Well, it's about time I wrote a post about launchd. launchd is the new catch-all services launcher in Tiger. It's the thing that makes everything you need in order to do anything on your computer launch, either at boot, or at the appropriate time. It's got some real problems and limitations right now, but hey, it's new. And there's plenty good to say about it, even in this early stage.
I won't go too much into how launchd works nor all of what it does. But if you want to get an idea of what services have been moved to launchd from other parts of the system, take a look at the files in:
/System/Library/LaunchDaemons
Here you'll find a list of preference files, or .plist files. These files are all named for services that used to be started and handled by init and other such UNIX daemons and config files. Among the most notable in this list are the periodic maintenance jobs once handled by cron. These, too, have now been taken over by launchd, though cron remains available for scheduling other jobs. For now.
Anyway, this should give you a good enough idea what launchd does. If you're really lost here, I suggest you do some homework. There is a great overview of launchd, complete with tutorial, over at AFP548. There's also a good launchd Developer document at Apple's Developer site. Between these two you should be able to get largely up to speed on launchd. And if all this services stuff is way over your head, well, just move along please.
One of the coolest features of launchd, though, is the fact that it is easily user-configurable, and user-specific. That is launchd items, or LaunchAgents and LaunchDaemons as they're called, can be placed in the user's home account and run on a per-user basis. The other remarkably cool feature of LaunchAgents and Daemons -- and the one I really want to talk about -- is that they can watch folders and/or files. These two features allow for a degree of conditional automation -- and by that I mean, If/when user does A, then System does B -- previously undreamt of on a Mac. Well, that may be an overstatement, but launchd will certainly make such customization a great deal easier. And it's not even that hard to use, once you get the hang of it.
If you check out the AFP548 demo, they have you create a LaunchDaemon that watches a folder for modification, and then, if the folder is modified, launchd automatically moves the items of this "watch folder" (and, yes, that is the technical term) to a new destination folder. Already you can see where this would be extremely useful for backups or versioning systems, or any kind of synchronization of files. The idea of the watch folder/file is remarkably powerful. You can have launchd run any shell script, AppleScript or command anytime any file or folder is changed. In our lab we were, at one point, using launchd to watch our list of automount user mounts, and automatically reload the automount daemon whenever a new user was added to the list. You could (and I may, at some point) make a launchd item that detects the mounting of external hard drives, and then runs a script to disable Spotlight on said drives. There are a million things you can do with this one feature alone. But to do any of it, you must know how to edit those pesky .plist files.
Being no stranger to editing XML .plist files, I started off the hard way, editing them by hand in a text editor. I then graduated to the Property List Editor included with the Developer Tools. But recently, I found an excellent and free utility called Lingon, which takes a lot of the grunt work out of creating LaunchDaemons and Agents. It even comes with an "Assistant" to help you get started creating your first LaunchDaemon. And it will blessedly load your items into launchd, so trips to the command-line and the arcane syntax of lines like this:
become a thing of the past.
Lingon: Launch Your Dreams Right Here!
(click for larger view)
So, okay, sometimes a GUI is a great thing. No offense to the Lingon guys and/or gals, but I'd really love to see Apple include a front-end to launchd sometime in the future, so that it could be really accessible to mere mortals without all the fussing around. Until then, Lingon makes a great addition to any Admin's toolbox who might have need of the sort of auto-magic attainable with launchd.
I do want to note one more thing. It's possible to seriously lock up your system with LaunchDaemons and Agents. To wit, I somehow created a LaunchAgent that perpetually opened one of my hard drives. This, for all intents and purposes, prevented me from accessing any application but the Finder. In fact, I couldn't even navigate the Finder to remove the item as every time I tried to do anything at all, the window to the drive would open and take over the system. Nor could I easily access the offending LaunchAgent via single-user mode, as my home account is stored on a second partition that is not mounted in single-user mode. Fortunately I keep a secondary user account for just such emergencies. In the end, I was able to unload and delete the offending LaunchAgent via this secondary account. So just a word to the wise: Use launchd with care. And keep a secondary account handy, just in case.
And one last thing: Hats off to the Tiger devs. I bitch a lot about 10.4, but launchd is a keeper. One of the best hidden surprises to this new OS.