inittab

Inittab is a configuration file used by traditional init systems, most notably SysVinit, to control system startup, runlevel transitions, and shutdown behavior. It tells the init process which programs to run, when to run them, and how to respond to system events.

Typically located at /etc/inittab, the file consists of lines with four fields: id, runlevels, action, and process. Lines beginning with a hash mark are comments and are ignored. The id is a unique identifier for the entry, the runlevels field specifies the runlevels during which the entry is active (such as 0–6 and S), the action field describes what init should do with the specified process, and the process field contains the command or script to execute.

Runlevel specification determines when entries apply. Actions control behavior, with common examples including sysinit (run once during system initialization), boot and bootwait (start items during boot, with an optional wait for completion), initdefault (designate the default runlevel), respawn (restart a process if it terminates), once (run once and do not restart), wait (start and wait for the process to finish), and ctrlaltdel (handle the Ctrl-Alt-Del sequence). The process field typically points to daemon startup scripts, getty processes for consoles, or other startup tasks.

While inittab is central to SysVinit-style systems, many modern Linux distributions have migrated to alternatives such as systemd or Upstart. Some environments retain inittab or provide compatible implementations (e.g., BusyBox) for legacy or compatibility reasons. Editing /etc/inittab should be done with care, as misconfigurations can prevent the system from booting or disrupt runlevel changes.