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This is a follow-on question to the How do you use ssh in a shell script? question. If I want to execute a command on the remote machine that runs in the background on that machine, how do I get the ssh command to return? When I try to just include the ampersand (&) at the end of the command it just hangs. The exact form of the command looks like this:

ssh user@target "cd /some/directory; program-to-execute &"

Any ideas? One thing to note is that logins to the target machine always produce a text banner and I have SSH keys set up so no password is required.

HoldOffHunger
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dagorym
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17 Answers17

354

I had this problem in a program I wrote a year ago -- turns out the answer is rather complicated. You'll need to use nohup as well as output redirection, as explained in the wikipedia artcle on nohup, copied here for your convenience.

Nohuping backgrounded jobs is for example useful when logged in via SSH, since backgrounded jobs can cause the shell to hang on logout due to a race condition [2]. This problem can also be overcome by redirecting all three I/O streams:

nohup myprogram > foo.out 2> foo.err < /dev/null &
Jax
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    Those files are created in the current directory. So the limit is the amount of free space on the partition. Of course you can also redirect to `/dev/null`. – Frank Kusters Feb 28 '13 at 13:46
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    Any ideas on backgrounding the process after it's finished asking for prompts? (like a gpg --decrypt that's finished asking for the password) – isaaclw Jun 07 '13 at 20:43
  • Trying to start a resque worker in the background using nohup, but it doesn;t work.. :( – Infant Dev Feb 24 '14 at 11:40
  • I used this solution to start jboss using plink – JJ Roman Feb 03 '15 at 09:19
  • @Jax, is the `&` necessary? It only works for me without it; oddly it does detach from the long running job. Here's my command: `ssh -t user@server "cd apps/myapp && nohup make my-make-target > myapp.log 2>&1 < /dev/null"`. – Tyler Collier Feb 27 '15 at 23:56
  • Ok, I figured out it was the `-t` on my ssh that was causing this. – Tyler Collier Feb 27 '15 at 23:57
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    Can you please explain what `< /dev/null` mean? Thanks. – Qian Chen Sep 20 '15 at 15:43
  • Wikipedia text changed a bit (though still discusses it); the detailed reference is http://www.snailbook.com/faq/background-jobs.auto.html – Beni Cherniavsky-Paskin Jun 30 '16 at 13:19
  • It seems your answer works for most of the cases. However, for google-chrome, it fails. I mean `ssh -n server 'nohup google-chome http://google.com 1>/dev/null 2>&1 &' ` – doraemon Oct 12 '17 at 07:57
  • Are you sure `nohup` is needed? I'm getting good results by running the process in the background and redirecting its output to files like `ssh "myprogram >foo.out 2>foo.err &"`. – ijt Feb 27 '20 at 16:17
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    Also from the wikipedia article on nohup: "Also note that a closing SSH session does not always send a HUP signal to depending processes. Among others, this depends on whether a pseudo-terminal was allocated or not." So while strictly the nohup might not always be needed, you're better long term with it than without. – Jax Mar 05 '20 at 14:58
  • Oh, you're so great!! I'm fighting with it for above 1 hour, thanks! Finally, your answer fixed it. – Clock ZHONG Apr 12 '20 at 13:32
  • @QianChen `< /dev/null` is explained here https://stackoverflow.com/a/19956266/336184 as avoiding of input wait – Max May 30 '20 at 05:12
273

This has been the cleanest way to do it for me:-

ssh -n -f user@host "sh -c 'cd /whereever; nohup ./whatever > /dev/null 2>&1 &'"

The only thing running after this is the actual command on the remote machine

carlspring
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Russ
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39

Redirect fd's

Output needs to be redirected with &>/dev/null which redirects both stderr and stdout to /dev/null and is a synonym of >/dev/null 2>/dev/null or >/dev/null 2>&1.

