simple linux commands basic

Linux commands :

1.add user in linux
useradd user_name

2.change password
passwd user_name

3.removing user/group
userdel/groupdel user_name/group_name

4.show all process
  ps 

5. Delete  running process
kill p_id    ( -9 for force quit )

6.Show user details 
 id user_name

7.How to find particular word in file

grep "find_String" file_name
egrup -r "s1|s2" file_name

8.show open ports in linux
netstat --listen OR netstat

9.show how many file are open
lsof

10.memory status
 free