.\"{{{}}} .\"{{{ Title .TH INIT 8 "9 March 1998" "" "Linux System Administrator's Manual" .\"}}} .\"{{{ Name .SH NAME init, telinit \- process control initialization .\"}}} .\"{{{ Synopsis .SH SYNOPSIS .B /sbin/init .RB [ " 0123456Ss " ] .br .B /sbin/telinit .RB [ " \-t sec " ] .RB [ " 0123456sSQqabcUu " ] .\"}}} .\"{{{ Description .SH DESCRIPTION .\"{{{ init .SS Init .B Init is the father of all processes. Its primary role is to create processes from a script stored in the file \fB/etc/inittab\fP (see \fIinittab\fP(5)). This file usually has entries which cause \fBinit\fP to spawn \fBgetty\fPs on each line that users can log in. It also controls autonomous processes required by any particular system. .PP .\"{{{ Runlevels .SH RUNLEVELS A \fIrunlevel\fP is a software configuration of the system which allows only a selected group of processes to exist. The processes spawned by \fBinit\fP for each of these runlevels are defined in the \fB/etc/inittab\fP file. \fBInit\fP can be in one of eight runlevels: \fB0\(en6\fP and \fBS\fP or \fBs\fP. The runlevel is changed by having a privileged user run \fBtelinit\fP, which sends appropriate signals to \fBinit\fP, telling it which runlevel to change to. .PP Runlevels \fB0\fP, \fB1\fP, and \fB6\fP are reserved. Runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel 1 is used to get the system down into single user mode. Runlevel \fBS\fP is not really meant to be used directly, but more for the scripts that are executed when entering runlevel 1. For more information on this, see the manpages for \fBshutdown\fP(8) and \fBinittab\fP(5). .PP Runlevels 7-9 are also valid, though not really documented. This is because "traditional" Unix variants don't use them. In case you're curious, runlevels \fIS\fP and \fIs\fP are in fact the same. Internally they are aliases for the same runlevel - this is just a leftover from the systems the author used to use when writing sysvinit. .\"}}} .PP .SH BOOTING After \fBinit\fP is invoked as the last step of the kernel boot sequence, it looks for the file \fB/etc/inittab\fP to see if there is an entry of the type \fBinitdefault\fP (see \fIinittab\fP(5)). The \fBinitdefault\fP entry determines the initial runlevel of the system. If there is no such entry (or no \fB/etc/inittab\fP at all), a runlevel must be entered at the system console. .PP Runlevel \fBS\fP or \fBs\fP bring the system to single user mode and do not require an \fB/etc/initttab\fP file. In single user mode, \fB/sbin/sulogin\fP is invoked on \fB/dev/console\fP. .PP When entering single user mode, \fBinit\fP reads the console's \fIioctl\fP(2) states from \fB/etc/ioctl.save\fP. If this file does not exist, \fBinit\fP initializes the line at \fB9600 baud\fP and with \fBCLOCAL\fP settings. When \fBinit\fP leaves single user mode, it stores the console's \fIioctl\fP settings in this file so it can re-use them for the next single-user session. .PP When entering a multi-user mode for the first time, \fBinit\fP performs the \fBboot\fP and \fBbootwait\fP entries to allow file systems to be mounted before users can log in. Then all entries matching the runlevel are processed. .PP When starting a new process, \fBinit\fP first checks whether the file \fI/etc/initscript\fP exists. If it does, it uses this script to start the process. .PP Each time a child terminates, \fBinit\fP records the fact and the reason it died in \fB/var/run/utmp\fP and \fB/var/log/wtmp\fP, provided that these files exist. .SH CHANGING RUNLEVELS After it has spawned all of the processes specified, \fBinit\fP waits for one of its descendant processes to die, a powerfail signal, or until it is signaled by \fBtelinit\fP to change the system's runlevel. When one of the above three conditions occurs, it re-examines the \fB/etc/inittab\fP file. New entries can be added to this file at any time. However, \fBinit\fP still waits for one of the above three conditions to occur. To provide for an instantaneous response, the \fBtelinit Q\fP or \fBq\fP command can wake up \fBinit\fP to re-examine the \fB/etc/inittab\fP file. .PP If \fBinit\fP is not in single user mode and receives a powerfail signal, special powerfail entries are invoked. .PP When \fBinit\fP is requested to change the runlevel, it sends the warning signal \s-1\fBSIGTERM\fP\s0 to all processes that are undefined in the new runlevel. It then waits 5 seconds before forcibly terminating these processes via the \s-1\fBSIGKILL\fP\s0 signal. Note that \fBinit\fP assumes that all these processes (and their descendants) remain in the same process group which \fBinit\fP originally created for them. If any process changes its process group affiliation it will not receive these signals. Such processes need to be terminated separately. .\"}}} .