A user recently brought to my attention that Universal Back Button isn't working on Mac OS X 10.6.5, nor is it working on the just-released 10.6.6. I spent some time trying to figure out the cause, and it comes down to this: Apparently Apple has begun actively filtering environment variables from the two pertinent places where they could be set and still have an effect on GUI applications: In ~/.MacOSX/environment.plist and ~/.launchd.conf. For posterity, attempting to set either of the following environment variables in either of these files will fail:

• AEDebug
• AEDebugSends
• AEDebugReceives

It's hard to say why the good folks at Apple have made this change. Possibly they saw the environment variables as potential security risks (after all, information about URLs being exchanged between applications is exposed in the user's console log - although the information isn't sent anywhere else, of course), or they felt that allowing these variables to be set could adversely affect the performance of the user's Mac. My guess is that it's a combination of these two concerns, though I suppose we'll never know the true answer.

So, how do we fix it? There is just one place left (that I've been able to discover, at least - if you know of anywhere else, please chime in). By editing /etc/launchd.conf, we can reinstate the environment variable we need. The downside to this solution is that it will be active for all users on your Mac. That's hardly ideal, but then again, I can't say I much care for Apple's solution to this problem in the first place. After all, it's my computer - why can't I set the environment variables I want?

To fix Universal Back Button on 10.6.5 and later, open Terminal (located in the Utilities folder inside your Applications folder), and then run the following command:

sudo bash -c "echo 'setenv AEDebugReceives 1' >> /etc/launchd.conf"

Note that the command is all one line, in case it wraps in your browser. You'll be prompted for your your administrator password in order to make the change. To reverse the change, execute the following (again, it's all on one line. I suggest copy-pasting it to Terminal to avoid any difficulties:

sudo bash -c "sed -e '/setenv AEDebugReceives 1/d' /etc/launchd.conf > /etc/launchd.conf-tmp &&
mv -f /etc/launchd.conf-tmp /etc/launchd.conf"

Once either the first or the second command has been executed, you will need to restart your Mac in order for the changes to take effect. All of this should make it into an update of Universal Back Button sooner or later, but I wanted to put the fix out here until I have time to post an update.