initdefault

Initdefault is a term used in System V style init systems to designate the line in the inittab configuration that tells the init process which runlevel to boot into by default. The inittab file, read by the init daemon at boot, contains entries that define how and when to start system services for each runlevel. The line labeled or associated with initdefault serves as the boot-time default, guiding init to switch to that runlevel automatically during startup.

In practice, an initdefault entry typically appears as a line such as id:3:initdefault: or id:5:initdefault:. The exact field order in inittab is id:runlevels:action:process, and the initdefault line uses the initdefault action to indicate the default runlevel. When the system boots, init scans inittab, finds this default directive, and then proceeds to initialize the system services defined for that runlevel.

This mechanism is part of the broader concept of runlevels, which are predefined states of the system (for example, single-user mode, multi-user mode, or halt). While widely used on traditional Unix and many Linux distributions, the approach has become less common with newer init systems. Modern systems using Upstart or systemd typically do not rely on inittab to set a default runlevel; systemd uses targets (such as multi-user.target) and a default.target, and Upstart uses its own job configurations. Nevertheless, initdefault remains a key concept in understanding historical SysVinit behavior and the boot process on legacy or compatibility-focused systems.