Parantheses

The best way is to use sh -c '( ( command ) & )' where command is anything.

ssh askapache 'sh -c "( ( nohup chown -R ask:ask /www/askapache.com &>/dev/null ) & )"'

Nohup Shell

You can also use nohup directly to launch the shell:

ssh askapache 'nohup sh -c "( ( chown -R ask:ask /www/askapache.com &>/dev/null ) & )"'

Nice Launch

Another trick is to use nice to launch the command/shell:

ssh askapache 'nice -n 19 sh -c "( ( nohup chown -R ask:ask /www/askapache.com &>/dev/null ) & )"'
AskApache Webmaster
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    I know this is a very old answer of yours, but could you add some comments on why the parentheses way is the best way, what (if any) difference adding `nohup` makes, and why and when you would use `nice`? I think that would add a lot to this answer. – Dr K Apr 20 '18 at 18:40
  • Maybe to partly answer this: With nohup, you don't need to append the & to the command to be run. – Cadoiz Jun 17 '19 at 14:24
26

If you don't/can't keep the connection open you could use screen, if you have the rights to install it.

user@localhost $ screen -t remote-command
user@localhost $ ssh user@target # now inside of a screen session
user@remotehost $ cd /some/directory; program-to-execute &

To detach the screen session: ctrl-a d

To list screen sessions:

screen -ls

To reattach a session:

screen -d -r remote-command

Note that screen can also create multiple shells within each session. A similar effect can be achieved with tmux.

user@localhost $ tmux
user@localhost $ ssh user@target # now inside of a tmux session
user@remotehost $ cd /some/directory; program-to-execute &

To detach the tmux session: ctrl-b d

To list screen sessions:

tmux list-sessions

To reattach a session:

tmux attach <session number>

The default tmux control key, 'ctrl-b', is somewhat difficult to use but there are several example tmux configs that ship with tmux that you can try.

chicks
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hometoast
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18

I just wanted to show a working example that you can cut and paste:

ssh REMOTE "sh -c \"(nohup sleep 30; touch nohup-exit) > /dev/null &\""
cmcginty
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8

Quickest and easiest way is to use the 'at' command:

ssh user@target "at now -f /home/foo.sh"
vinzee
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neil
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    This would be a great general solution if `at` accepted command line arguments after the time and not only from a file. – Tyler Collier Feb 27 '15 at 16:30
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    You can simulate a file with <<< like in: ssh user@target "at now -f <<< 'my_comnads'" – Nico Feb 24 '17 at 14:18
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    Involving a separate daemon (and using tooling that won't work unless that daemon is running) seems like a lot of unnecessary complexity for something that [doesn't _require_ an extra daemon at all](https://stackoverflow.com/a/60437534/14122). – Charles Duffy Oct 03 '21 at 16:02
8

I think you'll have to combine a couple of these answers to get what you want. If you use nohup in conjunction with the semicolon, and wrap the whole thing in quotes, then you get:

ssh user@target "cd /some/directory; nohup myprogram > foo.out 2> foo.err < /dev/null"

which seems to work for me. With nohup, you don't need to append the & to the command to be run. Also, if you don't need to read any of the output of the command, you can use

ssh user@target "cd /some/directory; nohup myprogram > /dev/null 2>&1"

to redirect all output to /dev/null.

8

You can do this without nohup:

ssh user@host 'myprogram >out.log 2>err.log &'
ijt
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6

This worked for me may times:

ssh -x remoteServer "cd yourRemoteDir; ./yourRemoteScript.sh </dev/null >/dev/null 2>&1 & "
fs82
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2

You can do it like this...

sudo /home/script.sh -opt1 > /tmp/script.out &
jeff porter
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user889030
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  • Perfect, works with a PuTTY SSH session. I can exit and the script continues on the machine. Thank you. – Satria Nov 05 '19 at 03:14
2

It appeared quite convenient for me to have a remote tmux session using the tmux new -d <shell cmd> syntax like this:

ssh someone@elsewhere 'tmux new -d sleep 600'

This will launch new session on elsewhere host and ssh command on local machine will return to shell almost instantly. You can then ssh to the remote host and tmux attach to that session. Note that there's nothing about local tmux running, only remote!

