prefsync 1.0

A little tool to help synchronize Mac OS X plist files (used for preferences for most Mac Apps) seamlessly, in a way that can be tracked by git.

# Background

Quite a few of us on GitHub upload our [dotfiles](http://en.wikipedia.org/wiki/Configuration_file) ([here are mine](https://github.com/asmeurer/dotfiles)). It’s a nice way to share configuration, track it with git, and with symlinking, it makes for a nice semi-manual synchronization scheme.

This trick doesn’t work so well with Mac OS X preferences. The issue is that as of Mac OS X 10.4, [plist files](http://en.wikipedia.org/wiki/Plist) are binary by default, making them very difficult to track with git.

A typical solution to this file is to symlink the plist files to Dropbox—and as far as I know that works. But you lose the ability to easily share your settings, and perhaps most importantly, track them with git.

The good news is that it’s actually possible to use binary plist files, and automatically convert them back and forth to human-readable (and git trackable) XML, using some tools that come with Mac OS X.

# Installing

You can install it with conda using

conda install -c asmeurer prefsync

Or using pip

pip install prefsync

Or from the git source

python setup.py install

Or you can just run it from the checkout, with

python -m prefsync

Once you install it, use the command prefsync.

# How it works

It’s actually quite simple. The heavy work is done for us by the operating system and tools it provides. The main tools are [plutil](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/plutil.1.html), which allows to convert binary plist files to xml plist files and xml plist files back to binary, and [launchd](http://en.wikipedia.org/wiki/Launchd), which provides a facility to automatically run a script when a file changes.

There are some catches, though.

First, if you just do this, it will run cyclically forever, because as soon as the binary file is updated, it will write to the xml version, which will register as a change, causing it to write the binary plist, and so on.

We get around this by writing a special extended attribute to each file when it is written, and ignoring files if they have that attribute (so that only changes by other things will register). This will break if something somehow preserves these attributes. In my tests, they don’t, but you can easily test if your program does by running

xattr -w test:Test test ~/Library/Preferences/preferencefile.plist

then, make a change to the preferences, and see if the atttribute is still there

xattr ~/Library/Preferences/preferencefile.plist

Second, in Mac OS X 10.9, preferences are cached, meaning that even if a program is closed and the plist file is written to, it will be overridden with the original preferences when it is opened again if the cache was not flushed.

Getting around this is easy: we just call

defaults read com.reversedns.preference

(where com.reversedns.preference is the reverse DNS for the preference file in question). This flushes the cache.

Finally, it generally doesn’t work to update the binary preference file while a program is running. You may be able to just close and reopen the program, but it may write to the preferences when you do that, overwriting the changes.

Therefore, it is highly recommended to track the XML changes in git, so that if they are overwritten they can be easily recovered. To force the changes to be written to the binary, the easiest way is to close the program, make sure the xml changes are the same (i.e., closing the program didn’t cause the preferences to be written and synced over to the xml), and then touch the xml file.

Note that the synchronization in each direction may take up to 10 seconds by default to take place. You can change this by setting the -t flag when running prefsync.

# Gotchas

According to [this page](http://managingosx.wordpress.com/2006/05/10/launchd-gotcha/) will stop watching a path if it ceases to exist. You can reset the process by logging out and logging back in, or by restarting. You can also try using launchctl, though this may not work reliably.

# Acknowledgments

The Lingon program is invaluable when working with launchd (the older, free version works just fine).

StackOverflow and Mac OS X Hints helped me debug some of the strangeness when trying to get this to work.

# License

BSD (see the LICENSE file).