How to prevent a background process from being stopped after closing SSH client in Linux [closed]

Question

I'm working on a Linux machine through SSH (Putty). I need to leave a process running during the night, so I thought I could do that by starting the process in background (with an ampersand at the end of the command) and redirecting stdout to a file.

To my surprise, that doesn't work. As soon as I close the Putty window, the process is stopped.

How can I prevent that from happening??

Answer 1

Check out the "nohup" program.

Answer 2

I would recommend using GNU Screen. It allows you to disconnect from the server while all of your processes continue to run. I don't know how I lived without it before I knew it existed.

Answer 3

When the session is closed the process receives the SIGHUP signal which it is apparently not catching. You can use the nohup command when launching the process or the bash built-in command disown -h after starting the process to prevent this from happening:

> help disown
disown: disown [-h] [-ar] [jobspec ...]
     By default, removes each JOBSPEC argument from the table of active jobs.
    If the -h option is given, the job is not removed from the table, but is
    marked so that SIGHUP is not sent to the job if the shell receives a
    SIGHUP.  The -a option, when JOBSPEC is not supplied, means to remove all
    jobs from the job table; the -r option means to remove only running jobs.

Answer 4

daemonize? nohup? SCREEN? (tmux ftw, screen is junk ;-)

Just do what every other app has done since the beginning -- double fork.

# ((exec sleep 30)&)
# grep PPid /proc/`pgrep sleep`/status
PPid:   1
# jobs
# disown
bash: disown: current: no such job

Bang! Done :-) I've used this countless times on all types of apps and many old machines. You can combine with redirects and whatnot to open a private channel between you and the process.

Create as coproc.sh:

#!/bin/bash

IFS=

run_in_coproc () {
    echo "coproc[$1] -> main"
    read -r; echo $REPLY
}

# dynamic-coprocess-generator. nice.
_coproc () {
    local i o e n=${1//[^A-Za-z0-9_]}; shift
    exec {i}<> <(:) {o}<> >(:) {e}<> >(:)
. /dev/stdin <<COPROC "${@}"
    (("\$@")&) <&$i >&$o 2>&$e
    $n=( $o $i $e )
COPROC
}

# pi-rads-of-awesome?
for x in {0..5}; do
    _coproc COPROC$x run_in_coproc $x
    declare -p COPROC$x
done

for x in COPROC{0..5}; do
. /dev/stdin <<RUN
    read -r -u \${$x[0]}; echo \$REPLY
    echo "$x <- main" >&\${$x[1]}
    read -r -u \${$x[0]}; echo \$REPLY
RUN
done

and then

# ./coproc.sh 
declare -a COPROC0='([0]="21" [1]="16" [2]="23")'
declare -a COPROC1='([0]="24" [1]="19" [2]="26")'
declare -a COPROC2='([0]="27" [1]="22" [2]="29")'
declare -a COPROC3='([0]="30" [1]="25" [2]="32")'
declare -a COPROC4='([0]="33" [1]="28" [2]="35")'
declare -a COPROC5='([0]="36" [1]="31" [2]="38")'
coproc[0] -> main
COPROC0 <- main
coproc[1] -> main
COPROC1 <- main
coproc[2] -> main
COPROC2 <- main
coproc[3] -> main
COPROC3 <- main
coproc[4] -> main
COPROC4 <- main
coproc[5] -> main
COPROC5 <- main

And there you go, spawn whatever. the <(:) opens an anonymous pipe via process substitution, which dies, but the pipe sticks around because you have a handle to it. I usually do a sleep 1 instead of : because its slightly racy, and I'd get a "file busy" error -- never happens if a real command is ran (eg, command true)

"heredoc sourcing":

. /dev/stdin <<EOF
[...]
EOF

This works on every single shell I've ever tried, including busybox/etc (initramfs). I've never seen it done before, I independently discovered it while prodding, who knew source could accept args? But it often serves as a much more manageable form of eval, if there is such a thing.

