I'm on an Ubuntu 10.04 box, and started a server in the background (myserver &) over ssh. It's been running fine, but I need a way to get at the server's stdin, as the only way to control the server is through this method.

Is there some way to get at the stdin of an already-running process so I can write to it (and hopefully read its stdout)? Obviously, if I were going to be doing this now, I'd start it with a FIFO redirecting to stdin, but unfortunately it's a little late for that now.

Any ideas?

  • Couldn't you just bring it back to the foreground? ('jobs' will list your current background process, 'fg $X' will bring the job back to the foreground, ctrl+b will pause the job and return you to your shell, while 'bg' will continue the paused process in the background) – symcbean May 4 '16 at 16:38

You could try writing to it's /proc pid directory. Say your daemons' pid is 2000, try writing to /proc/2000/fd/0

  • Thanks... I found that right after I posted this (after a day of looking--typical). That seems to work (as far as actually sending data to the program). Unfortunately, the program doesn't accept the commands. I tested it running the server on my local computer, and sure enough, I see the data appear, but the program doesn't recognize the commands. I have to manually press enter on the server terminal, and then it just says unrecognized command. Maybe some java weirdness? I'm stuck... – tajmorton Oct 8 '10 at 9:35
  • 1
    how about echo -e "something\n" > /proc/2000/fd/0? – katriel Oct 8 '10 at 9:39
  • Actually, this is not alway sworking as /proc/<pid>/fd/0 points to /dev/pts<some number> on at least some systems... – bk138 Mar 17 '15 at 13:13
  • The first answer to serverfault.com/questions/178457/… notes that this approach doesn't actually work. – user47584 Dec 13 '15 at 17:26
  • 4
    This does not actually work. Your shell normally (when no pipes or redirections are used) starts a command with file descriptors 0 through 2 set to the same file, which normally is a virtual terminal (something like /dev/pty/...). The command then reads from FD 0 and writes to FD 1 and 2 to communicate with the virtual terminal (e.g. over SSH or directly with your terminal emulator). If any other process accesses that file (e.g. through /proc), exactly the same thing happens, i.e. writing to it writes to the terminal and not to the command. – Feuermurmel Sep 19 '16 at 10:16

You could start you server with a named pipe (fifo) as its input:

mkfifo /tmp/srv-input
cat > /tmp/srv-input &
echo $! > /tmp/srv-input-cat-pid
cat /tmp/srv-input | myserver &

The cat > /tmp/srv-input & is important to avoid your server to receive a EOF. At least one process must have the fifo opened in writing so your server does not receive a EOF. The PID of this command is saved in the /tmp/srv-input-cat-pid file for latter kill.

In your case where you've already started your server, you have to use a debugger such as gdb to attach to your process to redirect its stdin to the fifo:

gdb -p PID
call close(0)
call open(0, "/tmp/srv-input", 0600)

And then do something like bellow to send input to your server (in another terminal window if necessary):

echo "command" > /tmp/srv-input

To send a EOF to your server, you need to kill the cat > /tmp/srv-input process which PID has been saved in the /tmp/srv-input-cat-pid file.

In the case of GDB, just quit GDB and EOF will be sent.

  • 1
    this is a much more portable approach than the one from @katriel as /proc/2000/fd/0 is not stdin on all systems. – Prior99 Mar 2 '15 at 13:47
  • The trick with the "cat > /tmp/srv-input &" saved me some headaches. Thank you! – Prior99 Mar 2 '15 at 13:47
  • What about mkfifo /tmp/srv-input; tail -f /tmp/srv-input | myserver & ? This'll keep the pipe open as well... – bk138 Mar 16 '15 at 21:58
  • @bk138: it looks to me as tail should work, but there is only one way to know for sure: test. – jfg956 Mar 17 '15 at 7:34
  • tail doesn't work, but appended this to finish the job: cat /tmp/srv-input | myserver; kill -9 cat /tmp/srv-input-cat-pid` && rm /tmp/srv-input-cat*` – Thiago Macedo Feb 1 '16 at 10:54

Same as above, but 'cat' did not work for me. The file got EOF and ended after sending one command.

This worked for me:


mkfifo /tmp/srv-input
tail -f /tmp/srv-input | myserver &

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.