How Linux Boots? 6 Stages of Linux Boot Process
How Linux Boots: First of all thanks for landing this article, if you are pointed for Linux Boots. I just tell you are in the right place. So without getting into query lets directly jump into the post of 6 Stages of Linux Boot Process. As it turns away, there isn’t a lot to the boot system.
6 Stages of Linux Boot Process
- A boot loader finds the kernel image on the disk, loads it keen on memory, and starts it.
- The kernel initializes the devices with its drivers.
- The kernel mounts the root file system.
- The kernel starts a program called init.
- Init sets the rest of the processes in activity.
- The final processes that init starts as part of the boot sequence allow you to log in.
Identify every phase of the boot procedure is invaluable in fixing boot troubles and accepting the system as a complete. To start, zero in on the boot loader, which is the primary screen or prompts you to get after the PC does its power-on-self-test, asking which operating system to run. Following you make an option, the boot loader runs the Linux kernel, handing control of the system to the kernel.
In attendance is a full argument of the kernel elsewhere in this book from which this article is excerpted. This article covers the kernel initialization phase, the phase when the kernel prints a bunch of messages about the hardware in attendance on the system. The kernel starts to init immediately after it displays a message proclaiming that the kernel has mounted the root file system:
VFS: Mounted Root (ext2 file system) Read Only
Rapidly after, you will see a message about init starting, followed by system service startup messages, and as a final point, you get a login prompt of some sort.
Note down On Red Hat Linux, the init note is particularly clear because it “welcomes” you to “Red Hat Linux.” All messages subsequently show success or failure in brackets at the right-hand side of the display.
The majority of this episode deals with init, because it is the component of the boot sequence where you have the main control.
There is nothing particular about init. It is a program just like some additional on the Linux system, and you’ll discover it in /sbin along with other system binaries. The most important point of init is to start and stop other programs in a particular series. Everyone you have to know is how this sequence works.
There are a small number of different variations, but most Linux distributions utilize the System V technique discussed here. Some distributions utilize a simpler version that resembles the BSD init, but you are unlikely to encounter this.
At any specified time on a Linux system, a certain base set of processes is running. This condition of the machine is called it’s run level, and it is denoted with a digit from 0 through 6. The system spends most of its moment in a particular run level. However, when you shut the machine down, init switches to a different runlevel in order to terminate the system services in an orderly fashion and to tell the kernel to stop. Yet one more runlevel is for single-user mode, discussed soon after.
The easiest method to get a handle on runlevels is to inspect the init configuration file, /etc/inittab. Look for a line like the following:
Id: 5: initdefault
This line means so as to the default runlevel on the system is 5. Every line in the inittab file takes this form, with four fields divided by colons occurring in the following arrange:
# A unique identifier (a short string, such like id in the preceding case)
# The applicable runlevel number(s)
# The action that init should receive (in the preceding instance, the action is to set the default runlevel to 5)
# A command to execute (optional)
There is no command to perform in the preceding initdefault example because a command doesn’t make sense in the context of setting the default runlevel. Look a little further down in inittab, awaiting you see a line like this:
This line triggers nearly every one of the system configuration and services during the RC*.d and init.d directories. You can notice that init is set to execute a command called /etc/rc.d/RC 5 when in runlevel 5. The wait action tells while and how to init runs the command: run RC 5 once when entering runlevel 5, and then wait for this command to finish before doing anything else.
Few particular actions in addition to initdefault and wait, for the most part pertaining to power management and the inittab (5) manual page tells you all about them. The ones that you’re most likely to encounter are explained in the subsequent sections.
The respawn action causes init to run the command that follows, and if the command finishes executing, to run it once more. You’re probably to see something alike to this line in your inittab file:
1:2345: respawn:/sbin/mingetty tty1
The Getty programs offer login prompts. The preceding line is for the first virtual console (/dev/tty1), the one you see when you push ALT-F1 or CONTROL-ALT-F1. The respawn action brings the login quick back after you log out.
The Ctrl-Alt-Del action controls what the system does when you push CONTROL-ALT-DELETE on a virtual console. On most systems, this is a few sorts of reboot command using the shutdown command.
The sysinit action is the especially first thing that init should run when it starts up, before incoming any runlevels.
How Processes in the runlevels launch
You are now prepared to discover how to init starts the system services, just before it lets you log in. Recall this inittab line from previous:
This small line triggers many additional programs. RC stands for run commands, and you will hear public pass on to the commands as scripts, programs, or services. So, where are these commands, anyhow?
Designed for runlevel 5, in this example, the commands are most likely also in /etc/rc.d/rc5.d or /etc/rc5.d. Runlevel 1 uses rc1.d, runlevel 2 uses rc2.d, and so on. You might discover the following items in the rc5.d directory:
S10sysklogd S20ppp S99gpm S12kerneld S25netstd_nfs S99httpd S15netstd_init S30netstd_misc S99rmnologin S18netbase S45pcmcia S99sshd S20acct S89atd S20logoutd S89cron
The RC 5 command starts programs within this runlevel directory by running the subsequent commands:
S10sysklogd start S12kerneld start S15netstd_init start S18netbase start ... S99sshd start
Notice the start argument in every command. The S in a command name means that the command should run in start method, and the number (00 through 99) determines where in the sequence RC starts the command.
The RC*.d commands are typically shell scripts that start programs in /sbin or /usr/sbin. Normally, you can build out what one of the commands truly does by looking at the script with less or one more pager program.
