#!/bin/bash
#
# rm2-backup
# John Simpson <jms1@jms1.net> 2023-06-29
#
# Back up the reMarkable 2 tablet, using rsync with the '--link-dest' option
# so each backup only "contains" the files which changed from the previous
# backup.
#
# Requirements:
#
# - SSH access to the tablet. Ideally you should have key-based authentication
#   set up, otherwise you'll have to type the tablet's password when running
#   the script.
#
# - The location where you're storing the backups should be on a UNIX-type
#   filesystem which supports "hard links" (i.e. multiple filenames pointing
#   to the same inode). This is true of most Linux filesytems, as well as the
#   macOS "HFS+" and "APFS" filesystems.
#
# - rsync version 3.1.1 or higher. In earlier versions (such as the version
#   that Apple includes with macOS), the '--link-dest' option didn't use hard
#   links, and every backup conained a full copy of the tablet's filesystem,
#   which took longer and used a LOT more disk space.
#
# If you're using macOS and are stuck with its ancient version of rsync, you
# can install a newer/working version from Homebrew. https://brew.sh/
#
# 2023-07-17 jms1 - Add 'latest' symlink in the backup directory.
#
# 2023-08-04 jms1 - Add serial number to backup directory
#
# 2023-08-05 jms1 - *always* use serial number in backup directory name
#
# 2023-08-18 jms1 - Adding a LIST of default backup directory locations.
#
###############################################################################
#
# The MIT License (MIT)
#
# Copyright (C) 2023 John Simpson
#
# Permission is hereby granted, free of charge, to any person obtaining a
# copy of this software and associated documentation files (the “Software”),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
# DEALINGS IN THE SOFTWARE.
#
###############################################################################

########################################
# How to access the tablet.

TABLET="10.11.99.1"

########################################
# Where you plan to store the backup images.
# - The script walks through this list. The first location which exists as a
#   directory, will be used.
# - If none of them exist, the script will fail.
# - The '-b' option overrides this list.

BACKUP_LOCATIONS=(
    "/Volumes/rm2-backups"
    "$HOME/rm2-backups"
)

###############################################################################
#
# Coloured line functions

function blueline {
    if [[ -t 1 ]]
    then
        printf "\e[0;1;37;44m%s\e[0K\e[0m\n" "$*"
    else
        echo "===== $* ====="
    fi
}

###############################################################################
#
# usage

function usage {
    MSG="${*:-}"

    cat <<EOF
$0 [options]

Create a time-based backup of a reMarkable or reMarkable 2 tablet.

Time-based backups are written to directories named after the time when the
backup started. The script will identify the most recent backup, and any files
which have not changed since that backup will be stored as "hard links" to the
same file in the previous backup, rather than copying and storing another copy
of the same file.

-t ___  Specify the tablet's IP address.
        Default: 10.11.99.1

-b ___  Specify the directory in which the time-based backups will be written.
        Default: the first of the following which exists as a directory:
EOF

    for D in ${BACKUP_LOCATIONS[@]}
    do
        echo "        - $D"
    done

    cat <<EOF

-h      Show this help message.

EOF

    if [[ -n "$MSG" ]]
    then
        echo "$MSG"
        exit 1
    fi

    exit 0
}

###############################################################################
###############################################################################
###############################################################################
#
# Handle the command line

BACKUPS=""

while getopts ':hb:t:' OPT
do
    case $OPT in
        h)  usage
            ;;
        b)  BACKUPS="${OPTARG}"
            ;;
        t)  TABLET="${OPTARG}"
            ;;
        *)  usage "ERROR: unknown option '-${OPTARG}'"
            ;;
    esac
done

shift $(( OPTIND - 1 ))

###############################################################################
#
# Figure out where we're storing the backups

if [[ -z "$BACKUPS" ]]
then
    for D in "${BACKUP_LOCATIONS[@]}"
    do
        if [[ -d "$D" ]]
        then
            BACKUPS="$D"
            break
        fi
    done

    if [[ -z "$BACKUPS" ]]
    then
        usage "ERROR: no backup location specified or found, cannot continue"
    fi
fi

########################################
# Make sure the backup location exists
# - may not be true if it was specified using the '-b' option

BACKUPS="${BACKUPS%/}"

if [[ ! -d "$BACKUPS" ]]
then
    usage "ERROR: backup location '$BACKUPS' does not exist or is not a directory, cannot continue"
fi

blueline "Backing up to $BACKUPS"

###############################################################################
#
# Get the connected tablet's serial number

BDEV=$( set -x ; ssh "root@$TABLET" mount | awk '/\/home/{print substr($1,1,index($1,"p")-1)}' )
SERIAL=$( set -x ; ssh "root@$TABLET" "dd if=${BDEV}boot1 bs=1 skip=4 count=15 2>/dev/null" )

########################################
# Make sure the tablet's serial number directory exists

mkdir -p "$BACKUPS/$SERIAL"

########################################
# Get current timestamp, used for the backup directory name.

NOW="$( date -u '+%Y-%m-%dT%H%M%SZ' )"

########################################
# Find the most recent previous backup.
# - This assumes that the timestamps all start with "2", and that the 'ls'
#   command sorts the names correctly. THIS WILL BREAK in the year 3000.

PREV="$( ls -d1 "$BACKUPS/$SERIAL"/2* | tail -1 )"
if [[ -n "$PREV" ]]
then
    LINKPREV="--link-dest=$PREV"
else
    LINKPREV=""
fi

###############################################################################
#
# Back up the files

set -x

rsync -avzHl $LINKPREV  \
    --exclude   /dev    \
    --exclude   /proc   \
    --exclude   /run    \
    --exclude   /sys    \
    "root@$TABLET:/"    \
    "$BACKUPS/$SERIAL/$NOW/"

rm -f "$BACKUPS/$SERIAL/latest"
ln -s "$NOW" "$BACKUPS/$SERIAL/latest"