rsync

A fast, versatile command-line utility for local and remote file synchronization using an efficient delta-transfer algorithm.

Engineered in 1996 by Andrew Tridgell and Paul Mackerras, rsync is the industry-standard tool for replicating files across local storage or secure SSH connections. Its signature delta-transfer algorithm minimizes network traffic by transmitting only the differences between source and destination files rather than copying entire payloads. Operators rely on its robust flag set (such as -a for archiving, -v for verbose output, and --delete to mirror directories precisely) to manage everything from daily incremental backups to massive server migrations with minimal overhead.

https://rsync.samba.org