How to attach terminal to detached process?

Question

I have detached a process from my terminal, like this:

$ process &

That terminal is now long closed, but process is still running and I want to send some commands to that process's stdin. Is that possible?

Answer 1

Yes, it is. First, create a pipe: mkfifo /tmp/fifo. Use gdb to attach to the process: gdb -p PID

Then close stdin: call close (0); and open it again: call open ("/tmp/fifo", 0600)

Finally, write away (from a different terminal, as gdb will probably hang):

echo blah > /tmp/fifo

Answer 2

I am quite sure you can not.

Check using ps x. If a process has a ? as controlling tty, you can not send input to it any more.

9942 ?        S      0:00 tail -F /var/log/messages
9947 pts/1    S      0:00 tail -F /var/log/messages

In this example, you can send input to 9947 doing something like echo "test" > /dev/pts/1. The other process (9942) is not reachable.

Next time, you'll can use screen or tmux to avoid this situation.

Answer 3

EDIT : As Stephane Gimenez said, it's not that simple. It's only allowing you to print to a different terminal.

You can try to write to this process using /proc. It should be located in /proc/pid/fd/0, so a simple :

echo "hello" > /proc/PID/fd/0

should do it. I have not tried it, but it should work, as long as this process still has a valid stdin file descriptor. You can check it with ls -l on /proc/pid/fd/.

• if it's a link to /dev/null => it's closed
• if it's a link to /dev/pts/X or a socket => it's open

See nohup for more details about how to keep processes running.

Answer 4

Just ending the command line with & will not completely detach the process, it will just run it in the background. (With zsh you can use &! to actually detach it, otherwise you have do disown it later).

When a process runs in the background, it won't receive input from it's controlling terminal anymore. But you can send it back into the foreground with fg and then it will read input again.

Otherwise, it's not possible to externally change it's filedescriptors (including stdin) or to reattach a lost controlling terminal… unless you use debugging tools (see Ansgar's answer, or have a look at the retty command).