How can I use ssh to run a command on a remote Unix machine and exit before the command completes?

How can I use ssh to run a command on a Unix machine and exit before the command completes?

For instance, if I type

ssh localhost 'sleep 60 &'

ssh hangs on my terminal until sleep completes, even though sleep is run in the background.

Richard Hoskins

Posted 2009-07-19T17:54:20.787

Reputation: 10 260

You probably want the & outside of the quotes. – Brad Gilbert – 2009-07-19T18:20:04.530

1No, that just puts the whole kit and kaboodle in the background and I am unable to type my password. – Richard Hoskins – 2009-07-19T18:55:09.330

Answers

SSH has an -f switch which does exactly the same as using nohup on your client but makes it possible that ssh asks for a password. Use it like that:

ssh localhost -f 'command'

But: SSH will still live in background, so if you shut down the client before the command on the server finished, it will be aborted. If you want the ssh connection to be closed after executing the command, you can use screen on the server side:

ssh localhost -f 'screen -d -m sleep 60'

Martin

Posted 2009-07-19T17:54:20.787

Reputation: 404

If I do 'ssh remotemachine -n 'long running command > withremoteredirects 2>&1 </dev/null &' and the client shuts down will the command complete? – Richard Hoskins – 2009-07-19T18:24:49.340

Afaik no. I ssh connections is lost, the ssh daemon should terminate all terminals "inside" ssh. Ans since the terminal started the command they would be terminated to, since thier parent process terminated. But it's worth trying ;) – Martin – 2009-07-19T18:32:19.477

What about 'nohup' instead of 'screen' on the server side? – Richard Hoskins – 2009-07-19T18:33:44.670

@Brad Gilbert Nothing? I can think of a few things, but one thing the don't have in common, is that 'screen' isn't available on all POSIX machines. – Richard Hoskins – 2009-07-19T18:44:24.940

http://www.gnu.org/software/screen/manual/ – Brad Gilbert – 2009-07-19T18:44:52.457

screen and nohup have different uses. You can't rejoin a nohup session, only view what it has done. – Brad Gilbert – 2009-07-19T18:57:45.470

@Brad Gilbert I know. Screen isn't always available, and anyway it was being used in the example to prevent sleep from being killed when the ssh connection closed. – Richard Hoskins – 2009-07-19T19:04:42.393

Use nohup, stdout/stderr redirection, and run the command in the background:

 nohup ssh localhost 'sleep 60' >/dev/null 2>/dev/null &

EDIT: Or even better, just use the ssh's -f parameter:

ssh -f localhost 'sleep 60'

neu242

Posted 2009-07-19T17:54:20.787

Reputation: 1 316

@BradGilbert That syntax is not POSIX-compatible; it's hard to argue for sacrificing compatibility in order to save a few characters. – tripleee – 2018-04-12T11:48:33.597

You could probably combine the two redirections with &>/dev/null. – Brad Gilbert – 2009-07-19T18:13:34.547

Or after login in using ssh, start screen if you may want to come back later, to look at the progress or output.

Arjan

Posted 2009-07-19T17:54:20.787

Reputation: 29 084

You can also use "tmux" which I personally find easier to configure. Both applications are pretty much equivalent with each having its pros and cons. – mzuther – 2016-02-14T13:08:31.467