Start command remotely and then close connection

Question

I'm trying to start a trim command on a bunch of servers remotely since these are appliances and we lose support if you change anything like cron jobs locally.

I'm trying to do this:

ssh [email protected] "nohup fstrim /data &"

That is executed but the problem is the secure shell is kept open despite nohup and everything. Can I force ssh to just drop the command and pull out immediately?

Just found a similar request via google. There it was solved with

screen -d -m ./script

Unfortunately screen isn't available on the appliances.

Answer 1

If you've got screen installed on the remote servers, try this, which will run the fstrim command inside a detached screen session. As far as ssh is concerned this will appear to exit immediately.

ssh [email protected] screen -S fstrim -md fstrim /data

You can reattach to the session with

ssh -tt [email protected]. screen -r fstrim

and disconnect again with Ctrl a(Ctrl) d

The reason your initial command isn't working is because nohup can see that stdout isn't a terminal and keeps the descriptor open for writing. (If you'd used ssh -tt it would have created a pseudotty on the remote side and nohup would have seen a terminal.) By redirecting stdout explicitly you can get the process to run in the background as you'd expect:

ssh -fn [email protected] "nohup fstrim /data >nohup.out &"

Read man nohup for the gory details on this now you know what you're looking for.

Answer 2

Never mind, believe I found the solution:

ssh [email protected] "nohup fstrim /data > /dev/null &"

Answer 3

Is atd running on the appliance? If so, you can schedule an immediate at job:

ssh [email protected] "echo 'fstrim /data' | at now"

Your remote session will terminate immediately, because all it's doing is scheduling the job. The atd daemon takes care of actually executing it.