System Run Level

• init 0 → shutdown
• init 1 → single user mode → troubleshoot
• init 6 → reboot the system
• init 2 → multiuser without networking
• init 3 → multiuser with networking
• init 5 → multiuser with networking with GUI

who -r

• To know which level

Boot Process

• CPU -> BIOS -> CMOS
• BIOS = Basic Input / Output System
• CMOS = Complementary Metal-Oxide Semiconductor
• ROM = Read Only Memory
• POST = Power On Self Test
• HDD → 1st sector = MBR
• MBR = Master Boot Record
• OS in RAM
• App goes to CPU

Bootstrap

• How the computer is going to power on