Shell tips and tricks (was: cat pipe)

[Steven W. Orr]

On Friday, Oct 5th 2007 at 10:04 -0000, quoth Ben Scott:

=>On 10/4/07, Michael ODonnell <michael.odonnell at comcast.net> wrote:
=>> Bash users can do this:
=>>
=>>    cp <( date ) /dev/tty
=>
=>  I had to look that up.  "Process Substitution".  Assuming I
=>understand correctly, that connects the output of the "date" command
=>to a named pipe (say, /tmp/foo), and then substitutes the name of said
=>pipe for the actual argument.  So after substitution, the command
=>would look like:
=>
=>	cp /tmp/foo /dev/tty
=>
=>  Obviously, your example is a pointless example, but even so... I'm
=>trying to think what possible use this would be.  Why would you want
=>to use Process Substitution for real?
=>
=>  The only thing I could think of would be for lame programs which
=>insist on read/writing to a named file, and will not use stdin/stdout
=>in the manner of a Unix filter.  But I've always just done:
=>
=>	foo | lameprog /dev/stdin
=>
=>or whatever for such cases.

Good question. It doesn't come up often, but the best way to get it is to 
think in C vs thinking in shell.

>From C, you can create subprocesses and pipes, but when you do it, you 
don't have to constrain your dataflow from left to right.

p1 | p2 | p3 

has data from from left to right. Now what happens if you want data to 
travel differently?

I want p1's arg1 to take its input from p2 piped to p3  and I want p1's 
second arg to come from p4.

p1 <(p2 | p3) <( p4)

Easy peasy, japanesy

Here are a few I found online:

sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin)
# Lists all the files in the 3 main 'bin' directories, and sorts by filename.
# Note that three (count 'em) distinct commands are fed to 'sort'.

diff <(command1) <(command2)    # Gives difference in command output.

tar cf >(bzip2 -c > file.tar.bz2) $directory_name

# Tar up a dir and compress the output (Yeah, I know, just use j opt.)

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steveo at syslang.net