bash: ram disk and trap shutdown

cache2ram.sh

#!/bin/bash

# Auto-filled variables
USER_NAME=$(/usr/bin/who am i | /usr/bin/awk '{print $1}')
SELF=$(/usr/bin/basename $0)
USER_ID=$(id | sed 's/uid=\([0-9]*\).*/\1/')
OUR_PID=$(ps -ef | grep ${SELF} | grep -v 'grep' | awk -v uid="$USER_ID" '$1==uid {print $2; exit;}') # Only first occurence from the current user.


# Constants
RSYNC="/usr/bin/rsync -aNHAXx --fileflags --protect-decmpfs "
SLEEP_DELAY=1
let SAFETY_BACKUP_INTERVAL=(60*60*2)/SLEEP_DELAY # Create backup in case of power failure every 2 hours.

# Settings
RAMDISK_SIZE=256 # Size in Mega Bytes.
RAMDISK_NAME="RamDisk"
SNAPSHOT_LOCATION="/Users/${USER_NAME}/Library/CachesSnapshot${RAMDISK_NAME}/"

# Variables
COUNT=

# Operational function declaration
function log
{
  echo "${SELF}: $1"
}

function quit
{
  log "Quiting: $1"
  exit 0
}

function lock_ramdisk
{
   exec 5>"/Volumes/${RAMDISK_NAME}/.lock.${USER_NAME}"
}

function unlock_ramdisk
{
   exec 5>&-
   rm "/Volumes/${RAMDISK_NAME}/.lock.${USER_NAME}"
}

function create_ramdisk
{
  # Check if ramdisk is not already created and mounted.
  if [ ! -d "/Volumes/${RAMDISK_NAME}" ]; then
     log "Creating Ram disk (\"${RAMDISK_NAME}\") with a size of: ${RAMDISK_SIZE}MB."
     let RAMDISK_BLOCKSIZE=2048*${RAMDISK_SIZE} # Size in blocks.

     BLOCK_DEVICE=$(hdiutil attach -nomount ram://${RAMDISK_BLOCKSIZE})
     diskutil eraseVolume HFS+ "$RAMDISK_NAME" $BLOCK_DEVICE

     # Update so every user can write to it while pertaining its own
     # user permissions.
     mount -u -o owners "/Volumes/${RAMDISK_NAME}"
     chmod g+w "/Volumes/${RAMDISK_NAME}"

  else
     log "Ram disk (\"${RAMDISK_NAME}\") already created."
  fi

}

function restore_contents_from_snapshot
{
  # Check if snapshot exists
  if [ -d ${SNAPSHOT_LOCATION} ]; then
     DEST="/Volumes/${RAMDISK_NAME}/${USER_NAME}/"

     log "Restoring snapshot from \"${SNAPSHOT_LOCATION}\" to \"${DEST}\"."
     $RSYNC "${SNAPSHOT_LOCATION}" "${DEST}"
  else
     log "No restore required: \"${SNAPSHOT_LOCATION}\" not found."
  fi
}

function store_contents_on_snapshot
{
  SOURCE="/Volumes/${RAMDISK_NAME}/${USER_NAME}/"
  if [ ! -d "$SOURCE" ]; then
   quit "Directory: \"$SOURCE\" does not exist."
  fi

  log "Storing snapshot from \"${SOURCE}\" to \"${SNAPSHOT_LOCATION}\"."
  $RSYNC "${SOURCE}" "${SNAPSHOT_LOCATION}"
}


function store_periodically
{
   let COUNT=COUNT+1
	if [ "0$COUNT" -gt $SAFETY_BACKUP_INTERVAL ]; then
	   log "Starting power failure preventive backup."
	   store_contents_on_snapshot
	   let COUNT=0
   fi
}


function shutdown
{
  log "Shutdown request received."
  store_contents_on_snapshot
  unlock_ramdisk
  quit "Shutdown request finished."
}


# Shutdown: Register terminate signals.
trap shutdown TERM



# Startup
create_ramdisk
lock_ramdisk
restore_contents_from_snapshot

# Renice ourself to a low priority
log "Lowering priority from $OUR_PID to 19."
/usr/bin/renice 19 ${OUR_PID}

# Main loop
while [ 1 ]; do
	sleep ${SLEEP_DELAY}

	# Store to snapshot every hour for extra safety (power failure, etc.)
	store_periodically
done