Mac Geekery

As others have noted, there’s nothing all that new about Time Machine other than its UI. UNIX admins have been doing similar incremental backups for ages immemorial. Well, OS X is a UNIX, isn’t it? So it should follow that we can get the same bang-for-buck in Tiger (and possibly prior versions, too). For this, rsync and cpio are our friends today. Adam Knight did most of the footwork here; I just hacked it up into OS X-compatibility.

The Original

The original script¹ comes to us from Mike Rubel, as linked in the footnotes. The script he provides is pretty specific to Linux, though, and assumes the GNU cp (which we do not have) and a few Linuxy paths. I’ve cleaned up the script to point to the correct paths in OS X. BSD cp doesn’t do some of the same magic as GNU cp, so it also takes a bit of working around to get the same effect. Specifically, the GNU version allows you to copy as links instead of creating a new whole copy of the file, whereas the BSD version does not. Never fear, after a fashion, cpio can.

Ours

Whereas GNU cp allows us to do this:

$ cp -al foo1 foo2

In a BSD environment, we get to do this:

$ cd foo1 && /usr/bin/find . -print | cpio -dplm ../foo2

The means are not as elegant, I know, but it accomplishes the same ends. I also removed the root user check, as well as some volume mounting and remounting work that don’t apply to OS X (or at least, not for our purposes). In the end, we have:

#!/bin/bash
	

		
———————————————————————————————————
		
jc’s handy rotating-home-snapshot utility. modified from
		
mike’s handy rotating-filesystem-snapshot utility from
		
http://www.mikerubel.org/computers/rsync_snapshots/
		
———————————————————————————————————
	


	
unset PATH	# suggestion from H. Milz: avoid accidental use of $PATH

	

		
——————- file locations ————————————————————-

SNAP_FROM=/Users/doce

SNAPSHOT_RW=/Volumes/Backup/snapshots

EXCLUDES=~/bin/backup_exclude
	

	

		
——————- system commands used by this script ——————————

ECHO=/bin/echo

RM=/bin/rm

MV=/bin/mv

CP=/bin/cp

TOUCH=/usr/bin/touch

CPIO=/usr/bin/cpio

RSYNC=/usr/bin/rsync
	

	

		
——————- the script itself ———————————————————
	

	
if [ -d $SNAPSHOT_RW ]; then

	
	# rotating the snapshots

	
	# step 1: delete the oldest snapshot, if it exists:
	if [ -d $SNAPSHOT_RW/snap.3 ]; then
		$ECHO “Clearing oldest snapshot…”
		$RM -rf $SNAPSHOT_RW/snap.3
	fi

	
	# step 2: shift the middle snapshots(s) back by one, if they exist
	if [ -d $SNAPSHOT_RW/snap.2 ]; then
		$ECHO “Aging snap.2…”
		$MV $SNAPSHOT_RW/snap.2 $SNAPSHOT_RW/snap.3
	fi

	
	if [ -d $SNAPSHOT_RW/snap.1 ] ; then
		$ECHO “Aging snap.1…”
		$MV $SNAPSHOT_RW/snap.1 $SNAPSHOT_RW/snap.2
	fi

	
	# step 3: make a hard-link-only (except for dirs) copy of the latest snapshot,
	# if that exists
	if [ -d $SNAPSHOT_RW/snap.0 ] ; then
		$ECHO “Aging snap.0…”
		cd $SNAPSHOT_RW/snap.0 && /usr/bin/find . -print | $CPIO -dplm $SNAPSHOT_RW/snap.1
	fi

	
	# step 4: rsync from the system into the latest snapshot (notice that
	# rsync behaves like cp —remove-destination by default, so the destination
	# is unlinked first.  If it were not so, this would copy over the other
	# snapshot(s) too!
	$RSYNC -va —delete —delete-excluded —exclude-from=”$EXCLUDES” $SNAP_FROM $SNAPSHOT_RW/snap.0

	
	# step 5: update the mtime of snap.0 to reflect the snapshot time
	$TOUCH $SNAPSHOT_RW/snap.0

	
	# and thats it for home.

else
	$ECHO “$SNAPSHOT_RW not present. Dying.”

fi

Change the paths for $SNAPSHOT_RW and others as per your needs. As is, this will create four revolving snapshots (one “current” and three historical). Lather, rinse, and repeat the sections with mv commands to create a longer chain, if you wish. This is automated easily by simple inclusion in your crontab.

Rubel provides for hourly, daily, and weekly rotations with a few other scripts, which you can see at his site. Similar changes could be made to those scripts to effect the same results. I leave that as an excercise for the reader.

¹ This tip is derived from Easy Automated Snapshot-style Backups with Rsync and Incremental Backups Using Rsync.