Transfer Tips, Part III

Jerry Peek

Thursday, May 15th, 2003

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This month, we’ll take our third and final look at transferring files between systems. The utilities we’ll check out this month, rsync and unison, analyze two sets of files and synchronize them, making the two sets identical with little or no help from you. Given a source and a destination, rsync makes all destination files match those at the source. unison works in both directions, automatically making any changes that don’t conflict and asking you about the rest.

Does wget Get It Right?

To keep up-to-date, local copies of some remote files, you could constantly copy and recopy all of them. However, that wastes a lot of bandwidth, especially if only a few (or none) of the remote files have changed recently.

How can you tell whether any remote files are different from your local copy? One obvious way is by comparing file sizes: If the files have different lengths, they’re different. You could also try comparing timestamps, specifically the last-modified date and time. wget (covered in April, available online at http://www.linux-mag.com/2003-04/power_01.html) uses both methods when it’s mirroring a remote server.

But those methods don’t always work. Why? First, two files with the same size and name can can have small (or great) differences. Second, the granularity of timestamps is too great. For instance, a typical timestamp from an FTP server looks like ls -l output: Only the date, hour, and minute that a file was last changed is recorded (or, for…

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