[Adoby]

OK!

I accepted the implied challenge and improved the script.

Now I use flock to ensure that only one instance of the script can run, use zenity to provide some feedback while the snapshot is written by rsync and write a small log to each snapshot folder. Also it is now all in one file, this is no longer a general folder snapshotter that is used to take snapshots of the calibre libraries.

It is now a calibre launcher script that also takes a snapshot after calibre exits.

Both flock and zenity are installed by default in Ubuntu 14.04.01, and also in many other dists, I think. So typically no need to do anything other than fill in the correct paths, place the script in the file ~/bin/calibre and make it executable.

~/bin/calibre

Spoiler:

#!/bin/sh
# calibre launcher that also takes a snapshot of the calibre libraries. 
# Call it just "calibre" and place in ~/bin and make executable
# Makes use of rsync and hardlinks to create snapshots that look like full backups. 
# Don't use or update files in a snapshot. Copy the snapshot first.
# Deletes snapshots older than a certain number of days.
# Uses flock(1) to ensure only one instance of this script can run.
# Uses zenity(1) to provide progress dialog and error message if needed.
# Both flock and zenity are installed by default in Ubuntu 14.04.

# Change these variables to customize:
snapshot_path='/full/path/to place to store snapshots'
source_path='/full/path/to place where the calibre library/libraries are'
age_snapshot='30'
snapshot_name=$(date +%F_%T)

# Use subshell and flock to ensure only one instance of this script can run
(
	# Exit if script is already running
	flock -n 9 || exit 1

 	# Run calibre
	/opt/calibre/calibre

	# Run the majority of the script in a subshell, and send the output from that subshell to zenity
	(
		# Output text to zenity progress dialog
		echo "10\n# Preparing for snapshot, please wait..."

		# Delete earlier failed attempt at making a snapshot, if any
		rm -rf "$snapshot_path"/unfinished		

		# Find latest snapshot, if any
		latest_snapshot=$(ls -rt "$snapshot_path" | tail -1)

		# Create folder for this snapshot, unfinished so far...
		mkdir "$snapshot_path"/unfinished

		# Grab new snapshot. Use rsync to create hard links to files already in latest previous snapshot(s).

		echo "40\n# Taking snapshot with rsync, please wait..."

		rsync -a --delete --stats \
		      --link-dest="$snapshot_path"/"$latest_snapshot" \
		      "$source_path" \
		      "$snapshot_path"/unfinished > "$snapshot_path"/unfinished/rsync.log

		# Finished!
		if [ $? -eq 0 ]; then
			mv "$snapshot_path"/unfinished "$snapshot_path"/"$snapshot_name"
		else
			exit 1
		fi

		# Delete snapshots older than $age_snapshot

		echo "# Deleting old snapshots, please wait..."

		find $snapshot_path -maxdepth 1 -type d -mtime +$age_snapshot -print0 | xargs -0 rm -rf

		echo "99\n# Snapshot done! Check rsync.log in snapshot folder."
		sleep 2
		echo "100"
	) |
	zenity --progress --auto-close --pulsate \
	  --title="Taking snapshot of calibre library" \
	  --text="Please wait..." \
	  --percentage=1

	# Snapshot failed?
	if [ "$?" = 1 ] ; then
	        zenity --error --text="Snapshot failed."
	fi

) 9>/var/lock/snapshot_calibre_lock