Last Updated: February 25, 2016
·
3.676K
· shawnrice

Skeleton Daemon written in Bash

I needed to write a lightweight daemon for a project recently, so I wrote a skeleton of one in Bash. Use it if it's useful. Also, find it as a gist.

To adapt this for yourself, just change anything needed above the first long line of #####'s.

To launch it, I'd recommend something like:

nohup sh daemon.sh start > /dev/null 2>&1 &

All of the relevant information should be logged, so you shouldn't miss anything from STDOUT or STDERR. However, when you're testing it at first, it might be good to run it without the "nohup" and keep the output.

 #!/bin/sh

 # This is a skeleton of a bash daemon. To use for yourself, just set the
 # daemonName variable and then enter in the commands to run in the doCommands
 # function. Modify the variables just below to fit your preference.

 daemonName="DAEMON-NAME"

 pidDir="."
 pidFile="$pidDir/$daemonName.pid"
 pidFile="$daemonName.pid"

 logDir="."
 # To use a dated log file.
 # logFile="$logDir/$daemonName-"`date +"%Y-%m-%d"`".log"
 # To use a regular log file.
 logFile="$logDir/$daemonName.log"

 # Log maxsize in KB
 logMaxSize=1024   # 1mb

 runInterval=60 # In seconds

 doCommands() {
   # This is where you put all the commands for the daemon.
   echo "Running commands."
 }

############################################################
# Below is the skeleton functionality of the daemon.
############################################################

 myPid=`echo $$`

 setupDaemon() {
   # Make sure that the directories work.
   if [ ! -d "$pidDir" ]; then
     mkdir "$pidDir"
   fi
   if [ ! -d "$logDir" ]; then
     mkdir "$logDir"
   fi
   if [ ! -f "$logFile" ]; then
     touch "$logFile"
   else
     # Check to see if we need to rotate the logs.
     size=$((`ls -l "$logFile" | cut -d " " -f 8`/1024))
     if [[ $size -gt $logMaxSize ]]; then
       mv $logFile "$logFile.old"
       touch "$logFile"
     fi
   fi
 }

 startDaemon() {
   # Start the daemon.
   setupDaemon # Make sure the directories are there.
   if [[ `checkDaemon` = 1 ]]; then
     echo " * \033[31;5;148mError\033[39m: $daemonName is already running."
     exit 1
   fi
   echo " * Starting $daemonName with PID: $myPid."
   echo "$myPid" > "$pidFile"
   log '*** '`date +"%Y-%m-%d"`": Starting up $daemonName."

   # Start the loop.
   loop
 }

 stopDaemon() {
   # Stop the daemon.
   if [[ `checkDaemon` -eq 0 ]]; then
     echo " * \033[31;5;148mError\033[39m: $daemonName is not running."
     exit 1
   fi
   echo " * Stopping $daemonName"
   log '*** '`date +"%Y-%m-%d"`": $daemonName stopped."

   if [[ ! -z `cat $pidFile` ]]; then
     kill -9 `cat "$pidFile"` &> /dev/null
   fi
 }

 statusDaemon() {
   # Query and return whether the daemon is running.
   if [[ `checkDaemon` -eq 1 ]]; then
     echo " * $daemonName is running."
   else
     echo " * $daemonName isn't running."
   fi
   exit 0
 }

 restartDaemon() {
   # Restart the daemon.
   if [[ `checkDaemon` = 0 ]]; then
     # Can't restart it if it isn't running.
     echo "$daemonName isn't running."
     exit 1
   fi
   stopDaemon
   startDaemon
 }

 checkDaemon() {
   # Check to see if the daemon is running.
   # This is a different function than statusDaemon
   # so that we can use it other functions.
   if [ -z "$oldPid" ]; then
     return 0
   elif [[ `ps aux | grep "$oldPid" | grep "$daemonName" | grep -v grep` > /dev/null ]]; then
     if [ -f "$pidFile" ]; then
       if [[ `cat "$pidFile"` = "$oldPid" ]]; then
         # Daemon is running.
         # echo 1
         return 1
       else
         # Daemon isn't running.
         return 0
       fi
     fi
   elif [[ `ps aux | grep "$daemonName" | grep -v grep | grep -v "$myPid" | grep -v "0:00.00"` > /dev/null ]]; then
     # Daemon is running but without the correct PID. Restart it.
     log '*** '`date +"%Y-%m-%d"`": $daemonName running with invalid PID; restarting."
     restartDaemon
     return 1
   else
     # Daemon not running.
     return 0
   fi
   return 1
 }

 loop() {
   # This is the loop.
   now=`date +%s`

   if [ -z $last ]; then
     last=`date +%s`
   fi

   # Do everything you need the daemon to do.
   doCommands

   # Check to see how long we actually need to sleep for. If we want this to run
   # once a minute and it's taken more than a minute, then we should just run it
   # anyway.
   last=`date +%s`

   # Set the sleep interval
   if [[ ! $((now-last+runInterval+1)) -lt $((runInterval)) ]]; then
     sleep $((now-last+runInterval))
   fi

   # Startover
   loop
 }

 log() {
   # Generic log function.
   echo "$1" >> "$logFile"
 }


 ###############################################################
 # Parse the command.
 ###############################################################

 if [ -f "$pidFile" ]; then
   oldPid=`cat "$pidFile"`
 fi
 checkDaemon
 case "$1" in
   start)
     startDaemon
     ;;
   stop)
     stopDaemon
     ;;
   status)
     statusDaemon
     ;;
   restart)
     restartDaemon
     ;;
   *)
   echo "\033[31;5;148mError\033[39m: usage $0 { start | stop | restart | status }"
   exit 1
 esac

 exit 0