Jump to content United States-English
HP.com Home Products and Services Support and Drivers Solutions How to Buy
» Contact HP
More options
HP.com home
HP-UX Reference > I


HP-UX 11i Version 3: February 2007

Technical documentation

» Feedback
Content starts here

 » Table of Contents

 » Index


init — process control initialization


/sbin/init [0|1|2 |3|4|5 |6|S|s |Q|q|a |b|c]


The init daemon and command is a general process spawner. Its primary role is to create processes from a script stored in the file /etc/inittab (see inittab(4)). This file usually has init spawn a getty on each line where users can log in. It also controls autonomous processes required by any particular system.

At boot time, init is started as a system daemon.

While the system is running, a user-spawned init directs the actions of the boot init. It accepts a one-character argument and signals the boot init with the kill() system call to perform the appropriate action.

The arguments have the following effect:


Place the system in one of the run levels 0 through 6.


Process the inittab entries that have the special "run level" a, b, or c, without changing the numeric run level.


Re-examine the inittab entries without changing the run level.


Enter the single-user environment. When this level change occurs, the logical system console /dev/syscon is changed to the terminal from which the command was executed.

Boot init considers the system to be in a run level at any given time. A run level can be viewed as a software configuration of the system, where each configuration allows only a selected group of processes to exist. The processes spawned by boot init for each of these run levels are defined in the inittab file. Boot init can be in one of eight run levels, 0-6, and S or s. The run level is changed by having a privileged user run the init command. This user-spawned init sends appropriate signals to the boot init.

Boot init is invoked inside the HP-UX system as the last step in the boot procedure. Boot init first performs any required machine-dependent initialization, such as setting the system context. Next, boot init looks for the inittab file to see if there is an entry of the type initdefault (see inittab(4)). If an initdefault entry is found, boot init uses the run level specified in that entry as the initial run level to enter. If this entry is not in inittab, or inittab is not found, boot init requests that the user enter a run level from the logical system console, /dev/syscon. If S or s is entered, boot init goes into the single-user level. This is the only run level that does not require the existence of a properly formatted inittab file. If inittab does not exist, then by default the only legal run level that boot init can enter is the single-user level.

In the single-user level, the logical system console terminal /dev/syscon is opened for reading and writing, and the command /usr/bin/su, /usr/bin/sh, or /sbin/sh is invoked immediately. To exit from the single-user run level, one of two options can be selected:

  • If the shell is terminated with an end-of-file, boot init reprompts for a new run level.

  • User init can signal boot init and force it to change the current system run level.

When attempting to boot the system, some processes spawned by boot init may send display messages to the system console (depending on the contents of inittab). If messages are expected but do not appear during booting, it may be caused by the logical system console (/dev/syscon) being linked to a device that is not the physical system console (/dev/systty). If this occurs, you can force boot init to relink /dev/syscon to /dev/systty by pressing the DEL (delete) key (ASCII 127) on the physical system console.

When boot init prompts for the new run level, you can only enter one of the digits 0 through 6 or the letter S or s. If you enter S, boot init operates as previously described in single-user mode with the additional result that /dev/syscon is linked to the user's terminal line, thus making it the logical system console. A message is generated on the physical system console, /dev/systty, identifying the new logical system console.

When boot init comes up initially, and whenever it switches out of single-user state to normal run states, it sets the states (see ioctl(2)) of the logical system console, /dev/syscon, to those modes saved in the file /etc/ioctl.syscon. This file is written by boot init whenever single-user mode is entered. If this file does not exist when boot init wants to read it, a warning is printed and default settings are assumed.

If 0 through 6 is entered, boot init enters the corresponding run level. Any other input is rejected and a new prompt is issued. If this is the first time boot init has entered a run level other than single-user, boot init first scans inittab for special entries of the type boot and bootwait. These entries are performed — provided that the run level entered matches that of the entry — before any normal processing of inittab takes place. In this way, any special initialization of the operating system, such as mounting file systems, can take place before users are allowed onto the system. The inittab file is scanned to find all entries that are to be processed for that run level.

Run levels in HP-UX are defined as follows:


Shut down HP-UX.


Use for system administration (also known as "single-user state"). When booting into run level S at powerup, the only access to the system is through a shell spawned at the system console as the root user. The only processes running on the system will be kernel daemons started directly by the HP-UX kernel, daemon processes started from entries of type sysinit in /etc/inittab, the shell on the system console, and any processes started by the system administrator. Administration operations that require the system to be in a quiescent state (such as the fsck(1M) operation to repair a file system) should be run in this state. Transitioning into run level S from a higher run level does not terminate other system activity and does not result in a "single-user state"; this operation should not be done.


Start a subset of essential system processes. This state can also be used to perform system administration tasks.


Start most system daemons and login processes. This state is often called the "multi-user state". Login processes either at local terminals or over the network are possible.


