Basic information/ knowledge about Linux Operating System and commands

Basic information /knowledge about Linux Operating System and commands  

Linux is an Open-Source Operating System...its 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
                     program

SHELL:- Interface between User and OS.

APPLICATION:- Helps users to intact with 
                               OS. 






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.
            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 destination 
      machine. 
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