexible
than the standard pipe mechanism.

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