CODEWITHSUNDEEP
sshprocessshremote-server
esantix
esantix

Reputation: 381

Detach command from SSH session

I need to run commands on a server via an SSH session and close the session without killing the process on the server. How can this be implemented correctly.

Server and client are linux systems.

Please note than I can't install additional software on server.

Upvotes: 5

Views: 3509

Answers (4)

F. Hauri - Give Up GitHub
F. Hauri - Give Up GitHub

Reputation: 70852

Before using special tools

  1. Undertanding

    Why do ssh not return when I try to background job on target.

    ssh user@host 'sleep 300 &'

    Then you have to hit Ctrl+C to get your session.

    This is because each connection hold 3 file descriptors: STDIN (0), STDOUT (1) and STDERR (2). The connection will stay open while at least one of this file descriptors are in use.

  2. Simply closing them before running background task

    This is aproximatively what nohup do.

    ssh user@host 'exec 0<&-;exec 1>&-;exec 2>&-; sleep 300 &'

    This will do the job.

    Or by using nohup:

    ssh user@host 'nohup sleep 300 &'

    Will do the job too.

  3. Redirecting output to static file(s)

    You could store output in files located somewhere on server side:

    ssh user@host 'exec 0<&-;exec 1>>/path/to/logfile;exec 2>>/path/to/errlog; sleep 300 &'

    >> for adding log to existing files... you could use uniq files by adding $$ or date +%F...

    ssh user@host 'exec 0<&-;exec 1>/path/to/logfile-$$;exec 2>&1; sleep 300 &'

    2>&1 cited after 1>file will redirect STDERR as STDOUT to file.

    ssh user@host 'exec 0<&-;exec 1>/path/to/file-$(date +%F-%T).$$;exec 2>&-;sleep 300 &'

    Only STDOUT will be stored in newfiles.

Using screen

  1. If screen is installed

    If you would be able to access interactive front end, you could:

    ssh user@host screen -dmS mySleep sh -c \
          "'x=0;while [ \$x -lt 300 ];do echo \$x;x=\$((x+1));sleep 1;done'"

    This will start a background job, printing 1 ling each 300 next second, then finish.

    To reconnect this, you could

    ssh -t user@host screen -x mySleep

    Then Ctrl+A, followed by d for leaving background task.

  2. Installing screen at user level

    If you can't install something in server, maybe* could you install something in your $HOME directory:

    (* This will be easy if your architecture is same than on server. If not, you even could find compatible binaries or cross compile yourself.)

     $ ssh user@host
 user@host:~$ mkdir bin lib
 user@host:~$ exit
 $ scp /path/to/bin/screen user@host:~/bin
 $ ssh user@host
 user@host:~$ bin/screen
 bin/screen: error while loading shared libraries: libutempter.so.0:
           cannot open shared object file: No such file or directory
 user@host:~$ exit
 $ scp /path/to/lib/libutempter.so.0 user@host:~/lib
 $ ssh user@host
 user@host:~$ LD_LIBRARY_PATH=~/lib bin/screen
 bin/screen: /lib/x86_64-linux-gnu/libcrypt.so.1: version `XCRYPT_2.0' 
           not found (required by bin/screen)
 user@host:~$ exit
 $ scp /lib/x86_64-linux-gnu/libcrypt.so.1  user@host:~/lib
 $ ssh user@host
 user@host:~$ LD_LIBRARY_PATH=~/lib bin/screen
 Cannot make directory '/run/screen': Permission denied
 user@host:~$ mkdir $HOME/.screen
 user@host:~$ SCREENDIR=$HOME/.screen LD_LIBRARY_PATH=~/lib bin/screen

    Finally, now it work...

     $ ssh user@host SCREENDIR=\~/.screen LD_LIBRARY_PATH=\~/lib bin/screen -dmS mySleep sh -c \
          "'x=0;while [ \$x -lt 300 ];do echo \$x;x=\$((x+1));sleep 1;done'"
 $ ssh -t user@host SCREENDIR=\~/.screen LD_LIBRARY_PATH=\~/lib bin/screen -x mySleep

There are lot of other alternatives, like tmux, byobu...

Upto create your own wrapper...

Upvotes: 6

Nicolas Kagami
Nicolas Kagami

Reputation: 140

nohup is the best tool for this, in my opinion.

Simply use it like:

nohup command &

As taken from nohup's man page:

If standard input is a terminal, redirect it from /dev/null. If standard output is a terminal, append output to 'nohup.out' if possible, '$HOME/nohup.out' otherwise. If standard error is a terminal, redirect it to standard output. To save output to FILE, use 'nohup COMMAND > FILE'.

Upvotes: 5

1218985
1218985

Reputation: 8022

Have a look at reptyr. It is a utility for taking an existing running program and attaching it to a new terminal. You just need to start a screen, use reptyr to grab it, and then kill the ssh session. reptyr works by attaching to the target program using ptrace(2), redirecting relevant file descriptors, and changing the program's controlling terminal. After attaching a program, the program will appear to be either backgrounded or suspended to the shell it was launched from (depending on the shell). It is preferable to use bg; disown in the old shell to remove the association with the program, but reptyr will attempt to ensure that the target program remains running even if you close the shell without doing so.

Upvotes: 1

Timothy Brackett
Timothy Brackett

Reputation: 181

You can use the screen or tmux utilities for this, if one is installed.

Upvotes: 3

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