Describe in a brief manner the main purpose of the following Unix/Linux commands; 1. Is -I /home/ 2. man 3. whatis 4. clear 5. cd- 6. pwd 7. cal 8. who 9. mkdir 10. rmdir 11. passwd 12. uname -a 13. cat> filename.txt 14. rm 15. more and less 16. head and tail 18. sftp 19. put and get 20. exit