INIT(8) | System Manager's Manual | INIT(8)

# NAME

`init` — process control initialization

# SYNOPSIS

`init` | [`-fs`]

# DESCRIPTION

The `init` program is the last stage of the boot process. It normally executes the sequence of events described in rc(8) and begins multi-user operation.

The kernel may pass the following options to `init`, usually when requested by the boot(8) program:

`-f`
  Activate fastboot mode. This is not currently supported by the OpenBSD kernel. Instead, use the /fastboot file as explained in the rc(8) manual.
`-s`
  Boot directly into single-user mode.

Single-user mode is also entered if the boot scripts fail.

In single-user mode, the rc(8) script is not run and normal daemons are not started, but instead a super-user shell is started on the system console. If the console entry in the ttys(5) file does not contain the “secure” flag, then `init` will require that the superuser password be entered before the system will start a single-user shell. The password check is skipped if the console is marked as “secure”.

In single-user mode, the system is quiescent for maintenance work and may later be made to go to multi-user by exiting the single-user shell (with ^D). This causes `init` to run the rc(8) startup command file in fastboot mode (skipping disk checks).

The kernel securelevel(7) is normally set to 0 while in single-user mode, and raised to 1 when the system begins multi-user operations. This action will not take place if the securelevel is -1, and can be modified via the /etc/rc.securelevel script.

In multi-user operation, `init` maintains processes for the terminal ports found in the file ttys(5). `init` reads this file, and executes the command found in the second field. This command is usually getty(8); getty opens and initializes the tty line and executes the login program. The login program, when a valid user logs in, executes a shell for that user. When this shell dies, either because the user logged out or an abnormal termination occurred (a signal), the `init` program wakes up, deletes the user from the utmp(5) file of current users and records the logout in the wtmp file. The cycle is then restarted by `init` executing a new getty for the line.

Line status (on, off, secure, getty, or window information) may be changed in the ttys file without a reboot by sending the signal `SIGHUP` to `init` with the command “`kill -s HUP 1`”. On receipt of this signal, `init` re-reads the ttys file. When a line is turned off in ttys, `init` will send a `SIGHUP` signal to the controlling process for the session associated with the line. For any lines that were previously turned off in the ttys file and are now on, `init` executes a new getty to enable a new login. If the getty or window field for a line is changed, the change takes effect at the end of the current login session (e.g., the next time `init` starts a process on the line). If a line is commented out or deleted from ttys, `init` will not do anything at all to that line. However, it will complain that the relationship between lines in the ttys file and records in the utmp file is out of sync, so this practice is not recommended.

`init` will terminate multi-user operations and resume single-user mode if sent a terminate (`TERM`) signal, for example, “`kill -s TERM 1`”. If there are processes outstanding that are deadlocked (because of hardware or software failure), `init` will not wait for them all to die (which might take forever), but will time out after 30 seconds and print a warning message.

`init` will cease creating new getty(8) and allow the system to slowly die away, if it is sent a terminal stop (`TSTP`) signal, i.e., “`kill -s TSTP 1`”. A later hangup will resume full multi-user operations, or a terminate will start a single-user shell. This hook is used by reboot(8) and halt(8).

`init` will terminate multi-user operations, kill all getty(8), and run /etc/rc.shutdown if a user-defined signal 1 (`USR1`), user-defined signal 2 (`USR2`), or interrupt (`INT`) signal is received. Following this, `USR1` will halt the system; `USR2` will request a powerdown; and `INT` will cause a reboot. /etc/rc.shutdown can specify that a powerdown is requested instead of the action specified by the signal.

The role of `init` is so critical that if it dies, the system will reboot itself automatically. If, at bootstrap time, the `init` process cannot be located, the system will panic with the message “panic: init died (signal %d, exit %d)”.

# RESOURCES

When `init` spawns a process it sets the process priority, umask, and resource limits based on /etc/login.conf. When starting the rc(8) files, the login class “daemon” is used. When starting a window system or getty(8), the login class “default” is used. No resource changes are made when entering single-user mode.

# FILES

/dev/console
  system console device
/dev/tty*
  terminal ports found in ttys
/etc/rc
  system startup commands
/etc/rc.securelevel
  commands that run before the security level changes
/etc/rc.shutdown
  script run at shutdown time
/etc/ttys
  terminal initialization information file
/fastboot
  tells rc(8) not to run fsck(8) during the next boot
/var/run/utmp
  record of users currently logged in
/var/log/wtmp
  record of all logins and logouts

# DIAGNOSTICS

getty repeating too quickly on port %s, sleeping
  A process being started to service a line is exiting quickly each time it is started. This is often caused by a ringing or noisy terminal line. Init will sleep for 30 seconds, then continue trying to start the process.
some processes would not die; ps axl advised.
  A process is hung and could not be killed when the system was shutting down. This condition is usually caused by a process that is stuck in a device driver because of a persistent device error condition.

# SEE ALSO

kill(1), login(1), sh(1), fbtab(5), login.conf(5), ttys(5), securelevel(7), crash(8), getty(8), halt(8), rc(8), rc.shutdown(8), reboot(8), shutdown(8)

# HISTORY

An `init` command appeared in Version 1 AT&T UNIX.

January 16, 2018 | Debian