Using commandoutput as a file descriptor

diff rpm_output_from_other_computer <(rpm -qa|sort)
Description is moved to "Sample output" because the html sanitizer for commandlinefu breaks the examples..
Sample Output
# Standard diff output...

Using <(command_here) will do the same job as "command_here > command_output" and then use command_output instead of <(command_here).
This trick can be used for much more than diff as the command that get this as a parameter wont see the different between the <( ) file descriptor and a normal file.
Eg: diff /etc/passwd <(ssh remotehost "cat /etc/passwd")

ls <(ls), will give you a little hint why this works :)

0
By: xeor
2011-06-25 11:45:15

These Might Interest You

  • taken from http://www.unix.com/shell-programming-scripting/158311-how-tee-stderr.html " What does it mean? The redirection operator n>&m makes file descriptor n to be a copy of file descriptor m. So, whe are: - Opening a new file descriptor, 3, that is a copy of file descriptor 1, the standard output; - Making file descriptor 1 a copy of file descriptor 2, the standard error output; - Making file descriptor 2 to be a copy of file descriptor 3 (the "backup" of the standard output) in a short: we swapped the standard output and the standard error output. "


    1
    command 3>&1 1>&2 2>&3 | tee file
    hute37 · 2012-10-30 10:53:21 1
  • the tee command does fine with file names, but not so much with file descriptors, such as &2 (stderr). This uses process redirection to tee to the specified descriptor. In the sample output, it's being used to tee to stderr, which is connected with the terminal, and to wc -l, which is also outputting to the terminal. The result is the output of bash --version followed by the linecount Show Sample Output


    5
    tee >(cat - >&2)
    camocrazed · 2010-07-20 17:22:31 3
  • Ever needed to test firewalls but didn't have netcat, telnet or even FTP? Enter /dev/tcp, your new best friend. /dev/tcp/(hostname)/(port) is a bash builtin that bash can use to open connections to TCP and UDP ports. This one-liner opens a connection on a port to a server and lets you read and write to it from the terminal. How it works: First, exec sets up a redirect for /dev/tcp/$server/$port to file descriptor 5. Then, as per some excellent feedback from @flatcap, we launch a redirect from file descriptor 5 to STDOUT and send that to the background (which is what causes the PID to be printed when the commands are run), and then redirect STDIN to file descriptor 5 with the second cat. Finally, when the second cat dies (the connection is closed), we clean up the file descriptor with 'exec 5>&-'. It can be used to test FTP, HTTP, NTP, or can connect to netcat listening on a port (makes for a simple chat client!) Replace /tcp/ with /udp/ to use UDP instead.


    15
    exec 5<>/dev/tcp/time.nist.gov/13; cat <&5 & cat >&5; exec 5>&-
    tyzbit · 2015-07-30 21:12:38 9
  • Press > or < to go to the next or previous track. Space to toggle play/pause, etc. It creates a temp file descriptor. To see where the file descriptor gets created type: echo <(echo foo) This works better than running find first, then piping to mplayer with xargs or something, because that won't let you use keyboard shortcuts.


    0
    mplayer -playlist <(find $PWD -type f)
    rkulla · 2010-04-17 00:20:08 3

What do you think?

Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

You must be signed in to comment.

What's this?

commandlinefu.com is the place to record those command-line gems that you return to again and again. That way others can gain from your CLI wisdom and you from theirs too. All commands can be commented on, discussed and voted up or down.

Share Your Commands



Stay in the loop…

Follow the Tweets.

Every new command is wrapped in a tweet and posted to Twitter. Following the stream is a great way of staying abreast of the latest commands. For the more discerning, there are Twitter accounts for commands that get a minimum of 3 and 10 votes - that way only the great commands get tweeted.

» http://twitter.com/commandlinefu
» http://twitter.com/commandlinefu3
» http://twitter.com/commandlinefu10

Subscribe to the feeds.

Use your favourite RSS aggregator to stay in touch with the latest commands. There are feeds mirroring the 3 Twitter streams as well as for virtually every other subset (users, tags, functions,…):

Subscribe to the feed for: