Linux Boot process is the process that happens right from power on until a shell appears for an user to start working on. Here I try to explain the process step by step.
1. Once the system is powered on, the control is taken by BIOS(Basic Input Output System).
2. This system triggers POST. POST is Power On System Test. It verifies that all hardware that are connected to the system are working fine.
3. If the verification succeeds, a bootable device is searched for. The bootable device could be a USB stick, a partition in hard drive, a CD ROM, via network etc.
4. When the bootable disk is identified, MBR is loaded from it. MBR is nothing but the Master Boot Record, which occupies the first sector (512 bytes) of the bootable device. Out of these 512 bytes, first 446 bytes consist of the primary boot loader information, next 64 bytes store the partition table information and the last 2 bytes store the magic for validating the MBR.
5. The MBR loads itself to the RAM and starts searching for the active partition. It also verifies no other partition is active at that point of time. It loads this in the RAM. In nut-shell it transfers control to GRUB.
6. The Grub (or earlier days it was lilo) uses /etc/grub/grub.cfg file. It opens a boot menu, which lists the set of available distributions for deployment based on grub.cfg. If nothing is selected in this menu, the distribution which is specified as ddefault in grub.cfg will be deployed on the system.
7. After the decision of which distribution is to be deployed, an attempt is made to search for the kernel in the predetermined path. It will be zImage (Compressed, less than 512 Kb) or a bzImage(Big compressed, more than 512 Kb).
8. The head of this image itself will have some code that can
a) Setup hardware
b) Decompress the kernel image
c) Copy the kernel to the memory.
9. Once the kernel is loaded into the memory,
a) It initializes the memory and configures hardware that are connected to the system
b) It finds the Initrd, decompresses it and mounts the Initrd.
c) It then loads the kernel modules/device drivers.
d) Initializes virtual devices.
After all this the smaller Initrd, which was used for bringing up the board is unmounted and the actual initrd is pivot_root ed. Some times it is possible to embed a small Initrd which contains only the bare essentials to bring up the board inside the kernel itself.
10. With this stage, The kernel is loaded and initialised. Now the control is transferred to user space.
a) The first thing started in user space is /sbin/init. It runs with a PID 1
b) Then /etc/rc.d/rc.sysinit script is executed.
c) The /etc/inittab is executed. This is a special file, which spawns/starts services (like for console logging, syslog etc). Here the run level can also be mentioned.
d) Then all the services i /etc/rc.d/rc<RUN LEVEL>.d is started.
e) The last step is execution of /etc/rc.d/rc.local. This script can be very useful if we need to customise the start up procedure.
Once all this is done, Shell is available for the user to start using the system.
Thanks,
Gomathi
1. Once the system is powered on, the control is taken by BIOS(Basic Input Output System).
2. This system triggers POST. POST is Power On System Test. It verifies that all hardware that are connected to the system are working fine.
3. If the verification succeeds, a bootable device is searched for. The bootable device could be a USB stick, a partition in hard drive, a CD ROM, via network etc.
4. When the bootable disk is identified, MBR is loaded from it. MBR is nothing but the Master Boot Record, which occupies the first sector (512 bytes) of the bootable device. Out of these 512 bytes, first 446 bytes consist of the primary boot loader information, next 64 bytes store the partition table information and the last 2 bytes store the magic for validating the MBR.
5. The MBR loads itself to the RAM and starts searching for the active partition. It also verifies no other partition is active at that point of time. It loads this in the RAM. In nut-shell it transfers control to GRUB.
6. The Grub (or earlier days it was lilo) uses /etc/grub/grub.cfg file. It opens a boot menu, which lists the set of available distributions for deployment based on grub.cfg. If nothing is selected in this menu, the distribution which is specified as ddefault in grub.cfg will be deployed on the system.
7. After the decision of which distribution is to be deployed, an attempt is made to search for the kernel in the predetermined path. It will be zImage (Compressed, less than 512 Kb) or a bzImage(Big compressed, more than 512 Kb).
8. The head of this image itself will have some code that can
a) Setup hardware
b) Decompress the kernel image
c) Copy the kernel to the memory.
9. Once the kernel is loaded into the memory,
a) It initializes the memory and configures hardware that are connected to the system
b) It finds the Initrd, decompresses it and mounts the Initrd.
c) It then loads the kernel modules/device drivers.
d) Initializes virtual devices.
After all this the smaller Initrd, which was used for bringing up the board is unmounted and the actual initrd is pivot_root ed. Some times it is possible to embed a small Initrd which contains only the bare essentials to bring up the board inside the kernel itself.
10. With this stage, The kernel is loaded and initialised. Now the control is transferred to user space.
a) The first thing started in user space is /sbin/init. It runs with a PID 1
b) Then /etc/rc.d/rc.sysinit script is executed.
c) The /etc/inittab is executed. This is a special file, which spawns/starts services (like for console logging, syslog etc). Here the run level can also be mentioned.
d) Then all the services i /etc/rc.d/rc<RUN LEVEL>.d is started.
e) The last step is execution of /etc/rc.d/rc.local. This script can be very useful if we need to customise the start up procedure.
Once all this is done, Shell is available for the user to start using the system.
Thanks,
Gomathi
Nice post. Thanks for posting this in step by step and simple form :)
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