init is a general process spawner. Its primary role is to create processes from information stored in the file /etc/inittab.
At any given time, the system is in one of eight possible run levels. A run level is a software configuration under which only a selected group of processes exists. Processes spawned by init for each of these run levels are defined in /etc/inittab. init can be in one of eight run levels, 0-6 and S or s (S and s are identical). The run level changes when a privileged user runs /sbin/init. This sends appropriate signals to the original init spawned by the operating system at boot time, saying which run level to invoke.
When the system is booted, init is invoked and the following occurs. First, it reads /etc/default/init to set environment variables. This is typically where TZ (time zone) and locale-related environments such as LANG or LC_CTYPE get set.
init then looks in /etc/inittab for the initdefault entry (see inittab(4)). If the initdefault entry:
init usually uses the run level specified in that entry as the initial run level to enter.
/etc/inittab, init asks the user to enter a run level from the system console.
init goes to the single-user state. In this state, the system console device (/dev/console) is opened for reading and writing and the command /sbin/su, (see su(1M)), is invoked. Use either init or telinit to change the run level of the system. Note that if the shell is terminated (using an end-of-file), init only re-initializes to the single-user state if /etc/inittab does not exist.
init enters the corresponding run level. Run levels 0, 5, and 6 are reserved states for shutting the system down. Run levels 2, 3, and 4 are available as multi-user operating states.
If this is the first time since power up that init has entered a run level other than single-user state, init first scans /etc/inittab for boot and bootwait entries (see inittab(4)). These entries are performed before any other processing of /etc/inittab takes place, providing that the run level entered matches that of the entry. 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. init then scans /etc/inittab and executes all other entries that are to be processed for that run level.
To spawn each process in /etc/inittab, 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 /etc/inittab, init waits for one of its descendant processes to die, a powerfail signal, or a signal from another init or telinit process to change the system's run level. When one of these conditions occurs, init re-examines /etc/inittab.
New entries can be added to /etc/inittab at any time; however, init still waits for one of the above three conditions to occur before re-examining /etc/inittab. To get around this, init Q or init q command wakes init to re-examine /etc/inittab immediately.
When init comes up at boot time and whenever the system changes from the single-user state to another run state, init sets the ioctl(2) states of the console to those modes saved in the file /etc/ioctl.syscon. init writes this file whenever the single-user state is entered.
When a run level change request is made, init sends
the warning signal (
all processes that are undefined in the target run level. init waits five seconds before forcibly terminating these processes
by sending a kill signal (
When init receives a signal telling it that a process it spawned has died, it records the fact and the reason it died in /var/adm/utmpx and /var/adm/wtmpx if it exists (see who(1)). A history of the processes spawned is kept in /var/adm/wtmpx.
If init receives a powerfail
SIGPWR) it scans /etc/inittab for special entries of the type powerfail and powerwait. These entries are invoked (if
the run levels permit) before any further processing takes place. In this
way init can perform various cleanup and recording functions
during the powerdown of the operating system.
Default values can be set for the following flags in /etc/default/init. For example: TZ=US/Pacific
Either specifies the timezone information (see ctime(3C)) or the name of a timezone information file /usr/share/lib/zoneinfo.
Character characterization information.
Monetary formatting information.
Numeric formatting information.
Time formatting information.
If set, all other LC_* environmental variables take-on this value.
If LC_ALL is not set, and any particular LC_* is also not set, the value of LANG is used for that particular environmental variable.
init uses pam(3PAM) for session management. The PAM configuration policy, listed through /etc/pam.conf, specifies the session management module to be used for init. Here is a partial pam.conf file with entries for init using the UNIX session management module.
init session required /usr/lib/security/pam_unix.so.1
If there are no entries for the init service, then the entries for the "other" service will be used.
Go into firmware.
Put the system in system administrator mode. All local file systems are mounted. Only a small set of essential kernel processes are left running. This mode is for administrative tasks such as installing optional utility packages. All files are accessible and no users are logged in on the system.
Put the system in multi-user mode. All multi-user environment terminal processes and daemons are spawned. This state is commonly referred to as the multi-user state.
Extend multi-user mode by making local resources available over the network.
Is available to be defined as an alternative multi-user environment configuration. It is not necessary for system operation and is usually not used.
Shut the machine down so that it is safe to remove the power. Have the machine remove power, if possible.
Stop the operating system and reboot to the state defined by the initdefault entry in /etc/inittab.
process only those /etc/inittab entries having the a, b, or c run level set. These are pseudo-states, which may be defined to run certain commands, but which do not cause the current run level to change.
Enter single-user mode. This is the only run level that doesn't require the existence of a properly formatted /etc/inittab file. If this file does not exist, then by default, the only legal run level that init can enter is the single-user mode. When in single-user mode, the filesystems required for basic system operation will be mounted. When the system comes down to single-user mode, these file systems will remain mounted (even if provided by a remote file server), and any other local filesystems will also be left mounted. During the transition down to single-user mode, all processes started by init or init.d scripts that should only be running in multi-user mode are killed. In addition, any process that has a utmpx entry will be killed. This last condition insures that all port monitors started by the SAC are killed and all services started by these port monitors, including ttymon login services, are killed.
controls process dispatching by init
user access and administration information
history of user access and administration information
system console device
See attributes(5) for descriptions of the following attributes:
|ATTRIBUTE TYPE||ATTRIBUTE VALUE|
If init finds that it is respawning an entry from /etc/inittab more than ten times in two minutes, assumes that there is an error in the command string in the entry, and generates an error message on the system console. It will then refuse to respawn this entry until either five minutes has elapsed or it receives a signal from a user-spawned init or telinit. This prevents init from eating up system resources when someone makes a typographical error in the inittab file, or a program is removed that is referenced in /etc/inittab.
init and telinit can be run only by a privileged user.
The S or s state must not be used indiscriminately in /etc/inittab. When modifying this file, it is best to avoid adding this state to any line other than initdefault.
If a default state is not specified in the initdefault entry in /etc/inittab, state 6 is entered. Consequently, the system will loop by going to firmware and rebooting continuously.
If the utmpx file cannot be created when booting the system, the system will boot to state “s” regardless of the state specified in the initdefault entry in /etc/inittab. This can occur if the /var file system is not accessible.