Prevent currently running linux task to be killed after logout from SSH

If your task is already launched, it is too late* to consider alternative solutions that insert an additional layer between your ssh session and the shell running the command, like screen, tmux, byobu, nohup and the likes.

If your process support to be placed in the background and particularly doesn't hang when stdout and stderr are unwritable/closed, you can put it in the background before logging out with ControlZ and bg then detach it from your shell with the disown builtin.

eg:

$ ssh localhost
You have new mail.
Last login: Fri Jun  6 11:26:56 2014

$ /bin/sleep 3600    


^Z[1] + Stopped                  /bin/sleep 3600
$ bg
[1] /bin/sleep 3600&
$ jobs
[1] +  Running                 /bin/sleep 3600
$ disown %1
$ exit
Connection to localhost closed.
$ ps -ef|grep sleep
jlliagre 12864     1  0 21:12 ?        00:00:00 /bin/sleep 3600
jlliagre 13056 12477  0 21:13 pts/18   00:00:00 grep sleep
$ 

* As Bob commented, there are actually several hackish ways to reparent a tty session under Linux. repty, retty, injcode and neercs. The most advanced looks to be reptyr but you might need root privileges to enable ptrace to hack your process.

One solution is to use GNU screen. You could start up screen, run your command, then detach with C-a d. Later, to reconnect, do screen -r, and you are back in your previous session.

Other benefits of screen are window management (so you can switch to other shells while your command is running, without needing a new SSH connection), and it allows your command to remain in the foreground, whether in the current session or a later one.

Edit: As noted in the comments, this will only work if you remember to start screen before running the command. If the command is already running, then you will need @jlliagre's solution.