Basic information knowledge about Linux Operating System and commands.  

Linux is an Open-Source Operating System, It’s available in various flavor such as RedHat, Cent, Ubuntu, Suse, Mint, Scientific Linux etc.

Linux is divided into four major part 

HARDWARE :- Physical Components of
                             Desktop/PC or Laptop.
KERNEL :- Layer between hardware and OS

SHELL:- Interface between User and OS.

APPLICATION:- Helps users to intact with 

LINUX Directory Structure 

/ (forward Slash) sub directory under root. 
/root –  #root (Admin) user directory.
/bin – Contain user Command Binaries. 
/boot – contain linux OS boot loader information.
/dev – Hardware devices related files.
/etc – System configuration files.
/home – Contain users files and directory.
/lib – Shared library and Kernel Modules.
/media – mount point for external media HDD,  SAN / NAS.
/mnt – Mount point for temp file system.
/opt – ADD-ON application packages.
/sbin – System binaries.
/srv – System provided service data.
/tmp – Temporary files.
/usr – Utilities and applications.
/var – Contain variables.
/proc – Contain Kernel Process status as text.
Linux Boot process :-  
As we switch on linux loaded machine its typically follow 6 major steps.

1. BIOS (basic Input Output System).

2. MBR (Master Boot Record). 

3. GRUB – (GRAND Unified Bootloader).

4. KERNEL initialization, load OS on main memory. 

5. init process reads /etc/inittab file and set the path,   
    start swapping, checks the file system. 

6. Run Level –  there are 7 run level for specific purpose such as
   0 – halt
     1 – Single user mode
     2 – Multi user without NFS(Network File System)
     3 – Full muntiuser mode
     4 – unused 
     5 – X11 (GUI-Graphic User Interface)
     6 – Reboot.
Note :- We can define specific run level in /etc/inittab file. 
Few frequently used  Linux Commands –:
1. ls – use to list all the file and folder. “ls -l” for more detail. 
2. cd – use to change directory.
3. pwd – print working directory. shows your present path.
4. cp –  helps you to copy file form one location to other place Example :- cp -r use for folder.  cp -rp copy with same permission. 
5. scp – secure copy command help to copy through network.
6. rm – use to delete file or folder, “rm -rf” for force delete.
7. date – shows system date.  
8. cal – shows current month calendar. cal 2018 for specific year.
9. exit – to close terminal/Shell. 
10. chmod – use to change file folder permission.
11. chown – use to change file folder ownership. 
12. mkdir – use to create directory.
13. ln -s – use to create symbolic link to file. 
14. more – show output of file.
15. tail –  shows output of last 10 lines of file.
16. tail -f shows current updated last 10 lines.
17. ssh –  secure login command to access other system.
18. grep – search for pattern in files.
19. locate – find files. 
20. whereis – search for file / folder.
21. ps –  shows current running process. 
22. uptime – shows  uptime of system in days and hours.
23. whoami – check for current user info.
24. w – check how many users are logged-in.
25. finger – shows detailed info about user.
26. man – shows detailed manual of command.
27. df -h – show disk usage and partition structure.   
28. free – memory info. 
29. du -hs – used disk usage. 
30. tar -xvf –  use to extract .tar file.
31. tar -xjvf – use to extract .tar.gz file.
32. ping – helps to check system on network status.
33. wget url path- download file on command mode. 
34. nslookup – check ip domain info,  vice versa.
35. traceroute – connectivity path from host to destinationmachine. 
36. dig – check DNS info.
37. uname -a – to check kernal version 
38.lsb_release -a  – to check version 
39. who -u (who logged in currently and process ID)
40. kill -HUP -1 kill all users 41. sudo apt-get remove –purge mysql-\* remove package