LaunchDaemon
The following article goes over what Launch Daemons and Agents are. Examples are provided along with actual launchd jobs.
LaunchDaemon and LaunchAgents
Daemon
A daemon is a program that runs in the background as part of the overall system (that is, it is not tied to a particular user). A daemon cannot display any GUI; more specifically, it is not allowed to connect to the window server. A web server is the perfect example of a daemon.
Historically, Mac OS X had a number of different ways to start daemons (for details, see Deprecated Daemonomicon). On current systems there is only one recommend way: launchd.
A launchd daemon is configured by a sophisticated property list file. This file allows the daemon to be launched based on a variety of criteria (connections to listening sockets, items being modified in the file system, periodically, and so on). For details, see the launchd.plist man page.
A third party launchd daemon should be installed by adding a property list file to the /Library/LaunchDaemons directory.
Agent
An agent is a process that runs in the background on behalf of a particular user. Agents are useful because they can do things that daemons can't, like reliably access the user's home directory or connect to the window server. A calendar monitoring program is a good example of an agent because:
- there should be an instance of the program per GUI login session
- each agent has access to that user's calendar
- the agent can launch the calendar application when certain events occur
The difference between an agent and a daemon is that an agent can display GUI if it wants to, while a daemon can't. The difference between an agent and a regular application is that an agent typically displays no GUI (or a very limited GUI).
A launchd agent is like a launchd daemon, except that it runs on behalf of a particular user. It is launched by launchd, typically as part of the process of logging in the user.
A third party launchd agent should be installed by adding a property list file to the ~/Library/LaunchAgents directory (to be invoked just for this user) or /Library/LaunchAgents directory (to be invoked for all users).