Linux from the command line -- How to Master the rsync Command in Linux

FTA

"As its name suggests, rsync is a synchronization tool. The first time you use it, it simply copies files and directories from one location to another. Subsequent uses, however, synchronize only the differences between these locations. So, if you add one file to the source directory, rsync will copy only the new file to the destination. This saves a lot of time and bandwidth, especially when copying to a remote location.

Imagine you have 50GB of data you have to synchronize to a remote server. You can schedule rsync to run daily. Instead of having to copy the whole 50GB over and over again, the utility will only send the differences. This might mean it only has to upload a few megabytes of data every day, instead of gigabytes...."

https://www.maketecheasier.com/use-rsync-command-linux/