You can launch one of these services by hand. For example, if you want to start the http Web server program physically, run S99httpd start. Equally, if you ever need to kill one of the services when the machine is on, you can run the command in the RC*.d directory with the stop argument (S99httpd stop, for instance). Some RC*.d directories include commands that start with K (for “kill,” or stop mode). In this case, RC runs the command with the stop argument as a substitute to the start. You are most probable to encounter K commands in runlevels that shut the system down.
Adding and Removing Services
If you wish to add, delete, or modify services in the RC*.d directories, you require to take a closer look at the files inside. A long listing reveals an arrangement like this:
lrwxrwxrwx . . . S10sysklogd ->../init.d/sysklogd lrwxrwxrwx . . . S12kerneld ->../init.d/kerneld lrwxrwxrwx . . . S15netstd_init ->../init.d/netstd_init lrwxrwxrwx . . . S18netbase ->../init.d/netbase ...
The commands in an RC*.d directory are really symbolic links to files in an init.d directory, frequently in /etc or /etc/rc.d. Linux distributions contain these links so that they can utilize similar startup scripts for all runlevels. This convention is by no means a necessity, but it often makes the organization a slight easier.
To avoid one of the commands in the init.d directory from running in a particular runlevel, you might imagine of removing the symbolic link in the appropriate RC*.d directory. This does work, but if you create a mistake and still require putting the link back in place, you might have difficulty remembering the correct name of the link. As a result, you shouldn’t remove links in the RC*.d directories, but fairly, add an underscore (_) to the beginning of the link name like this:
mv S99httpd _S99httpd
On boot time, RC ignores _S99httpd because it doesn’t start with S or K. Furthermore, the unique name is still obvious, and you have rapid access to the command if you’re in a touch and require to start it by hand.
To add a service, you necessity create a script like the others in the init.d directory and then create a symbolic link in the accurate RC*.d directory. The easiest method to write a script is to inspect the scripts already in init.d, make a copy of one that you recognize and modify the copy.
When adding a service, make sure that you decide an appropriate place in the boot sequence to start the service. If the service starts too quickly, it may not work, due to a dependency on several other services. For unnecessary services, most systems administrators wish numbers in the 90s, after most of the services that came with the system.
Linux distributions typically come with a command to enable and disable services in the RC*.d directories. For case in point, in Debian, the command is update-rc.d, and in Red Hat Linux, the command is chkconfig. Graphical user interfaces are also presented. Using these programs helps carry on the startup directories reliable and helps with upgrades.
HINT: One of the most ordinary Linux installation troubles is an offensively configured XFree86 server that flicks on and off, manufactures the system unusable on the console. To stop this performance, boot into single-user mode and alter your runlevel or runlevel services. Seem for something containing xdm, gdm, or kdm in your RC*.d directories, or your /etc/inittab.
Intermittently, you want to give init a little kick to tell it to switch runlevels, to re-read the inittab file, or just to shut down the system. Because init is at all times the first process on a system, its process ID is always 1.
You can control init through telinit. For instance, if you want to switch to runlevel 3, use this command:
When switching runlevels, init tries to execute off any processes that aren’t in the inittab file for the new runlevel. Therefore, you should be alert about changing runlevels.
When you want to add or remove respawning jobs or create any other change to the inittab file, you must tell init concerning the change and cause it to re-read the file. Some public uses kill -HUP 1 to tell init to do this. This traditional technique works on most versions of UNIX, as long as you type it correctly. Though, you can also run this telinit command:
You can also utilize telinit s to switch to single-user mode.
Init also controls how the system shuts down and reboots. The suitable method to shut down a Linux machine is to utilize the shutdown command.
There are two essential ways to utilize shutdown. If you want to halt the system, it shuts the machine down and keeps it down. To make the machine halt right away, utilize this command:
Shutdown -h now
On most new machines with logically recent versions of Linux, a halt cuts the power to the machine. You can also reboot the machine. For a reboot, use -r instead of -h.
The shutdown method takes some seconds. You should never reset or power off a machine during this phase. In the previous example, now is the time to shut down. This argument is compulsory, but there are a lot of ways of specifying it. If you want the machine to go down a little bit in the future, one method is to use +n, where n is the number of minutes shutdown should wait before doing its work. For additional options, look at the shutdown (8) manual page.
To Create the System Reboot in 10 Minutes, Run this Command:
Shutdown -r +10
On Linux, shutdown notifies anybody logged on that the machine is leaving down, but it does little genuine work. If you identify a time further than now, shutdown creates a file called /etc/nologin. While this file is in attendance, the system prohibits logins by anyone except the superuser.
When system shutdown time lastly arrives, shutdown tells init to switch to runlevel 0 for a halt and runlevel 6 for a reboot. When init enters runlevel 0 or 6, all of the following takes place, which you can verify by looking at the scripts inside rc0.d and rc6.d:
Init kills each process that it can (as it would when switching to some other runlevel).
# The initial rc0.d/rc6.d commands run, locking system files into position and making other arrangements for the shutdown.
# The next rc0.d/rc6.d commands unmount each and every one file systems other than the root.
# Further rc0.d/rc6.d commands remount the root file system read-only.
# Still more rc0.d/rc6.d commands write all buffered data out to the file system with the sync program. # The last rc0.d/rc6.d commands notify the kernel to reboot or stop with the reboot, halt, or power off the program.
The reboot and halt programs act another way for every runlevel, potentially causing confusion. By default, these programs call shutdown with the -r or -h options, but if the system is already at the halt or reboot runlevel, the programs inform the kernel to shut itself off instantly. If you really want to shut your machine down in a hurry (disregarding any probable damage from a disorderly shutdown), utilize the -f option.
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