# All About Linux

**What is Linux?**

Linux is a free open-source operating system that manages a computer's hardware and resources,like memory,storage and cpu.

**Why linux?**

It is a free,open source and secure platform with flexibility,customization and fast compared with other os.

**What is Linux Boot process?**

The Linux boot process has 6 levels:

**BIOS:** Basic input output system

\-Perform integrity checks

\-Search,load and execute the boot load program from cd-rom,hard drive,floppy

\-In simple terms, the BIOS loads and executes the Master Boot Record (MBR) boot loader

**MBR:** Master boot record

\-Located in first sector of bootable disk /dev/hda or /dev/sda

\-It has 3 components 1.primary boot loader 2.partition table 3.mbr validation check

\-It is responsible for loading and executing the GRUB boot loader.

**GRUB:**Grand Unified Bootloader executes kernel-It has multiple kernel images on your system

\-Knowledge about file system

\-Grub configuration file is /boot/grub/grub.conf

**Kernel**:Core of the operating system.

\-Mounts root file system and executes /sbin/init program

\-As it is the first program executed by kernel,its process id is 1

\-initrd(initial RAM disk) is a used as a temporary root file system until kernel is booted.

**Init:**Looks at the /etc/inittab file to decide the Linux run level.

\-0 – halt,1 – Single user mode, 2 – Multiuser, without NFS ,3 – Full multiuser mode , 4 – unused ,5 – X11 , 6 – reboot

**Runlevel**: When the Linux system is booting up, you might see various services getting started, depending on your default init level setting.

\-Programs start with S are used during startup and K for shutdown.

**Top 50 linux Commands:**

1. ls-Displays list of files or directories
    
2. pwd-print current working directory
    
3. cd-change directory ,cd ..-change directory to previous,cd ~-change directory to home
    
4. mkdir-create a new directory
    
5. mv-move or rename files
    
6. cp-copy a file
    
7. rm-remove file or directory
    
8. touch-creates a blank file
    
9. clear-clear the display
    
10. cat-Display the content of the file
    
11. ln - Create symbolic links (shortcuts) to other files 
    
12. echo - Print any text that follows the command 
    
13. less - Display paged outputs in the terminal 
    
14. man - Access manual pages
    
15. uname - Gets basic information about the OS 
    
16. whoami - Get the active username 
    
17. tar - Command to extract and compress files in linux 
    
18. grep - Search for a string within an output 
    
19. head - Return the specified number of lines from the top 
    
20. tail - Return the specified number of lines from the bottom 
    
21. diff - Find the difference between two files 
    
22. cmp - Allows you to check if two files are identical 
    
23. comm - Combines the functionality of diff and cmp 
    
24. sort - sort the content of a file while outputting 
    
25. export - Export environment variables
    
26. zip - Zip files in Linux 
    
27. unzip - Unzip files in Linux
    
28. ssh - Secure Shell command in Linux 
    
29. service - Linux command to start and stop services 
    
30. ps - Display active processes 
    
31. kill and killall - Kill active processes by process ID or name 
    
32. df - Display disk filesystem information 
    
33. mount - Mount file systems in Linux
    
34.  chmod -change file permissions,4-read,2-write,1-execute
    
35. chown - granting ownership of files or folders 
    
36. ifconfig - Display network interfaces and IP addresses 
    
37. traceroute - Trace all the network hops to reach the destination 
    
38. wget - Direct download files from the internet 
    
39. ufw - Firewall command 
    
40. iptables - Base firewall for all other firewall utilities to interface with 
    
41. apt, pacman, yum, rpm - Package managers depending on the distribution 
    
42. sudo - Command to escalate privileges in Linux 
    
43. cal - View a command-line calendar 
    
44. alias - Create custom shortcuts for your regularly used commands 
    
45. dd - Majorly used for creating bootable USB sticks 
    
46. whereis - Locate the binary, source, and manual pages for a command 
    
47. whatis - Find what a command is used for 
    
48. top - View active processes live with their system usage 
    
49. useradd and usermod - Add a new user or change existing user data 
    
50. passwd - Create or update passwords for existing users 
    

**Linux File System:**

![](https://www.linuxfoundation.org/hs-fs/hubfs/Imported_Blog_Media/standard-unix-filesystem-hierarchy-1.png?width=1817&height=1001&name=standard-unix-filesystem-hierarchy-1.png align="left")