Linux - interior version

HAL - HW abstraction layer.Regardless of brand names it linux uses its own naming system for similar hardawre. HAL database is stored in /sys dir
 Ex: If you plug 3 diff Ethernet cards it will be eth0, eth1 and eth2. Thats it

dbus -  via dbus hw can communicate between applications and HAL

lsusb - lists all the usbs plugged in

lspci - lists all the pci cards plugged in

lsmod - lists all kernel modules

/proc - a virtual dir created in the RAM. But can be seen in the real dir structure. Only root can manipulate. Temporary configs can be made in here until reboot.

Booting up via run levels

Boot loader - Earlier linux used LILO. But now it is using GRUB

Boot loading menu entries 

UUID => partition where OS is

Kernel => where the kernel is 

quiet splash - dont verbose and show a logo

initrd =>initial ram disk. load essesntial modules from kernel into RAM when booting

init - first process when kernel is booted up. The master process of all processes.

pstree - shows all the processes available

/boot - kernel is stored in here. GRUB also stays here


Run levels 

0 - halt
1 - single user mode/ recovery mode
2 - default level for Debian OSs - GUI mode
3 - default level for Redhat OSs - text mode
4 - Available. But not used normally
5 - Redhat GUI mode
6 - Reboot. go to run level 0

/ect/inittab - run level details are here. can configure run levels and default level here.

 /etc/rcX.d - [X can be 0 to 5] shows sun level process starts and kills