added cront-rsync file

Backups-and-Security.md

@@ -1,6 +1,6 @@
 Having an external backup of your home directory is mandatory. Here are a few ways to back up your data, from simplest to most feature rich.
 
-- Using cron + rsync (can be used to backup to a server)
+- Using [cron + rsync](cron-rsync) (can be used to backup to a server)
 - On Gnome: using DejaDup (duplicity)
 - On KDE/Plasma: using KBackup or kup
 - (advanced) using zfs or btrfs snapshots

cron-rsync.md

@@ -0,0 +1,37 @@
+# Backups with cron and rsync
+
+Here is a sample script for incremental backups with rsync. It uses physical
+links to avoid data duplication.
+
+```bash
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+if [ $# -ne 2 ]; then
+    echo "usage: $0 source destination"
+    exit 1
+fi
+
+SRCDIR=$(realpath $1)
+BACKUPDIR=$(realpath $2)
+CURRENTDIR="$BACKUPDIR/current"
+
+POSTFIX=$(date --iso-8601=seconds)
+CURRENT_BCKP="$BACKUPDIR/$POSTFIX/"
+
+# Backup nextcloud data
+rsync -Aax --info=STATS1 --delete --link-dest="$CURRENTDIR" "$SRCDIR" "$CURRENT_BCKP"
+
+# Create "current" symbolic link to latest backup
+ln -fns $(basename "$CURRENT_BCKP") "$CURRENTDIR"
+```
+
+One can use cron to execute a daily backup to an external drive. To add a new
+scheduled task, use `crontab -e` and something like:
+
+```
+0 12 * * * /path/to/backup.sh $HOME /mnt/drive/path/to/backupdir
+```
+
+This will run the backup task every day at noon.