Mac "item in use" error when installing a new app?

I know exactly what you're talking about, and it's actually something designed for your safety, believe it or not. The problem is that once you have a program running the system needs the file "contents" to stay untouched. Part of the issue is that what seems to be an application is actually a folder full of different data, graphics images, and much more. You can see this by Ctrl-Clicking on an icon in your Applications folder and choosing "Show Package Contents".

Now, imagine that you've written an application that pulls resources from the system on an as-needed basis and you delete those assistive files or rename them without warning, while the program is actually running. Not good. Great software will at least recognize the error situation and pop up a meaningful message, but what if it's related to saving a file you've been working on, a graphic you've produced or a film you've just finished editing?

As I said, the Mac's limitation on saving new versions of applications is to help you avoid data loss, file corruption or running programs that hiccup and glitch. It's good. Really.

However, you're right that sometimes you still want to circumvent the in-use problem, particularly if it's a helper application or utility and you know it can safely be killed and restarted once you have the new version installed.

I encountered this a few days ago when I went to update my version of the terrific iTunes utility Synergy. As usual, I dragged the program icon (which as you now know represents a folder full of stuff) into the same folder that contains the outdated version:

Looks normal so far, and when it pops up this warning message, it does seem like things are all going to work properly:

But then when everything's proceeding smoothly we get the dreaded Mac OS X "application in use" error:

In this case, however, I know that Synergy runs in the background, communicating with iTunes and displaying stuff on my screen, so I can safely kill the app and restart it once I've updated the actual program code and package contents.

To do so, my preferred route is to use Activity Monitor, which you can find in your "Utilities" folder within "Applications". Start it up and you'll see something like this:

On the top right search box, type in the name of the application you want to kill. In this case, I type in "synergy":

Click on the application, then click on the red stop sign (well, actually it's called "Quit Process", but I like 'stop!' instead) and it'll confirm that's what you want to do:

At this point I can wander off into a discussion of Unix signal processing, but let's save that for a different article and instead I'll just observe that just about every program you run on your Mac should be able to gracefully and promptly stop once it receives the "quit" signal: that's what you should use. Click on "Quit". If that doesn't work and you're sure you really want to - and can safely - kill the app, click on "Force Quit", but the latter is guaranteed to not safe temp files, work in progress, etc. It's dangerous. We Unix geeks think of it as "terminate with extreme prejudice", but that's another story.

Now that the process is terminated, you can quit Activity Monitor and safely copy the new version of the application file across. You get the same warning message we saw earlier:

This time once you click "Replace" you do just that and the program is replaced by its newer version. No trace of "in use" errors appear.

That's all there is to it. Once you get the new version of the program in place, double-click it and for most apps, it'll start up as if you'd restarted your computer.

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