I've created a simple process that monitors if certain processes are being executed, and, if they don't, restart them again.

My first purpose for this is for monitor two minecraft server I am running in my server; so there are specific glitches to work only for this case. However, there are easily changed to fit other purposes.

#!/bin/bash
#
# watchdog - monitors a process
#
#
#

pidfileList[0]="/home/minecraft/pidfile"
pidfileList[1]="/home/minecraft2/pidfile"

startcmd[0]="/etc/init.d/minecraft start"
startcmd[1]="/etc/init.d/minecraft2 start"

logfile=/var/log/watchdog.log

tries=0

umask 022

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin


# first we delete the log file
rm "$logfile"

# to write a message to the log
function log()
{
    now=$(date +"%Y-%m-%d %H:%M:%S")
    echo "$now $1" >> $logfile
} # log


# do a lazy start: the first time it will wait
# 20 minutes to let the sistem to stabilize
log "Waiting 20 minutes to let the sistem stabilize..."
sleep 20m
log "Recovering..."

while [ true ] ; do


  for(( i = 0; i < 50; i++ )) ; do

    its_ok_to_launch=0
    if [ -n "${pidfileList[i]}" ] ; then

      # get the pidfile value
      pidfile="${pidfileList[i]}"
      log "Checking pidfile $pidfile..."
      # check the existence of this pidfile
      if [ -e "$pidfile" ] ; then
        # get the pid value
        pidvalue=$(cat $pidfile)
        log "The file exists and contains the value $pidvalue"
        # check existence of this pidvalue
        line=$(ps aux | grep $pidvalue | grep minecraft | grep -v grep)
        if [ -z "$line" ] ; then
          # the process doesn't exist
          # or the pid number doesn't correspond
          # to a minecraft server
          log "There is no process with id: $pidvalue"
          its_ok_to_launch=1
        fi # -z "$line"
      else
        # if the pidfile doesn't exist,
        # it is correct to launch it
        log "The file doesn't exist"
        its_ok_to_launch=1
      fi # -e $pidfile

    fi # -n pidfileList[i]

    if [ $its_ok_to_launch -eq 1 ] ; then

      tries=$((tries+1))
      if [ $tries -le 6 ]; then
        # attempt to start the process
        # if the maximum reach attempts
        # haven't been reached
        log "Attempting to start command ${startcmd[i]} (this is time $tries out
 of 6)"
        ${startcmd[i]}
      else
       if [ $tries -eq 6 ] ; then
         log "This is time number $tries, giving up"
       fi # tries -eq 6
      fi # tries -le 6

    fi # its_ok_to_launc -eq 1

  done # for

  log "Sleeping for 10 minutes...."
  sleep 10m
done # true

First, it waits 20 minutes, to save the case this program is configured to be run in the booting of the server and the monitored processes aren't being started:

# do a lazy start: the first time it will wait
# 20 minutes to let the sistem to stabilize
log "Waiting 20 minutes to let the sistem stabilize..."
sleep 20m
log "Recovering..."

Next, it will run forever, awakening for every ten minutes:

while [ true ] ; do

  ....

  log "Sleeping for 10 minutes...."
  sleep 10m
done # true

Next, a for loop is run to traverse the arrah pidfileList:

  for(( i = 0; i < 50; i++ )) ; do

  ....

  done # for

For every element in the array that is not empty….

    its_ok_to_launch=0
    if [ -n "${pidfileList[i]}" ] ; then

    ....

    fi # its_ok_to_launc -eq 1

Comes the real part. Get the content of the pidfile and put into a pidvalue variable:

      # get the pidfile value
      pidfile="${pidfileList[i]}"
      log "Checking pidfile $pidfile..."
      # check the existence of this pidfile
      if [ -e "$pidfile" ] ; then
        # get the pid value
        pidvalue=$(cat $pidfile)
        log "The file exists and contains the value $pidvalue"
        ....

Verify that this pidvalue correspond to a real, existing process:

        # check existence of this pidvalue
        line=$(ps aux | grep $pidvalue | grep minecraft | grep -v grep)
        if [ -z "$line" ] ; then
          # the process doesn't exist
          # or the pid number doesn't correspond
          # to a minecraft server
          log "There is no process with id: $pidvalue"
          its_ok_to_launch=1
        fi # -z "$line"
      else
        # if the pidfile doesn't exist,
        # it is correct to launch it
        log "The file doesn't exist"
        its_ok_to_launch=1
      fi # -e $pidfile

    fi # -n pidfileList[i]
    

And if the process doesn't exist for whatever reason (the file doesn't exist, or the pid number doesn't correspond to a real process), try to restart the file up to a limit of six times:

    if [ $its_ok_to_launch -eq 1 ] ; then

      tries=$((tries+1))
      if [ $tries -le 6 ]; then
        # attempt to start the process
        # if the maximum reach attempts
        # haven't been reached
        log "Attempting to start command ${startcmd[i]} (this is time $tries out
 of 6)"
        ${startcmd[i]}
      else
       if [ $tries -eq 6 ] ; then
         log "This is time number $tries, giving up"
       fi # tries -eq 6
      fi # tries -le 6
      

You have to configure the pid files to be monitorized (here is my example with the minecraft server):

pidfileList[0]="/home/minecraft/pidfile"
pidfileList[1]="/home/minecraft2/pidfile"

How this command are run in the event of a failure:

startcmd[0]="/etc/init.d/minecraft start"
startcmd[1]="/etc/init.d/minecraft2 start"

The location of the logfile:

logfile=/var/log/watchdog.log

And, in the case you have to use it for other purposes, how it's identified each process:

line=$(ps aux | grep $pidvalue | grep minecraft | grep -v grep)

I've need to add this grep minecraft to avoid errors: in some ocasion, the process failed and another program start to occupy this process number.

I've used the file /etc/rc.local to start this file and I've put this under /usr/local/sbin, but you can pick whatever directory best suits you.