\"{{{ telinit .SH TELINIT \fB/sbin/telinit\fP is linked to \fB/sbin/init\fP. It takes a one-character argument and signals \fBinit\fP to perform the appropriate action. The following arguments serve as directives to \fBtelinit\fP: .IP "\fB0\fP,\fB1\fP,\fB2\fP,\fB3\fP,\fB4\fP,\fB5\fP or \fB6\fP" tell \fBinit\fP to switch to the specified run level. .IP \fBa\fP,\fBb\fP,\fBc\fP tell \fBinit\fP to process only those \fB/etc/inittab\fP file entries having runlevel \fBa\fP,\fBb\fP or \fBc\fP. .IP "\fBQ\fP or \fBq\fP" tell \fBinit\fP to re-examine the \fB/etc/inittab\fP file. .IP "\fBS\fP or \fBs\fP" tell \fBinit\fP to switch to single user mode. .IP "\fBU\fP or \fBu\fP" tell \fBinit\fP to re-execute itself (preserving the state). No re-examining of \fB/etc/inittab\fP file happens. Run level should be one of \fBSs12345\fP, otherwise request would be silently ignored. .PP \fBtelinit\fP can also tell \fBinit\fP how long it should wait between sending processes the SIGTERM and SIGKILL signals. The default is 5 seconds, but this can be changed with the \fB-t sec\fP option. .PP \fBtelinit\fP can be invoked only by users with appropriate privileges. .PP The \fBinit\fP binary checks if it is \fBinit\fP or \fBtelinit\fP by looking at its \fIprocess id\fP; the real \fBinit\fP's process id is always \fB1\fP. From this it follows that instead of calling \fBtelinit\fP one can also just use \fBinit\fP instead as a shortcut. .\"}}} .\"}}} .SH ENVIRONMENT \fBInit\fP sets the following environment variables for all its children: .IP \fBPATH\fP \fI/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin\fP .IP \fBINIT_VERSION\fP As the name says. Useful to determine if a script runs directly from \fBinit\fP. .IP \fBRUNLEVEL\fP The current system runlevel. .IP \fBPREVLEVEL\fP The previous runlevel (useful after a runlevel switch). .IP \fBCONSOLE\fP The system console. This is really inherited from the kernel; however if it is not set \fBinit\fP will set it to \fB/dev/console\fP by default. .SH BOOTFLAGS It is possible to pass a number of flags to \fBinit\fP from the boot monitor (eg. LILO). \fBInit\fP accepts the following flags: .TP 0.5i .B S, single Single user mode boot. In this mode \fI/etc/inittab\fP is examined and the bootup rc scripts are usually run before the single user mode shell is started. .PP .TP 0.5i .B 1-5 Runlevel to boot into. .PP .TP 0.5i .B -b, emergency Boot directly into a single user shell without running any other startup scripts. .PP .SH INTERFACE Init listens on a \fIfifo\fP in /dev, \fI/dev/initctl\fP, for messages. \fBTelinit\fP uses this to communicate with init. The interface is not very well documented or finished. Those interested should study the \fIinitreq.h\fP file in the \fIsrc/\fP subdirectory of the \fBinit\fP source code tar archive. .SH SIGNALS Init reacts to several signals: .TP 0.5i .B SIGHUP Init looks for .I /etc/initrunlvl and .I /var/log/initrunlvl. If one of these files exist and contain an ASCII runlevel, init switches to the new runlevel. \fI This is for backwards compatibility only! \fP. In the normal case (the files don't exist) init behaves like \fB telinit q\fP was executed. .PP .TP 0.5i .B SIGUSR1 On receipt of this signals, init closes and re-opens it's control fifo, \fB/dev/initctl\fP. Useful for bootscripts when /dev is remounted. .TP 0.5i .B SIGINT Normally the kernel sends this signal to init when CTRL-ALT-DEL is pressed. It activates the \fIctrlaltdel\fP action. .TP 0.5i .B SIGWINCH The kernel sends this signal when the \fIKeyboardSignal\fP key is hit. It activates the \fIkbrequest\fP action. \"{{{ Conforming to .SH CONFORMING TO \fBInit\fP is compatible with the System V init. It works closely together with the scripts in the directories \fI/etc/init.d\fP and \fI/etc/rc{runlevel}.d\fP. If your system uses this convention, there should be a \fIREADME\fP file in the directory \fI/etc/init.d\fP explaining how these scripts work. .\"}}} .\"{{{ Files .SH FILES .nf /etc/inittab /etc/initscript /dev/console /etc/ioctl.save /var/run/utmp /var/log/wtmp /dev/initctl .fi .\"}}} .\"{{{ Warnings .SH WARNINGS \fBInit\fP assumes that processes and descendants of processes remain in the same process group which was originally created for them. If the processes change their group, \fBinit\fP can't kill them and you may end up with two processes reading from one terminal line. .\"}}} .\"{{{ Diagnostics .SH DIAGNOSTICS If \fBinit\fP finds that it is continuously respawning an entry 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 has elapsed or it receives a signal. This prevents it from eating up system resources when someone makes a typographical error in the \fB/etc/inittab\fP file or the program for the entry is removed. .\"}}} .\"{{{ Author .SH AUTHOR Miquel van Smoorenburg (miquels@cistron.nl), initial manual page by Michael Haardt (u31b3hs@pool.informatik.rwth-aachen.de). .\"}}} .\"{{{ See also .SH "SEE ALSO" .BR getty (1), .BR login (1), .BR sh (1), .BR who (1), .BR shutdown(8), .BR kill (1), .BR inittab (5), .BR initscript (5), .BR utmp (5) .\"}}}