Shell script to take periodically ZFS snapshot

z-snap.sh

#!/bin/sh

set -eu

# 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=3

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=$((count + 1))

for pool in $pools
do
    snapname="${time_tag}_$(date '+%Y-%m-%dT%H:%M:%S')"
    zfs snapshot -r "$pool@$snapname" || exit 1

    expired=$(
        zfs list -H -t snapshot -o name -d 1 "$pool" |
        grep "^${pool}@${time_tag}_" |
        sort -r |
        tail -n +"$count"
    )

    if [ -n "$expired" ]; then
        for e in $expired
        do
            zfs destroy -nv -r "$e" || exit 1
        done
    fi
done

exit 0