[Linux] Startup Process Analysis

The first process started by Linux kernel is/sbin/init, and its configuration file is/etc/inittab. This article will analyze how Linux starts it according to this configuration file, the following is a typical example of this file:

## inittab       This file describes how the INIT process should set up#               the system in a certain run-level.## Author:       Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>#               Modified for RHS Linux by Marc Ewing and Donnie Barnes## Default runlevel. The runlevels used by RHS are:#   0 - halt (Do NOT set initdefault to this)#   1 - Single user mode#   2 - Multiuser, without NFS (The same as 3, if you do not have networking)#   3 - Full multiuser mode#   4 - unused#   5 - X11#   6 - reboot (Do NOT set initdefault to this)# id:3:initdefault:# System initialization.si::sysinit:/etc/rc.d/rc.sysinitl0:0:wait:/etc/rc.d/rc 0l1:1:wait:/etc/rc.d/rc 1l2:2:wait:/etc/rc.d/rc 2l3:3:wait:/etc/rc.d/rc 3l4:4:wait:/etc/rc.d/rc 4l5:5:wait:/etc/rc.d/rc 5l6:6:wait:/etc/rc.d/rc 6# Trap CTRL-ALT-DELETEca::ctrlaltdel:/sbin/shutdown -t3 -r now# When our UPS tells us power has failed, assume we have a few minutes# of power left.  Schedule a shutdown for 2 minutes from now.# This does, of course, assume you have powerd installed and your# UPS connected and working correctly.  pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"# If power was restored before the shutdown kicked in, cancel it.pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"# Run gettys in standard runlevels1:2345:respawn:/sbin/mingetty tty12:2345:respawn:/sbin/mingetty tty2#3:2345:respawn:/sbin/mingetty tty3#4:2345:respawn:/sbin/mingetty tty4#5:2345:respawn:/sbin/mingetty tty5#6:2345:respawn:/sbin/mingetty tty6# Run xdm in runlevel 5x:5:respawn:/etc/X11/prefdm -nodaemon

The following is a one-to-one explanation.

1) set the run level

# Default runlevel. The runlevels used by RHS are:#   0 - halt (Do NOT set initdefault to this)#   1 - Single user mode#   2 - Multiuser, without NFS (The same as 3, if you do not have networking)#   3 - Full multiuser mode#   4 - unused#   5 - X11#   6 - reboot (Do NOT set initdefault to this)# id:3:initdefault:

The preceding seven levels of run level are described in detail. Generally, 3 (multi-user mode) and 5 (image Interface) are commonly used ).

2) set the system environment (/etc/rd. d/rc. sysinit)

# System initialization.si::sysinit:/etc/rc.d/rc.sysinit

The rc. sysinit script is used to set the system environment. If you are interested, you can check the script content.

3) Start the System Service (/etc/rc. d/rc N)

l0:0:wait:/etc/rc.d/rc 0l1:1:wait:/etc/rc.d/rc 1l2:2:wait:/etc/rc.d/rc 2l3:3:wait:/etc/rc.d/rc 3l4:4:wait:/etc/rc.d/rc 4l5:5:wait:/etc/rc.d/rc 5l6:6:wait:/etc/rc.d/rc 6

Linux will start the corresponding system service according to the corresponding run level. In this example, we run level = 3, so we will start the service in the/etc/rc. d/rc3.d directory:

Find K ?? * For Files starting with S ?? * Start the file.

Among them ,?? It is a two-digit integer that represents the execution sequence of start or stop.

If you are careful, you will find that all the files under the/etc/rc. d/rc3.d directory are linked to the files under the/etc/init. d directory.

Do you need to process these link files starting with S and K? Of course no. The chkconfig command is used to process these link files.

4) User-Defined boot Startup Program (/etc/rc. d/local)

In/etc/rc. the d/rc d directory must have an S99local file pointing to/etc/rc. d/rc. local, which is used to configure the User-Defined boot Startup Program.

5) set the terminal

# Run gettys in standard runlevels1:2345:respawn:/sbin/mingetty tty12:2345:respawn:/sbin/mingetty tty2#3:2345:respawn:/sbin/mingetty tty3#4:2345:respawn:/sbin/mingetty tty4#5:2345:respawn:/sbin/mingetty tty5#6:2345:respawn:/sbin/mingetty tty6

In the preceding configuration table, when run level = 2345, two terminals (1, 2) are started, and terminals 3, 4, 5, and 6 are commented out, so they are not enabled.