GNU Screen – Using -dmS Option to Run Background Jobs Stably

gnu-screenlinux

I saw many place introduce screen to run background job stably even log out. They use

screen -dmS name

According to screen -h, this option means

-dmS name Start as daemon: Screen session in detached mode.

What is daemon? I don't understand.

I found that if I simply type screen, I can enter automatically into a screen. After I run some command, and press Ctrl+a d, and then log off. The job is still running fine. So is this simple approach OK? Do I really need -dmS to make background job stable?

Let me try to give a summary:

Anything run in screen is safe to logging out (but you should detach the screen, not quit screen when you log out), no matter what the option you have set to screen.

-dmS is just an option convienient for submitting jobs in background noniteractively. That is

screen -dmS nameOfScreen command

Best Answer

You would only use -dm if you want to run a command in a screen session and not enter it interactively

-S is just to give the session a usable name so you can reconnect to it again easily later

If you want to use it interactively and don't want to give it a human readable name, you can omit all of those arguments safely.

For example, if you just want to start up screen to run the command, say, /path/to/longTime and you don't want to watch it run you could do it either as

screen -dmS longSession /path/to/longTime

or you could do

screen -S longSession
$ /path/to/longTime

ctrlad

Both would accomplish the same thing, but one is both easier to script and a bit less typing.

Related Solutions

Bash – Why `> the.log 2>&1 &` causes the job to sustain log out

Normally if you put something in the background it will continue to run even after its parent shell exits. In fact, I can create a test case like this:

/bin/sh -c 'while true; do echo hi; sleep 5; done' &

and then exit the shell and using ps I can see it's still running, forever, until I kill it.

There is one difference between redirecting the output and not: if you don't redirect the output and you exit your shell, then the stdout/stderr file descriptors are closed and the next time you try to write to them the write operation will fail. If your script is checking for that, or running with the -e (exit on error) option set, then your script will stop. Compare the behavior of the above with this version:

/bin/sh -c 'while true; do echo hi || exit 1; sleep 5; done' &

/bin/sh -ec 'while true; do echo hi; sleep 5; done' &

If you leave this running, exit the shell, and use ps you'll see that when it tries to run the echo it will fail, and exit.

If you were to redirect the output to a file then obviously the write will not fail anymore and the script will continue to run.

Programmatically run background tasks in a split screen

You can do it with tmux by stringing together split-window and send-keys like this:

tmux new session \; split-window -v \; send-keys 'ping -t 5 google.com' C-m \; split-window -h \; send-keys 'ping -t 10 microsoft.com' C-m \;

Best Answer

Related Solutions

Bash – Why `> the.log 2>&1 &` causes the job to sustain log out

Programmatically run background tasks in a split screen

Related Question