Edit: First you may need to go to the HaikuDepot and install rsync.
I wrote a script and tested it. You’ll need to modify the paths inside the script to your own needs, because my setup is different from yours. 
Here’s the script:
#!/bin/bash
if [ "${1:+x}" != "x" ]; then
echo "argument must be source directory" >>/dev/stderr
exit 1
fi
set -o errexit
set -o nounset
set -o pipefail
SRC="$1"
if [ "$1" != "/" ];then SRC="${1%%/}"; fi
readonly NAS="/Haiku-data"
readonly SOURCE_DIR="$SRC"
readonly BACKUP_DIR="$NAS/Haiku-backups/$SRC"
readonly DATETIME="$(date '+%Y-%m-%d_%H-%M-%S')"
readonly BACKUP_PATH="${BACKUP_DIR}/${DATETIME}"
readonly LATEST_LINK="${BACKUP_DIR}/latest"
options=(
# -aAXv --delete
-aXv --delete # -A (ACL) is not supported on Haiku
"${SOURCE_DIR}/"
--link-dest "${LATEST_LINK}"
--exclude=".cache"
--exclude=".DS_Store"
"${BACKUP_PATH}"
)
mkdir -p "${BACKUP_DIR}"
rsync "${options[@]}"
rm -rf "${LATEST_LINK}"
ln -s "${BACKUP_PATH}" "${LATEST_LINK}"
The easiest way to install the script, would be to copy it into the non-packaged/bin/ directory after changing the NAS path:
chmod +x DoBackup
cp -RPp DoBackup ~/config/non-packaged/bin/
Then you can try it out. First create a test-folder for holding the files to backup:
cd
mkdir Documents
cd Documents
echo "hello" >testfile
Then back up the Documents folder by switching to your home directory, then issue the DoBackup command:
cd
DoBackup Documents
Now change the file in the Documents folder and back it up again …
cd $HOME/Documents
echo "I just added this line" >>testfile
cd
DoBackup Documents
Now go to the NAS/Haiku-backups/Documents and see; there are two folders with each their own timestamp and one symbolic link named “latest”
The ‘latest’ is just a symlink to the last backup you made
The folders are incremental backups and thus they use very little space.
Edit2:
–link-dest is the secret behind the incremental backup; this is where rsync is putting the new files.
-A is for ACL (access control lists) and is not available in Haiku.
-a is for archiving
-X is for xattrs
-v is verbose, so you can see what’s going on.
–delete means that files should be deleted when you delete them. This is not as destructive as it might sound, because you’ll still have the files in the dated folders. Try it out with a test-directory (eg. delete the ‘testfile’ in the Documents folder, then make a backup and look at the contents of /NAS/Haiku-backups/Documents/latest …