Answer 5

nohup blah &

Substitute your process name for blah!

Answer 6

Personally, I like the 'batch' command.

$ batch
> mycommand -x arg1 -y arg2 -z arg3
> ^D

This stuffs it in to the background, and then mails the results to you. It's a part of cron.

Answer 7

For most processes you can pseudo-daemonize using this old Linux command-line trick:

# ((mycommand &)&)

For example:

# ((sleep 30 &)&)
# exit

Then start a new terminal window and:

# ps aux | grep sleep

Will show that sleep 30 is still running.

What you have done is started the process as a child of a child, and when you exit, the nohup command that would normally trigger the process to exit doesn't cascade down to the grand-child, leaving it as an orphan process, still running.

I prefer this "set it and forget it" approach, no need to deal with nohup, screen, tmux, I/o redirection, or any of that stuff.

Answer 8

As others have noted, to run a process in the background so that you can disconnect from your SSH session, you need to have the background process properly disassociate itself from its controlling terminal - which is the pseudo-tty that the SSH session uses.

You can find information about daemonizing processes in books such as Stevens' "Advanced Network Program, Vol 1, 3rd Edn" or Rochkind's "Advanced Unix Programming".

I recently (in the last couple of years) had to deal with a recalcitrant program that did not daemonize itself properly. I ended up dealing with that by creating a generic daemonizing program - similar to nohup but with more controls available.

Usage: daemonize [-abchptxV][-d dir][-e err][-i in][-o out][-s sigs][-k fds][-m umask] -- command [args...]
  -V          print version and exit
  -a          output files in append mode (O_APPEND)
  -b          both output and error go to output file
  -c          create output files (O_CREAT)
  -d dir      change to given directory
  -e file     error file (standard error - /dev/null)
  -h          print help and exit
  -i file     input file (standard input - /dev/null)
  -k fd-list  keep file descriptors listed open
  -m umask    set umask (octal)
  -o file     output file (standard output - /dev/null)
  -s sig-list ignore signal numbers
  -t          truncate output files (O_TRUNC)
  -p          print daemon PID on original stdout
  -x          output files must be new (O_EXCL)

The double-dash is optional on systems not using the GNU getopt() function; it is necessary (or you have to specify POSIXLY_CORRECT in the environment) on Linux etc. Since double-dash works everywhere, it is best to use it.

You can still contact me (firstname dot lastname at gmail dot com) if you want the source for daemonize.

However, the code is now (finally) available on GitHub in my SOQ (Stack Overflow Questions) repository as file daemonize-1.10.tgz in the packages sub-directory.

Answer 9

On a Debian-based system (on the remote machine) Install:

sudo apt-get install tmux

Usage:

tmux

run commands you want

To rename session:

Ctrl+B then $

set Name

To exit session:

Ctrl+B then D

(this leaves the tmux session). Then, you can log out of SSH.

When you need to come back/check on it again, start up SSH, and enter

tmux attach session_name

It will take you back to your tmux session.

Answer 10

If you use screen to run a process as root, beware of the possibility of privilege elevation attacks. If your own account gets compromised somehow, there will be a direct way to take over the entire server.

If this process needs to be run regularly and you have sufficient access on the server, a better option would be to use cron the run the job. You could also use init.d (the super daemon) to start your process in the background, and it can terminate as soon as it's done.

Answer 11

nohup is very good if you want to log your details to a file. But when it goes to background you are unable to give it a password if your scripts ask for. I think you must try screen. its a utility you can install on your linux distribution using yum for example on CentOS yum install screen then access your server via putty or another software, in your shell type screen. It will open screen[0] in putty. Do your work. You can create more screen[1], screen[2], etc in same putty session.

Basic commands you need to know:

To start screen

screen

---

To create next screen

ctrl+a+c

---

To move to next screen you created

ctrl+a+n

---

To detach

ctrl+a+d

---

During work close your putty. And next time when you login via putty type

screen -r

To reconnect to your screen, and you can see your process still running on screen. And to exit the screen type #exit.

