Daemonizing a Process in Linux

04 / May / 2016 by Nitin Bhadauria 0 comments

Most of the times when we need to run some Java, Node.js or python program in background, so it could stay running even after you leave console all we do is put “&” in the end of the command.

$java -jar SimpleService.jar &

The problem here is that when you leave the bash shell your process will become “Orphan” and it’s up to the OS if INIT should adopt the process or kill the process.

To fix this we can force the process to be detached from your shell and become child process of INIT by putting nohup in front of command.

$nohup java -jar SimpleService.jar &

It’s all good for testing purposes, but services are supposed to run as “daemons” under some service supervisor. You can find many service supervisors with which you manage your services on Linux and we will cover few of the defaults that come with Linux.

Why do you need it ?

a. auto start/stop the service on startup/shutdown
b. controlling which service should start first
c. controlling which service should stop first is equally important
d. INIT controlled processes directly which provides more options like respawn, post-start, etc..

Ubuntu services using Upstart

Modify the script according to your setup:

$sudo vi /etc/init/java-app.conf

author "nitin"
description "start and stop java-app for Ubuntu (upstart)"
version "1.0"

start on started networking
stop on runlevel [!2345]

# Automatically Respawn a process if fails, try max 5 times in 60 sec then give up
respawn
respawn limit 5 60

# Open files Limit
limit nofile 32768 32768

env APPUSER="admin"
env APPBIN="/usr/bin/java"
env APPARGS="-jar /opt/app/SimpleService.jar"

script
  exec su - $APPUSER -c "$APPBIN $APPARGS"
end script

post-stop script
  sleep 2
end script

Check the configuration:
$init-checkconf /etc/init/java-app.conf

Reload New configuration:
$sudo initctl reload-configuration

Check is service is known to Upstart:
$sudo initctl list |grep java

Start the service with:
$sudo start java-app

Note: For any errors look into /var/log/syslog and /var/log/upstart/*

Debian and Ubuntu services using SysVinit

Modify the script according to your setup.

$sudo vi /etc/init.d/java-app

#!/bin/sh

### BEGIN INIT INFO
# Provides: daemon
# Required-Start: $local_fs $network $syslog
# Required-Stop: $local_fs $network $syslog
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description: Java-app
# Description: Java-app start-stop-daemon - Debian
### END INIT INFO

NAME="java-app"
PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin"
APPDIR="/opt/app"
APPBIN="/usr/bin/java"
APPARGS="-jar /opt/app/SimpleService.jar"
USER="admin"
GROUP="admin"

# Include functions
set -e
. /lib/lsb/init-functions

start() {
  printf "Starting '$NAME'... "
  start-stop-daemon --start --chuid "$USER:$GROUP" --background --make-pidfile --pidfile /var/run/$NAME.pid --chdir "$APPDIR" --exec "$APPBIN" -- $APPARGS || true
  printf "done\n"
}

#We need this function to ensure the whole process tree will be killed
killtree() {
  local _pid=$1
  local _sig=${2-TERM}
  for _child in $(ps -o pid --no-headers --ppid ${_pid}); do
    killtree ${_child} ${_sig}
  done
  kill -${_sig} ${_pid}
}

stop() {
  printf "Stopping '$NAME'... "
  [ -z `cat /var/run/$NAME.pid 2>/dev/null` ] || \
  while test -d /proc/$(cat /var/run/$NAME.pid); do
    killtree $(cat /var/run/$NAME.pid) 15
  sleep 0.5
  done
  [ -z `cat /var/run/$NAME.pid 2>/dev/null` ] || rm /var/run/$NAME.pid
  printf "done\n"
}

status() {
  status_of_proc -p /var/run/$NAME.pid "" $NAME && exit 0 || exit $?
}

case "$1" in
    start)
    start
  ;;
    stop)
    stop
  ;;
    restart)
    stop
    start
  ;;
    status)
    status
  ;;
    *)
    echo "Usage: $NAME {start|stop|restart|status}" >&2
    exit 1
  ;;
esac

exit 0

Make sure the script is executable:
$chmod +x /etc/init.d/java-app

Enable the service to auto start in runlevel 2345:
$update-rc.d java-app defaults

Start the service with:
$service java-app start

CentOS/Redhat services using SysVinit

Modify the parameter according to your setup:
$sudo vi /etc/init.d/example

#!/bin/sh
#
# example start stop daemon for CentOS (sysvinit)
#
# chkconfig: - 64 36
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 2 3 4 6
# Required-Start:
# description: java-app start stop daemon for CentOS
# processname: java-app
# pidfile: none
# lockfile: /var/lock/subsys/java-app

# Source function library.
. /etc/rc.d/init.d/functions

# Source networking configuration.
. /etc/sysconfig/network

# Check that networking is up.
[ "$NETWORKING" = "no" ] && exit 0

USER="admin"
APPNAME="java-app"
APPBIN="/usr/bin/java"
APPARGS="-jar /opt/app/SimpleService.jar"
LOGFILE="/var/log/$APPNAME/error.log"
LOCKFILE="/var/lock/subsys/$APPNAME"

LOGPATH=$(dirname $LOGFILE)

start() {
  [ -x $prog ] || exit 5
  [ -d $LOGPATH ] || mkdir $LOGPATH
  [ -f $LOGFILE ] || touch $LOGFILE

  echo -n $"Starting $APPNAME: "
  daemon --user=$USER "$APPBIN $APPARGS >>$LOGFILE &"
  RETVAL=$?
  echo
  [ $RETVAL -eq 0 ] && touch $LOCKFILE
  return $RETVAL
}

stop() {
  echo -n $"Stopping $APPNAME: "
  killproc $APPBIN
  RETVAL=$?
  echo
  [ $RETVAL -eq 0 ] && rm -f $LOCKFILE
  return $RETVAL
}

restart() {
  stop
  start
}

rh_status() {
  status $prog
}

rh_status_q() {
  rh_status >/dev/null 2>&1
}

case "$1" in
    start)
    rh_status_q && exit 0
    $1
  ;;
    stop)
    rh_status_q || exit 0
    $1
  ;;
    restart)
    $1
  ;;
    status)
    rh_status
  ;;
    *)
  echo $"Usage: $0 {start|stop|status|restart}"
  exit 2
esac

Make sure the script is marked as executable:
$sudo chmod +x /etc/init.d/java-app

Enable the service to auto start at runlevels 2345:
$sudo chkconfig java-app on

Start the service with:
$sudo service java-app start

This is how simple it is to daemonize a process. In my next blog, I will talk about more interesting features available in Linux OS.

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