Export filesystems and start other system processes. In this state NFS filesystems are often exported, as may be required for an NFS server.


Activate graphical presentation managers and start other system processes.


These states are available for user-defined operations.

The default run level is usually run level 3 or 4, depending on the system configuration.

When init transitions into a new run level 0-6, the master sequencer script rc is invoked. rc in turn invokes each of the start or kill scripts for each installed subsystem for each intervening run level. When transitioning to a higher run level start scripts are invoked, and when transitioning to a lower run level kill scripts are invoked. See rc(1M).

In a multiuser environment, the inittab file is usually set up so that boot init creates a process for each terminal on the system.

For terminal processes, ultimately the shell terminates because of an end-of-file either typed explicitly or generated as the result of hanging up. When boot init receives a child death signal telling it that a process it spawned has died, it records the fact and the reason it died in the utmps database, /etc/utmp, /var/adm/wtmps, and /var/adm/wtmp if it exist (see who(1)). A history of the processes spawned is kept in /var/adm/wtmps and /var/adm/wtmp if it exists.

To spawn each process in the inittab file, boot init reads each entry and, for each entry that should be respawned, it forks a child process. After it has spawned all of the processes specified by the inittab file, boot init waits for one of its descendant processes to die, a powerfail signal, or until it is signaled by a user init to change the system's run level. When one of the above three conditions occurs, boot init re-examines the inittab file. New entries can be added to the inittab file at any time. However, boot init still waits for one of the above three conditions to occur. For an instantaneous response, use the init Q (or init q) command to wake up boot init to re-examine the inittab file without changing the run level.

If boot init receives a powerfail signal (SIGPWR) and is not in single-user mode, it scans inittab for special powerfail entries. These entries are invoked (if the run levels permit) before any other processing takes place by boot init. In this way, boot init can perform various cleanup and recording functions whenever the operating system experiences a power failure. Note, however, that although boot init receives SIGPWR immediately after a power failure, boot init cannot handle the signal until it resumes execution. Since execution order is based on scheduling priority, any eligible process with a higher priority executes before boot init can scan inittab and perform the specified functions.

When boot init is requested to change run levels via a user init, it sends the warning signal SIGTERM to all processes that are undefined in the target run level. Boot init waits 20 seconds before forcibly terminating these processes with the kill signal SIGKILL. Note that boot init assumes that all these processes (and their descendants) remain in the same process group that boot init originally created for them. If any process changes its process group affiliation with either setpgrp() or setpgrp2() (see setsid(2) and setpgid(2)), it will not receive these signals. (Common examples of such processes are the shells csh and ksh (see csh(1) and ksh(1).) Such processes need to be terminated separately.

A user init can be invoked only by users with appropriate privileges.


Boot Authentication

The system administrator can enable the boot authentication feature. If enabled, the system cannot be booted into single user mode until the password of a user authorized to boot the system in single user mode is provided. Refer to the /etc/default/security file in the security(4) manual page for detailed information on configurable parameters that affect the behavior of this command. The currently supported parameters for boot authentication are:


On systems that have been converted to trusted mode, use the System Administration Manager (SAM) program (see sam(1M)).


If boot init finds that it is continuously respawning an entry from inittab more than 10 times in 2 minutes, it will assume that there is an error in the command string, generate an error message on the system console, and refuse to respawn this entry until either 5 minutes have elapsed or it receives a signal from a user init. This prevents boot init from using up system resources if there is a typographical error in the inittab file or a program is removed that is referenced in inittab.


Boot init assumes that processes and descendants of processes spawned by boot init remain in the same process group that boot init originally created for them. When changing init states, special care should be taken with processes that change their process group affiliation, such as csh and ksh.

One particular scenario that often causes confusing behavior can occur when a child csh or ksh is started by a login shell. When boot init is asked to change to a run level that would cause the original login shell to be killed, the shell's descendant csh or ksh process does not receive a hangup signal since it has changed its process group affiliation and is no longer affiliated with the process group of the original shell. Boot init cannot kill this csh or ksh process (or any of its children).

If a getty process is later started on the same tty as this previous shell, the result may be two processes (the getty and the job control shell) competing for input on the tty.

To avoid problems such as this, always be sure to manually kill any job control shells that should not be running after changing init states. Also, always be sure that user init is invoked from the lowest level (login) shell when changing to an init state that may cause your login shell to be killed.

If init is unable to write to /etc/ioctl.syscon, a message is logged to the console. This may lead to the corruption of console settings.

HP-UX 11i Version 3 is the last release to support trusted systems functionality.


/dev/syscon /dev/systty /etc/default/security /etc/inittab /etc/ioctl.syscon /etc/utmp /var/adm/wtmp /var/adm/wtmps


init: SVID2, SVID3

Printable version
Privacy statement Using this site means you accept its terms Feedback to webmaster
© 1983-2007 Hewlett-Packard Development Company, L.P.