For more details see man screen.

Answer 12

Nohup allows a client process to not be killed if a the parent process is killed, for argument when you logout. Even better still use:

nohup /bin/sh -c "echo \$\$ > $pidfile; exec $FOO_BIN $FOO_CONFIG  " > /dev/null

Nohup makes the process you start immune to termination which your SSH session and its child processes are kill upon you logging out. The command i gave provides you with a way you can store the pid of the application in a pid file so that you can correcly kill it later and allows the process to run after you have logged out.

Answer 13

Use screen. It is very simple to use and works like vnc for terminals. http://www.bangmoney.org/presentations/screen.html

Answer 14

There's also the daemon command of the open-source libslack package.

daemon is quite configurable and does care about all the tedious daemon stuff such as automatic restart, logging or pidfile handling.

Answer 15

If you're willing to run X applications as well - use xpra together with "screen".

Answer 16

i would also go for screen program (i know that some1 else answer was screen but this is a completion)

not only the fact that &, ctrl+z bg disown, nohup, etc. may give you a nasty surprise that when you logoff job will still be killed (i dunno why, but it did happened to me, and it didn't bother with it be cause i switched to use screen, but i guess anthonyrisinger solution as double forking would solve that), also screen have a major advantage over just back-grounding:

screen will background your process without losing interactive control to it

and btw, this is a question i would never ask in the first place :) ... i use screen from my beginning of doing anything in any unix ... i (almost) NEVER work in a unix/linux shell without starting screen first ... and i should stop now, or i'll start an endless presentation of what good screen is and what can do for ya ... look it up by yourself, it is worth it ;)

Answer 17

Append this string to your command: >&- 2>&- <&- &. >&- means close stdout. 2>&- means close stderr. <&- means close stdin. & means run in the background. This works to programmatically start a job via ssh, too:

$ ssh myhost 'sleep 30 >&- 2>&- <&- &'
# ssh returns right away, and your sleep job is running remotely
$

Answer 18

Accepted answer suggest using nohup. I would rather suggest using pm2. Using pm2 over nohup has many advantages, like keeping the application alive, maintain log files for application and lot more other features. For more detail check this out.

To install pm2 you need to download npm. For Debian based system

sudo apt-get install npm

and for Redhat

sudo yum install npm

Or you can follow these instruction. After installing npm use it to install pm2

npm install pm2@latest -g

Once its done you can start your application by

$ pm2 start app.js              # Start, Daemonize and auto-restart application (Node)
$ pm2 start app.py              # Start, Daemonize and auto-restart application (Python)

For process monitoring use following commands:

$ pm2 list                      # List all processes started with PM2
$ pm2 monit                     # Display memory and cpu usage of each app
$ pm2 show [app-name]           # Show all informations about application

Manage processes using either app name or process id or manage all processes together:

$ pm2 stop     <app_name|id|'all'|json_conf>
$ pm2 restart  <app_name|id|'all'|json_conf>
$ pm2 delete   <app_name|id|'all'|json_conf>
     

Log files can be found in

$HOME/.pm2/logs #contain all applications logs

---

Binary executable files can also be run using pm2. You have to made a change into the jason file. Change the "exec_interpreter" : "node", to "exec_interpreter" : "none". (see the attributes section).

#include <stdio.h>
#include <unistd.h>  //No standard C library
int main(void)
{
    printf("Hello World\n");
    sleep (100);
    printf("Hello World\n");
   
    return 0;
}

Compiling above code

gcc -o hello hello.c  

and run it with np2 in the background

pm2 start ./hello

Answer 19

I used screen command. This link has detail as to how to do this

https://www.rackaid.com/blog/linux-screen-tutorial-and-how-to/#starting

Answer 20

On systemd/Linux, systemd-run is a nice tool to launch session-independent processes.