Diff: NamedPipesInBash

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version 3

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.:

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

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:

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

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.