rsync is a utility for transferring and synchronising files between a computer and a storage drive and across networked computers by delta-transfer algorithm (using modification time and size by default, or checksums with the --checksum option). It is commonly found on Unix-like operating systems and is under the GPL-3.0-or-later license.

rsync -avz source_dir/your_file.txt user@server_ip:target_dir/

• -a: to transfer the file in archive mode, which preserves file permissions, ownership, timestamps, and symbolic links.

• -v: to enables verbose output, providing more details about the transfer process.

• -z: to compress the data during transfer, saving bandwidth and potentially speeding things up.

• -h : converts the bytes to a more understandable unit based on the size. (1048576 bytes → 1.0M)

• -p : preserve permissions from source to destination

• --chmod : for more granular control over permissions

  # Set all files to 644
  rsync -avzh --chmod=F644 /path/to/local/files/ user@server_ip:/dest
  
  # Set directories to 755 and files to 644:
  rsync -avzh --chmod=D0755,F644 /path/to/local/files/ user@server_ip:/dest
  

How rsync works with SSH

Unlike ssh accesses config file to find user private key for signing challenge, rsync uses a different way.

• Initiating the Connection:
  • You execute the rsync command with the source, destination (including username and server address), and options.
  • rsync uses SSH to establish a secure connection to the remote server.
• SSH Handshake:
  • During the SSH handshake, the server does indeed send a challenge (a random piece of data) to the client (your machine).
• Client-side Private Key Usage:
  • However, rsync is not directly involved in this process. Your SSH client software (like OpenSSH) handles the handshake.
  • Your SSH client software (which might be using an SSH agent in the background) uses your private key (based on its default location or an SSH agent configuration) to cryptographically sign the received challenge.
• Public Key Verification by Server:
  • The signed challenge is sent back to the server.
  • The server has a copy of your public key (previously installed by you) associated with your username. This public key is typically stored in the ~/.ssh/authorized_keys file on the server, but the location can vary depending on server configuration.
  • The server uses its copy of your public key to verify the signature of the challenge it sent.
• Authentication Success (if Public Key Matches):
  • If the signature verification is successful, the SSH handshake is complete, and the server grants access to the authenticated user.
• Rsync Data Transfer:
  • rsync then uses the established secure SSH connection to transfer data efficiently.