Launchd

launchd is Apple's unified service management framework for macOS and Darwin-based systems. It is responsible for starting, supervising, and stopping daemons, agents, and other background processes, replacing older mechanisms such as init and cron with a single, centralized model.

Introduced with Mac OS X 10.4 Tiger in 2005, launchd centralizes the system startup and task scheduling. It runs as the first process (PID 1) and serves as the parent of all other processes. It supports both system-wide services and per-user agents, enabling background daemons and user tasks to be managed by a single framework.

Configuration is done with property list (.plist) files that describe individual jobs. Core keys include Label, ProgramArguments, RunAtLoad, KeepAlive, StartInterval, and StartCalendarInterval. Jobs are loaded from standard directories such as /System/Library/LaunchDaemons and /System/Library/LaunchAgents for Apple components, /Library/LaunchDaemons and /Library/LaunchAgents for system-wide services, and per-user agents reside in ~/Library/LaunchAgents.

Management is performed with the launchctl command-line tool, which loads or unloads jobs, queries status, or adjusts configurations at runtime. launchd can launch jobs at boot, on demand, or on a schedule. KeepAlive enables automatic respawning; StartInterval and StartCalendarInterval provide timed scheduling.

Security and deployment: macOS requires code signing and appropriate entitlements for many launchd jobs, and system integrity protections influence what can be installed in system directories. Administrators generally place custom, system-level services under the LaunchDaemons directory and user agents under LaunchAgents.

Impact: As the core service manager since early macOS, launchd coordinates startup, background processing, and resource management across the system and user sessions, contributing to reliability and resource efficiency.