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DESCRIPTION

init is executed automatically by the kernel, after the kernel has initialised all devices and mounted the root file system. The kernel may try multiple possible paths for init, including /sbin/init, /sbin/oinit, /sbin/init.bak, and /rescue/init.

The following table describes the state machine used by init:

1. Single user shell. If the kernel is booted in single user mode (see boothowto(9)), then the kernel will pass the -s option to init to prevent the system from going multi-user and to instead execute a single user shell without starting the normal daemons. If the kernel is in a secure mode, init will downgrade it to securelevel 0 (insecure mode). The system is then quiescent for maintenance work and may later be made to go to state 2 (multi-user) by exiting the single-user shell (with ^D).
2. Multi-user boot (default operation). Executes /etc/rc (see rc(8)). If this was the first state entered (as opposed to entering here after state 1), then /etc/rc will be invoked with its first argument being ‘autoboot’. If /etc/rc exits with a non-zero (error) exit code, commence single user operation by giving the super-user a shell on the console by going to state 1 (single user). Otherwise, proceed to state 3.

   If value of the “init.root” sysctl node is not equal to / at this point, the /etc/rc process will be run inside a chroot(2) indicated by sysctl with the same error handling as above.

   If the administrator has not set the security level to -1 to indicate that the kernel should not run multiuser in secure mode, and the /etc/rc script has not set a higher level of security than level 1, then init will put the kernel into securelevel mode 1. See rc.conf(5) and secmodel_securelevel(9) for more information.

3. Set up ttys as specified in ttys(5). See below for more information. On completion, continue to state 4. If we did chroot in state 2, each getty(8) process will be run in the same chroot(2) path as in 2 (that is, the value of “init.root” sysctl is not re-read).
4. Multi-user operation. Depending upon the signal received, change state appropriately; on SIGTERM, go to state 7; on SIGHUP, go to state 5; on SIGTSTP, go to state 6.
5. Clean-up mode; re-read ttys(5), killing off the controlling processes on lines that are now ‘off’, and starting processes that are newly ‘on’. On completion, go to state 4.
6. ‘Boring’ mode; no new sessions. Signals as per state 4.
7. Shutdown mode. Send SIGHUP to all controlling processes, reap the processes for 30 seconds, and then go to state 1 (single user); warning if not all the processes died.

If the ‘console’ entry in the ttys(5) file is marked “insecure”, 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”.

It should be noted that while init has the ability to start multi-user operation inside a chroot(2) environment, the init process itself will always run in the “original root directory”. This also implies that single-user mode is always started in the original root, giving the possibility to create multi-user sessions in different root directories over time. The “init.root” sysctl node is fabricated by init at startup and re-created any time it is found to be missing. Type of the node is string capable of holding full pathname, and is only accessible by the superuser (unless explicitly destroyed and re-created with different specification).

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); it opens and initializes the tty line and executes the login(1) program. The login(1) 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) and utmpx(5) files of current users and records the logout in the wtmp(5) and wtmpx(5) files. The cycle is then restarted by init executing a new getty(8) for the line.

Line status (on, off, secure, getty, or window information) may be changed in the ttys(5) file without a reboot by sending the signal SIGHUP to init with the command “kill -s HUP 1”. This is referenced in the table above as state 5. On receipt of this signal, init re-reads the ttys(5) file. When a line is turned off in ttys(5), 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(5) file and are now on, init executes a new getty(8) 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(5), init will not do anything at all to that line. However, it will complain that the relationship between lines in the ttys(5) file and records in the utmp(5) 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)'s 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).

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, or exits during its initialisation, the system will panic with the message “panic: init died (signal %d, exit %d)”.

If /dev/console does not exist, init will cd to /dev and run “MAKEDEV -MM init”. MAKEDEV(8) will use mount_tmpfs(8) or mount_mfs(8) to create a memory file system mounted over /dev that contains the standard devices considered necessary to boot the system.