Getting ssh to execute a command in the background on target machine

Question

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   amp   at the end of the command it just hangs   The exact form of the command looks like this  ssh user target  quot cd  some directory  program-to-execute  amp  quot   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

User · Accepted Answer

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 &

User · Answer

YOUR-COMMAND  amp  gt  YOUR-LOG log  amp        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

User · Answer

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  amp    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  amp    To detach the tmux session  ctrl-b d  To list screen sessions   tmux list-sessions   To reattach a session   tmux attach  lt session number gt    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

User · Answer

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   gt  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   gt  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   gt  cat  ssh id rsa pub   ssh b B  cat  gt  gt   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   gt  ssh b B   then this will work without entering a password   ssh b B  cd  some directory  program-to-execute  amp

User · Answer

Redirect fd s  Output needs to be redirected with  amp  gt  dev null which redirects both stderr and stdout to  dev null and is a synonym of  gt  dev null 2 gt  dev null or  gt  dev null 2 gt  amp 1   Parantheses  The best way is to use sh -c      command    amp     where command is anything   ssh askapache  sh -c      nohup chown -R ask ask  www askapache com  amp  gt  dev null    amp        Nohup Shell  You can also use nohup directly to launch the shell   ssh askapache  nohup sh -c      chown -R ask ask  www askapache com  amp  gt  dev null    amp        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  amp  gt  dev null    amp

User · Answer

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  lt  lt ENDSSH1     COMMAND 1                       COMMAND n ENDSSH1 done  lt  lt     HERE     PASS    PORT    USER    IP                                                                                                  PASS    PORT    USER    IP         HERE

User · Answer

I just wanted to show a working example that you can cut and paste   ssh REMOTE  sh -c    nohup sleep 30  touch nohup-exit   gt   dev null  amp

User · Answer

You can do it like this     sudo  home script sh -opt1  gt   tmp script out  amp

User · Answer

You can do this without nohup   ssh user host  myprogram  gt out log 2 gt err log  amp

User · Answer

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  gt  output txt 2 gt  amp 1  amp       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  gt  output txt 2 gt  amp 1  amp       Process process   b start       then read from process getInputStream   and close it    It took a bit of trial  amp  error to get this working  but it seems to work well now

User · Answer

This worked for me may times   ssh -x remoteServer  cd yourRemoteDir    yourRemoteScript sh  lt  dev null  gt  dev null 2 gt  amp 1  amp

User · Answer

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  lt some value gt    awk -F       print  8     And then I just run this command on the source machine   ssh user ip   path to simple script sh

User · Answer

Quickest and easiest way is to use the  at  command  ssh user target  quot at now -f  home foo sh quot

User · Answer

This has been the cleanest way to do it for me -  ssh -n -f user host  sh -c  cd  whereever  nohup   whatever  gt   dev null 2 gt  amp 1  amp      The only thing running after this is the actual command on the remote machine

User · Answer

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  gt  foo out 2 gt  foo err  lt   dev null    which seems to work for me  With nohup  you don t need to append the  amp  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  gt   dev null 2 gt  amp 1    to redirect all output to  dev null

User · Answer

It appeared quite convenient for me to have a remote tmux session using the tmux new -d  lt shell cmd gt  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

[bash] Getting ssh to execute a command in the background on target machine

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