Keeping a linux process running after I logout

Solution 1:

The best method is to start the process in a terminal multiplexer. Alternatively you can make the process not receive the HUP signal.

A terminal multiplexer provides "virtual" terminals which run independent from the "real" terminal (actually all terminals today are "virtual" but that is another topic for another day). The virtual terminal will keep running even if your real terminal is closed with your ssh session.

All processes started from the virtual terminal will keep running with that virtual terminal. When you reconnect to the server you can reconnect to the virtual terminal and everything will be as if nothing happened, other than the time which passed.

Two popular terminal multiplexers are screen and tmux.

Screen has a steep learning curve. Here is a good tutorial with diagrams explaining the concept: http://www.ibm.com/developerworks/aix/library/au-gnu_screen/

The HUP signal (or SIGHUP) is sent by the terminal to all its child processes when the terminal is closed. The common action upon receiving SIGHUP is to terminate. Thus when your ssh session gets disconnected all your processes will terminate. To avoid this you can make your processes not receive SIGHUP.

Two easy methods to do so are nohup and disown.

For more information about how nohup and disown works read this question and answer: https://unix.stackexchange.com/questions/3886/difference-between-nohup-disown-and

Note: although the processes will keep running you can no longer interact with them because they are no longer attached to any terminal. This method is mainly useful for long running batch processes which, once started, no longer need any user input.

Solution 2:

There are a few ways to do this, but the one I find most useful is to use GNU Screen.

After you ssh in, run screen. This will start another shell running within screen. Run your command, then do a Ctrl-a d.

This will "disconnect" you from the screen session. At this point, you can log out or do anything else you'd like.

When you want to re-connect to the screen session, just run screen -RD from the shell prompt (as the same use user that created the session).

Solution 3:

In bash, the disown keyword is perfectly suited to this. First, run your process in the background (either use &, or ^Z then type bg):

$ wget --quiet http://server/some_big_file.zip &
[1] 1156

By typing jobs you can see that the process is still owned by the shell:

$ jobs
[1]+  Running  wget

If you were to log out at this point, the background task would also get killed. However, if you run disown, bash detaches the job and allows it to continue running:

$ disown

You can confirm this:

$ jobs
$ logout

You can even combine the & and disown on the same line, like:

$ wget --quiet http://server/some_big_file.zip & disown
$ logout

This is better than running nohup in my opinion because it doesn't leave nohup.out files littered all over your filesystem. Also, nohup must be run before you run the command — disown can be used if you only decide later on that you want to background and detach the task.