#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

# Backup a ZFS Filesystem.
#
# Uses ZFS `send` and `receive` to perform incremental backups for a filesystem
# and its descendents. Snapshots created to facilitate this are formatted as
# "manualbackup-YYYY-MM-DD".

[[ $# != 2 ]] && echo "Usage: ${0##*/} target_filesys backup_filesys" && exit 1
fsFrom=$1
fsTo=$2

# Check that filesystems exist. Also checks that `zfs` exists.
zfs list -Hd 0 $fsFrom >/dev/null
zfs list -Hd 0 $fsTo >/dev/null
echo "Source:        $fsFrom"
echo "Destination:   $fsTo"

snapPrefix="manualbackup"

# Look for a previous snapshot created by this script.
snapLine=$(zfs list -Ht snapshot | grep $snapPrefix | head -n 1 || true)
prevSnap=$(echo $snapLine | sed 's/.*@\([^ ]*\)\ *.*/\1/')
echo "Prev Snapshot: ${prevSnap:-none}"

# Next snapshot will be marked with today's date.
nextSnap=$snapPrefix-$(date +%Y-%m-%d)
echo "Next Snapshot: $nextSnap"
echo ""

# Same snapshot? Nothing to do.
[[ $prevSnap = $nextSnap ]] && echo "Not backing up." && exit

# pv check: displays transfer speed during backup.
hasPv=$(command -v pv || true)
[[ -z $hasPv ]] && echo "Note: Install \`pv\` for transfer speed during backup."

# tmux check: helps avoid accidentally stopping the backup process.
if [[ -z ${TMUX:-} ]]; then
  echo "Backing up can take a while! You really should run this in \`tmux\`."
  echo "Type 'No need for tmux' to run anyway, or press Enter to exit."
  read ANS
  [[ $ANS != "No need for tmux" ]] && exit
fi

echo "Ready. Proceed? (y/n)"
read ANS
[[ $ANS != "y" ]] && exit

echo "Creating snapshot $fsFrom@$nextSnap"
zfs snapshot -r $fsFrom@$nextSnap || true

# Previous snapshot exists?
# No = perform an initial full backup.
# Yes = perform an incremental backup.
if [[ -z $prevSnap ]]; then
  echo "Initial full backup starting..."
  if [[ -n $hasPv ]]; then
    zfs send -R $fsFrom@$nextSnap | pv | zfs receive -F $fsTo
  else
    zfs send -R $fsFrom@$nextSnap | zfs receive -F $fsTo
  fi
else
  echo "Incremental backup starting..."
  if [[ -n $hasPv ]]; then
    zfs send -R -i $fsFrom@$prevSnap $fsFrom@$nextSnap | pv | zfs receive -F $fsTo
  else
    zfs send -R -i $fsFrom@$prevSnap $fsFrom@$nextSnap | zfs receive -F $fsTo
  fi
fi
echo "ZFS Sending/Receiving done."

echo $'\nDestroying previous snapshot and finalising backup.'
zfs destroy -R $fsFrom@$prevSnap
zfs destroy -R $fsTo@$prevSnap

echo $'\nScrubbing backup filesystem'
zpool scrub $fsTo