#!/usr/bin/bash
# Path to ZFS executable:
ZFS=/usr/sbin/zfs
# Parse arguments:
TARGET=$1
SNAP=$2
COUNT=$3
mount=`$ZFS get -H -o value mountpoint $TARGET`
# Function to display usage:
usage() {
    scriptname=`/usr/bin/basename $0`
    echo "$scriptname: Take and rotate snapshots on a ZFS file system"
    echo
    echo "  Usage:"
    echo "  $scriptname target snap_name count"
    echo
    echo "  target:    ZFS file system to act on"
    echo "  snap_name: Base name for snapshots, to be followed by a '.' and"
    echo "             an integer indicating relative age of the snapshot"
    echo "  count:     Number of snapshots in the snap_name.number format to"
    echo "             keep at one time.  Newest snapshot ends in '.0'."
    echo
    exit
}
# Basic argument checks:
if [ -z $COUNT ] ; then
    usage
fi
if [ ! -z $4 ] ; then
    usage
fi
# Snapshots are number starting at 0; $max_snap is the highest numbered
# snapshot that will be kept.
max_snap=$(($COUNT -1))
# Clean up oldest snapshot:
if [ -d /$mount/.zfs/snapshot/$SNAP.$max_snap ] ; then
    $ZFS destroy -r $TARGET@$SNAP.$max_snap
fi
# Rename existing snapshots:
dest=$max_snap
while [ $dest -gt 0 ] ; do
    src=$(($dest - 1))
    if [ -d /$mount/.zfs/snapshot/$SNAP.$src ] ; then
    $ZFS rename -r $TARGET@$SNAP.$src $TARGET@$SNAP.$dest
    fi
    dest=$(($dest - 1))
done
# Create new snapshot:
$ZFS snapshot -r $TARGET@$SNAP.0