#!/bin/bash
#
# btrfs-mksnapshot -- Make snapshots of btrfs filesystems
# Copyright (c)2013-2020 Alexander Barton (alex@barton.de)
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#

NAME=$(basename "$0")

Usage()
{
	echo "Usage: $NAME [-1] [<path>]"; echo
	echo " -1      Create only one shapshot per day."
	echo " -n      Dry run, perform a trial run with no changes made."
	echo " <path>  Volume root path (default: \"/\")."
	echo; exit 1
}

ONLY_ONE_PER_DAY=
DRY_RUN=

while [ $# -ge 1 ]; do
	case "$1" in
	  [^-]*)
		break
		;;
	  -1)
		ONLY_ONE_PER_DAY=1
		;;
	  -n)
		DRY_RUN=1
		;;
	  *)
		Usage
	esac
	shift
done
[ $# -gt 1 ] && Usage

[ -z "$DRY_RUN" ] || echo "ONLY_ONE_PER_DAY=$ONLY_ONE_PER_DAY"

# The subvolume to snapshot, "/" by default
[ $# -ge 1 ] && VOLUME_PATH="$1" || VOLUME_PATH="/"

[ -z "$DRY_RUN" ] || echo "VOLUME_PATH=$VOLUME_PATH"

# Detect btrfs filesystem
FS_NAME=$(btrfs filesystem show -m "$VOLUME_PATH" 2>/dev/null | head -1 | cut -d"'" -f2)
if [ -z "$FS_NAME" ]; then
	echo "$NAME: Failed to detect btrfs filesystem label for \"$VOLUME_PATH\"!"
	exit 1
fi

[ -z "$DRY_RUN" ] || echo "FS_NAME=$FS_NAME"

# Detect btrfs subvolume name
VOLUME_NAME=$(findmnt -nf -o source "$VOLUME_PATH" | grep -F '[' | sed -r 's|^.*\[/(.*)\]|\1|')
if [ -z "$VOLUME_NAME" ] && [ "$VOLUME_PATH" = "/" ]; then
	VOLUME_NAME=$(btrfs subvolume get-default / | cut -d' ' -f9)
fi
if [ -z "$VOLUME_NAME" ]; then
	echo "$NAME: Failed to detect btrfs subvolume name for \"$VOLUME_PATH\"!"
	exit 1
fi
BASE_VOLUME_NAME=$(echo "$VOLUME_NAME" | cut -d'@' -f1)

if [ -n "$DRY_RUN" ]; then
	echo "VOLUME_NAME=$VOLUME_NAME"
	echo "BASE_VOLUME_NAME=$BASE_VOLUME_NAME"
fi

# Detect mount point of the whole btrfs filesystem
FS_MOUNT_PATH="/media/$FS_NAME"
if [ ! -d "$FS_MOUNT_PATH" ]; then
	echo "$NAME: Directory \"$FS_MOUNT_PATH\" does not exist!"
	exit 1
fi
if ! grep -q " $FS_MOUNT_PATH btrfs " /proc/mounts; then
	echo "$NAME: btrfs \"$FS_NAME\" seems not to be mounted on \"$FS_MOUNT_PATH\"!"
	exit 1
fi

[ -z "$DRY_RUN" ] || echo "FS_MOUNT_PATH=$FS_MOUNT_PATH"

# Generate snapshot name
i=1
date=$(date +%Y%m%d)
while true; do
	id="$date-$i"
	[ -e "$FS_MOUNT_PATH/$BASE_VOLUME_NAME@$id" ] || break
	if [ -n "$ONLY_ONE_PER_DAY" ]; then
		echo "Snapshot \"$BASE_VOLUME_NAME@$id\" of \"$VOLUME_PATH\" (\"$VOLUME_NAME\") in btrfs \"$FS_NAME\" already exists."
		exit 0
	fi
	i=$((i + 1))
done
NEW_VOLUME_NAME="$BASE_VOLUME_NAME@$id"

[ -z "$DRY_RUN" ] || echo "NEW_VOLUME_NAME=$NEW_VOLUME_NAME"

[ -z "$DRY_RUN" ] || exit 0

echo "Creating snapshot of \"$VOLUME_PATH\" (\"$VOLUME_NAME\") in btrfs \"$FS_NAME\" ..."
set -x

cd "$FS_MOUNT_PATH" || exit 1
btrfs subvolume snapshot -r "$VOLUME_NAME" "$NEW_VOLUME_NAME"
find . -maxdepth 1 \( -name "$BASE_VOLUME_NAME" -o \
 -name "$BASE_VOLUME_NAME"@'*' \) -exec ls -1d {} \; | sort | column