Dry Run Rsync

6 min read Oct 10, 2024

Dry Run rsync: A Preview of Your File Transfers

Have you ever wondered what changes would be made before you execute a critical rsync command? Perhaps you're unsure about potential file deletions or unintended modifications. Fear not, because rsync offers a powerful tool called dry run that lets you preview these changes without actually moving any data.

What is a Dry Run in rsync?

A dry run with rsync is like a rehearsal before the main performance. It simulates the entire file transfer process but doesn't actually move or modify any files. You can observe the changes rsync intends to make, allowing you to verify the command's accuracy before committing to the actual transfer.

Why Use a Dry Run?

There are several compelling reasons to employ a dry run before running your rsync command:

Safety First: Avoid accidental deletions or modifications to your valuable data.
Verification: Ensure that the rsync command will transfer the correct files and make the desired changes.
Troubleshooting: Identify potential issues early and adjust your command accordingly.
Planning: Gain a clear understanding of the file transfer scope and potential impact.

How to Execute a Dry Run

Implementing a dry run in rsync is straightforward. Simply add the --dry-run or -n option to your regular rsync command:

rsync -avz --dry-run /path/to/source /path/to/destination

Let's break down the command:

-a: Archive mode, preserving permissions, timestamps, and owner/group information.
-v: Verbose mode, providing detailed output about the transfer process.
-z: Compresses data during transfer, potentially speeding up the process.
--dry-run or -n: Executes the dry run, simulating the transfer without modifying files.
/path/to/source: The location of the files you want to transfer.
/path/to/destination: The target location for the transferred files.

Understanding the Output

When you execute a dry run command, rsync will display a list of files it would transfer, delete, or modify. For example:

sending incremental file list
./
./file1.txt
./file2.jpg

sent 18 bytes  received 0 bytes  144.00 bytes/sec
total size is 0  speedup is 0.00

This output indicates that rsync would transfer the files file1.txt and file2.jpg to the destination directory.

Tips for Effective Dry Runs

Run the command multiple times: Execute the dry run several times with different options to refine your understanding of the transfer behavior.
Review the output carefully: Pay close attention to the details of the output to ensure the command aligns with your expectations.
Combine with other options: You can use the dry run in conjunction with other rsync options, such as --delete, to get a comprehensive preview.

Beyond Basic Dry Runs

While the --dry-run option is the most common way to preview rsync operations, it's not the only one. Here are some additional techniques:

--itemize-changes: This option provides a detailed list of changes rsync would make, including file modifications, deletions, and creations.
--checksum: Use this option to verify the checksums of files before transferring them, ensuring data integrity.
--archive-dir: Specify a temporary directory to store a copy of the files that would be transferred during the dry run.

Conclusion

The dry run feature is an essential tool for working with rsync. It allows you to confidently verify your commands, prevent unintended data loss, and plan your file transfer operations efficiently. By taking advantage of this feature, you can ensure smooth and predictable file transfers every time.