User & group commands in -nix systems

To change permission
chmod -R 755 folder_name
with which,755 -> ABC
A = User
B = Group
C = Others
and -R = recursive to all sub-folders.

To change group and owner of folder
chown -R group:user folder_name

Create group
groupadd group_name

Create user
adducer user_name

Add an existing user to an existing group
usermod -a -G group_name user_name
with which -a=append, -G=supplementary_group, -g=primary_group

Remove an existing user from an existing group (will not delete the user nor the group)
deluser user_name group_name

Validate user group settings
id user_name, or
groups user_name