Also, if you want your session to persist after the job is done, simply add a shell launcher after your command, but don't forget to enclose in quotes:

ssh someone@elsewhere 'tmux new -d "~/myscript.sh; bash"'
zebrilo
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1

Actually, whenever I need to run a command on a remote machine that's complicated, I like to put the command in a script on the destination machine, and just run that script using ssh.

For example:

# simple_script.sh (located on remote server)

#!/bin/bash

cat /var/log/messages | grep <some value> | awk -F " " '{print $8}'

And then I just run this command on the source machine:

ssh user@ip "/path/to/simple_script.sh"
MarkoCen
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PaulT
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0

I was trying to do the same thing, but with the added complexity that I was trying to do it from Java. So on one machine running java, I was trying to run a script on another machine, in the background (with nohup).

From the command line, here is what worked: (you may not need the "-i keyFile" if you don't need it to ssh to the host)

ssh -i keyFile user@host bash -c "\"nohup ./script arg1 arg2 > output.txt 2>&1 &\""

Note that to my command line, there is one argument after the "-c", which is all in quotes. But for it to work on the other end, it still needs the quotes, so I had to put escaped quotes within it.

From java, here is what worked:

ProcessBuilder b = new ProcessBuilder("ssh", "-i", "keyFile", "bash", "-c",
 "\"nohup ./script arg1 arg2 > output.txt 2>&1 &\"");
Process process = b.start();
// then read from process.getInputStream() and close it.

It took a bit of trial & error to get this working, but it seems to work well now.

j0k
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Randy Wilson
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  • Why do you have literal quotes inside the argument following `-c`? I don't see how that could possibly work. It's like `ssh -i keyFile bash -c '"nohup ./script arg1 arg2 > output.txt 2>&1 &"'` in bash, which doesn't work either and for the same reason (incorrect literal quotes nested inside the necessary and correct syntactic ones). – Charles Duffy Oct 03 '21 at 23:04
0
YOUR-COMMAND &> YOUR-LOG.log &

This should run the command and assign a process id you can simply tail -f YOUR-LOG.log to see results written to it as they happen. you can log out anytime and the process will carry on

richprice316
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  • `&>your-log.log` only works if the remote system's shell is bash (or otherwise has the extension at hand). If it could possibly be sh, `>your-log.log 2>&1` is better. – Charles Duffy Oct 03 '21 at 16:00
0

If you are using zsh then use program-to-execute &! is a zsh-specific shortcut to both background and disown the process, such that exiting the shell will leave it running.

Sathesh
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-2

First follow this procedure:

Log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:

a@A:~> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/a/.ssh/id_rsa): 
Created directory '/home/a/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/a/.ssh/id_rsa.
Your public key has been saved in /home/a/.ssh/id_rsa.pub.
The key fingerprint is:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a@A

Now use ssh to create a directory ~/.ssh as user b on B. (The directory may already exist, which is fine):

a@A:~> ssh b@B mkdir -p .ssh
b@B's password:

Finally append a's new public key to b@B:.ssh/authorized_keys and enter b's password one last time:

a@A:~> cat .ssh/id_rsa.pub | ssh b@B 'cat >> .ssh/authorized_keys'
b@B's password:

From now on you can log into B as b from A as a without password:

a@A:~> ssh b@B

then this will work without entering a password

ssh b@B "cd /some/directory; program-to-execute &"

Kevin Duterne
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I think this is what you need: At first you need to install sshpass on your machine. then you can write your own script:

while read pass port user ip; do
sshpass -p$pass ssh -p $port $user@$ip <<ENDSSH1
    COMMAND 1
    .
    .
    .
    COMMAND n
ENDSSH1
done <<____HERE
    PASS    PORT    USER    IP
      .      .       .       .
      .      .       .       .
      .      .       .       .
    PASS    PORT    USER    IP    
____HERE
MLSC
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