Attach to a processes output for viewing

aggitan_46857 0
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How would I 'attach' a console/terminal-view to an applications output so I can see what it may be saying?

How would I detach from an applications output without killing the application?

Normally if you fire up a talkative application using the command line you get to see all kinds of wonderful output. However, let’s say I have a particularly chatty programming running, like KINO, and I want to view its output at any given moment without restarting it through the command line. I cannot; at least I don't know how.

command-line-interface logging linux
8 Answers

There are a few options here. One is to redirect the output of the command to a file, and then use 'tail' to view new lines that are added to that file in real time.

Another option is to launch your program inside of 'screen', which is a sort-of text-based Terminal application. Screen sessions can be attached and detached, but are nominally meant only to be used by the same user, so if you want to share them between users, it's a big pain in the ass.

Don_Werve_852870

For me, this worked:

  1. Login as the owner of the process (even root is denied permission)

    ~$ su - process_owner

  2. Tail the file descriptor as mentioned in many other answers.

    ~$ tail -f /proc/<process-id>/fd/1 # (0: stdin, 1: stdout, 2: stderr)
user_30616860

You can use reptyr:

sudo apt install reptyr
reptyr pid
user_109493370

How would I 'attach' a console/terminal-view to an applications output so I can see what it may be saying?

About this question, I know it is possible to catch the output, even when you didn't launch sceen command before launching the processus.

While I never tried it, I've found an interesting article which explains how to do using GDB (and without restarting your process).

redirecting-output-from-a-running-process

Basically:

  1. Check the open files list for your process, thanks to /proc/xxx/fd
  2. Attach your process with GDB
  3. While it is paused, close the file you are interested in, calling close() function (you can any function of your process in GDB. I suspect you need debug symbols in your process..)
  4. Open the a new file calling the create() or open() function. (Have a look in comments at the end, you'll see people suggest to use dup2() to ensure the same handle will be in use)
  5. Detach the process and let in run.

By the way, if you are running a linux OS on i386 box, comments are talking about a better tool to redirect output to a new console : 'retty' . If so, consider its use.

yves_Baumes_833310

An even simpler option is cat:

cat /proc/<pid>/fd/1

Especially if tail does not work for you due to permission errors (even using sudo), or if the process to monitor display a progress bar using curses or an equivalent library.

user_28934080
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