Keeping Up To Date with Unison

Last month I talked about the tough censorship issues, facing Google. The company claims that they are ethically superior to their peers while doing nothing different than their peers to achieve that goal.

Many Linux developers responded in Google's defense. Here are some of the opinions and my responses:

• A company is only answerable to itself: there is no absolute right or wrong that can allow a discussion. Author and apologist C. S. Lewis argued that the existence of common sense implies that some things must be universally better than others. The idea of "no absolutes" is like saying all operating systems (like Linux and Windows) are the same. This is really moral laziness, a reluctance to talk about the best alternatives.
• Harm done to people which doesn't impact us should not be our concern. When a person takes no effort to protect the welfare of others that he knows are in danger, that person is a participant in the crime. Six degrees of separation: there is one world and we're all in this together.
• Linux companies are good and proprietary commercial software companies are evil. So Google is automatically right no matter what they do. How can the underlying software dictate the uses it will be put to? The same Linux used to fight overpriced, poor quality commercial software is the same Linux that can be used in a bomb targeting innocent civilians.
• Buy Google stock or you have no right to question their policies. Does a person need to give money to and become a member of the Liberals, Conservatives or NDP in order to ask questions about their party policies? A person who did not vote may not have a right to complain about the country's state, but that doesn't mean they can't engage in open dialog about party policies. Why should a person buy stock in any company they don't believe in?
• Google is an "easy target" because they are big. Google is making decisions affecting millions around the world, claiming to be more ethical than other companies but resulting in the loss of free speech, censorship, protests, cover-ups and lawsuits. Since Google sets an example that impacts almost everyone on the Internet, how can they not be a topic for open, intelligent conversation? They are representatives of Linux.

I was surprised by so many responses to my column. However, the common theme seemed to be plain old Canadian timidity, a relucance to go out on an emotional limb and engage in free dialog because of not wanting to be blasted and labeled by peers. Linux people need to defend their right and obligation to stand up talk about these kind of issues, to set a higher bar for Linux and its community. It's unfortunate that the FUD was coming from the Linux community this time. When people who fight FUD (Fear, Uncertainty and Doubt) cave in when they are labeled incorrectly as "bashers", nobody wins. Especially not the truth.

This month's column is a review of Unison. Available for Windows, Cygwin, Mac and Linux, Unison is an open source program that synchronize file folders on two different computers. The Unison home page is http://www.cis.upenn.edu/~bcpierce/unison/index.html . Binaries versions are available for a text-only or GTK desktop version (shown here).

Unlike tools such as rsync which copy one computer folder to another folder, Unison is bidirectional: new files on either computer are replicated to the other. Files are exchanged through secure shell (SSH) to protect your privacy. SSH is available out-of-the-box on most versions of Linux.

For example, say you want to synchronize saved email between your laptop computer and your home computer. Plug your laptop into your home network and if your home computer is, say, 192.168.100.2, you can synchronize email folders with a command like this:

$ unison ssh://192.168.100.2/email ~/email

Unison first displays a summary of what needs to be synchronized. An arrow shows where an update will occur. If there is a question mark, Unison isn't sure what do with the file: using the icon bar, you can tell Unison to what action to take. If you don't give Unison any instructions, it will ignore uncertain files.

After reviewing the files, click on "Go" to synchronize them. As Unison copies files, the status column will be marked with check-marks to show successful transfers.

The first time Unison is run, it will warn you that it has no history about the folders. After that, Unison maintains a history of the transfers to help identify what needs to be updated.

Unison can synchronize files over the Internet, provided your firewall allows SSH access. It also synchronizes between directories on the same machine. Use this feature to backup files to an external device such as a USB drive or a floppy disk. Use these as intermediaries and synchronize against them when you get to the other computer.

For administrators, Unison can be used as a simple tool to keep programs up-to-date across multiple machines. Use a star topology by synchronizing all the computers to one machine (so that machine contains a copy of all the updates) and then synchronize all the computers to the that machine again to make sure they are all up-to-date.

Unison does not merge changes between files. There is a sister project, Harmony, synchronizes the content XML files. This can be used, for example, to synchronize web browser bookmarks.

Unison can cause problems with source control software. When Unison is used, the copied files will have identical content but their time stamps will be different. This can confuse software like CVS that expects the time stamps to reflect file changes against the shared source code repository. Unison should not be used on source code controlled directories.

The wide display can cause trouble for small screens such as the 1024x768 LCD screen on my Thinkpad laptop.

Unison is a useful tool for Linux consultants who are always on the go.

Talk back on the Linux Cafe under the General category.

March 14, 2006 

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