A template to write a quick daemon as a bash script

skeleton-daemon.sh

#!/bin/bash

# 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"

myPid=$$
pidDir="."
pidFile="$pidDir/$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.
################################################################################


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=$(( $(stat --printf="%s" "$logFile") / 1024 ))
    if [[ $size -gt $logMaxSize ]]; then
      mv $logFile "$logFile.$(date +%Y-%m-%dT%H-%M-%S).old"
      touch "$logFile"
    fi
  fi
}

startDaemon() {
  # Start the daemon.
  setupDaemon # Make sure the directories are there.
  if ! checkDaemon; then
    echo 1>&2 " * Error: $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; then
    echo 1>&2 " * Error: $daemonName is not running."
    exit 1
  fi
  echo " * Stopping $daemonName"

  if [[ ! -z $(cat "$pidFile") ]]; then
    kill -9 $(cat "$pidFile") &> /dev/null
    log '*** '$(date +"%Y-%m-%d")": $daemonName stopped."
  else
    echo 1>&2 "Cannot find PID of running daemon"
  fi
}

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

restartDaemon() {
  # Restart the daemon.
  if checkDaemon; 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 "$oldPid" ]]; then
    return 0
  elif ps -ef | grep "$oldPid" | grep -v grep &> /dev/null ; then
    if [[ -f "$pidFile" && $(cat "$pidFile") -eq $oldPid ]]; then
      # Daemon is running.
      return 1
    else
      # Daemon isn't running.
      return 0
    fi
  elif ps -ef | grep "$daemonName" | grep -v grep | grep -v "$myPid" | grep -v "0:00.00" | grep bash &> /dev/null ; then
    # Daemon is running but without the correct PID. Restart it.
    log '*** '$(date +"%Y-%m-%d")": $daemonName running with invalid PID; restarting."
    restartDaemon
    return 1
  else
    # Daemon not running.
    return 0
  fi
  return 1
}

loop() {
  while true; do
    # 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
  done
}

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 1>&2 " * Error: usage $0 { start | stop | restart | status }"
  exit 1
esac

exit 0

Made changes and fixed bugs

• Changed from sh to bash
• Relocated "myPid" variable to top of file and removed unnecessary "echo"
• Removed extra "pidFile" definition that was overriding correct pidfile location
• Changed backticks to newer/preferred "$()" form for command substitution
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• Used "stat" instead of "ls" to find log file size
• Added date and time to rotated log file to avoid overwriting
• Fixed conditionals that were checking output of "checkDaemon" function rather than it's exit code
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• Removed shell formatting codes in error output and printed them to stderr
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• Quoted "pidFile" variable to make safe for spaces in filename
• Moved log message for when daemon is stopped to after it is actually stopped
• Added condition for if pid cannot be read from file
• Fixed conditions in "checkDaemon" using "ps" command to find status of running daemon
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• Changed the "loop" function to actually loop rather than recurse
• Changed single bracket to the preferred double brackets for extended test commands
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This version fixed several bugs I was experiencing with the parent copy. Thank you!