A script is to be created that runs on a TrueNAS server.
The server contains several datasets and storage hard drives.
The script is to back up an entire directory from another external server.
To do this:
1) The entire directory is to be copied locally (to the storage hard drives) using rsync.
2) Then, all changes should be versioned with a git commit (local git command).

This workflow is given and will not be changed. 
No backup tool or Git server will be used. Simply rsync to the disk + git commit. It's that simple.

# Configuration
No configuration must be in the script. 
A env file must be used for config.

# Git commit

Only create a new git commit, if something has changed.

# Rsync

No visual feedback or verbos mode needed.

recommended options:

-a  archive mode
-v  verbose
-z  compression
--delete remove deleted files