Attach to a processes output for viewing

Question

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

User · Answer

I was looking for this exact same thing and found that you can do:

strace -ewrite -p $PID

It's not exactly what you needed, but it's quite close.

I tried the redirecting output, but that didn't work for me. Maybe because the process was writing to a socket, I don't know.

User · Answer

For me  this worked    Login as the owner of the process  even root is denied permission      su - process owner  Tail the file descriptor as mentioned in many other answers      tail -f  proc  lt process-id gt  fd 1    0  stdin  1  stdout  2  stderr

User · Answer

I think I have a simpler solution here  Just look for a directory whose name corresponds to the PID you are looking for  under the pseudo-filesystem accessible under the  proc path  So if you have a program running  whose ID is 1199  cd into it     cd  proc 1199   Then look for the fd directory underneath    cd fd   This fd directory hold the file-descriptors objects that your program is using  0  stdin  1  stdout  2  stderr  and just tail -f the one you need - in this case  stdout      tail -f 1

User · Answer

I wanted to remotely watch a yum upgrade process that had been run locally  so while there were probably more efficient ways to do this  here s what I did   watch cat  dev vcsa1   Obviously you d want to use vcsa2  vcsa3  etc   depending on which terminal was being used   So long as my terminal window was of the same width as the terminal that the command was being run on  I could see a snapshot of their current output every two seconds  The other commands recommended elsewhere did not work particularly well for my situation  but that one did the trick

User · Answer

You can use reptyr  sudo apt install reptyr reptyr pid

User · Answer

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

User · Answer

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    Check the open files list for your process  thanks to  proc xxx fd Attach your process with GDB 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    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  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

[linux] Attach to a processes output for viewing

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