Shell script to take periodically ZFS snapshot

z-snap.sh

#!/bin/sh

# crontab
# ---
# */5   *   *   *   *   z-snap.sh minutely
# 1     *   *   *   *   z-snap.sh hourly
# 2     3   *   *   *   z-snap.sh daily
# 3     4   *   *   0   z-snap.sh weekly
# 4     5   1   *   *   z-snap.sh monthly

monthly_count=12
weekly_count=4
daily_count=7
hourly_count=24
minutely_count=12

pools="tank"

PATH="/bin:/usr/bin:/sbin:/usr/sbin:/usr/local/bin:$PATH"

time_tag=$1

case $time_tag in
    "monthly")
        count=$monthly_count;;
    "weekly")
        count=$weekly_count;;
    "daily")
        count=$daily_count;;
    "hourly")
        count=$hourly_count;;
    "minutely")
        count=$minutely_count;;
    *)
        exit 1;;
esac
count=`expr $count + 1`

for pool in $pools
do
    snapname=${time_tag}_`date '+%Y-%m-%dT%H:%M:%S'`
    zfs snapshot -r $pool@$snapname
    
    expired=`zfs list -H -t snapshot -o name -d 1 | fgrep $pool@$time_tag | sort -r | tail -n +$count`
    if [ -n "$expired" ]; then
        for e in $expired
        do
            zfs destroy -r $e
        done
    fi
done

exit 0