What is launchd?

What is a Daemon

A daemon is a program running in the background without requiring user input. A typical daemon might for instance perform daily maintenance tasks or scan a device for malware when it is connected.

Daemons and Agents

launchd differentiates between agents and daemons. The main difference is that an agent is run on behalf of the logged in user while a daemon runs on behalf of the root user or any user you specify with the UserName key.

Job Definitions

The behavior of a daemon/agent is specified in a special XML file called a property list. Depending on where it is stored it will be treated as a daemon or an agent.

Job definitions crucial for the operation of the operating system are stored below /System/Library. You should never need to create a daemon or agent in these directories. Third-Party definitions which are relevant for every user are stored below /Library. Job definitions for a specific user are stored below the respective user's Library directory.

Type Location Run on behalf of User Agents ~/Library/LaunchAgents Currently logged in user Global Agents /Library/LaunchAgents Currently logged in user Global Daemons /Library/LaunchDaemons root or the user specified with the key UserName System Agents /System/Library/LaunchAgents Currently logged in user System Daemons /System/Library/LaunchDaemons root or the user specified with the key UserName

The ~ character stands for the users home directory.

The following example shows a complete job definition with only three keys:

• Label This key is required for every job definition. It identifies the job and has to be unique for the launchd instance. Theoretically it is possible for an agent to have the same label as a daemon, as daemons are loaded by the root launchd whereas agents are loaded by a user launchd, but it is not recommended.
• Program This key defines what to start, in this case a shell script /Users/Me/Scripts/cleanup.sh.
• RunAtLoad This is one of several optional keys specifying when the job should be run, in this case right after it has been loaded.

launchd supports more than 36 different configuration keys. Most of them are explained in the Configuration-Section of this primer.

Automatic Loading of Job Definitions

Upon system start the root launchd process will scan the daemon directories /System/Library/LaunchDaemons and /Library/LaunchDaemons for job definitions and load them depending on the existence/value of the Disabled key and the contents of the override database.

When a user logs in a new launchd process will be started for this user. This launchd process will scan the agent directories /System/Library/LaunchAgents, /Library/LaunchAgents and ~/Library/LaunchAgents for job definitions and load them depending on the existence/value of the Disabled key and the contents of the override database.

Loading vs Starting

Loading a job definition does not necessarily mean to start the job. When a job is started is determined by the job definition. In fact, only when RunAtLoad or KeepAlive have been specified,launchd will start the job unconditionally when it has been loaded.