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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?

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2 Answers 2

up vote 5 down vote accepted

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

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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
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 at 13:13

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.

And then do something like bellow to send input to your server:

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.

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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 at 13:47
The trick with the "cat > /tmp/srv-input &" saved me some headaches. Thank you! –  Prior99 Mar 2 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 at 21:58
@bk138: it looks to me as tail should work, but there is only one way to know for sure: test. –  jfgagne Mar 17 at 7:34

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