#!/bin/bash
#
# backup-rm2
# 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", "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/

########################################
# How to access the tablet.
# - The last character should be "/".

TABLET="root@10.11.99.1:/"

########################################
# Where you plan to store the backup images.

BACKUPS="$HOME/rm2-backups/"

###############################################################################
###############################################################################
###############################################################################
#
# Make sure the location where we're storing the backups, exists.

if [[ ! -d "$BACKUPS" ]]
then
    echo "ERROR: '$BACKUPS' does not exist or is not a directory, cannot continue"
    exit 1
fi

########################################
# 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"/2* | tail -1 )"
if [[ -n "$PREV" ]]
then
    LINKPREV="--link-dest=$PREV"
else
    LINKPREV=""
fi

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

set -x

rsync -avzHl $LINKPREV  \
    --exclude   /dev    \
    --exclude   /proc   \
    --exclude   /run    \
    --exclude   /sys    \
    "$TABLET"         \
    "${BACKUPS%/}/$NOW/"