I'm been looking for an explanation for the difference between screen's -dm and -Dm options. I know that both options are used to make the screen run in the background / detach it after creation. I've read that the difference has something to do with forking, but I don't understand it.
I use screen to contain the console of my Minecraft server. When I called it directly in my simple systemd service, everything worked fine. However, when I moved the call to a script and called the script from the service, the service would exit when the script exits, stopping the Minecraft server with the ExecStop command. Some Googling revealed that I should use a forking service to prevent this, but that doesn't solve the issue, and I cannot use a oneshot service that doesn't exit, as I need the system to be able to detect if the Minecraft server is still running.
I currently call screen the following way:
screen -DmS minecraft java -jar server.jar nogui &I use the & to fork the process and keep the screen from freezing the shell.
Since the service fails due to not forking properly, and the difference between the -Dm and -dm options has something to do with forking, I suspect that's where the problem is.
Could someone explain to me what the practical difference between these options is, and how I can use screen properly in a forking service?
Thanks in advance.
21 Answer
I'm just quoting the manual here:
-d -m Start screen in "detached" mode. This creates a new session but doesn't attach to it. This is useful for system startup scripts.
-D -m This also starts screen in "detached" mode, but doesn't fork a new process. The command exits if the session terminates.
