chmod command in linux is used to change existing permissions of files/directories. chmod [OPTION]… MODE… FILE… . e.g. $chmod u+x file.txt
groupdel command is used to delete all entries related to a group in the system. The named group must exist. Syntax: groupdel [options] GROUP
groupmod command in linux is used to modify the information related to any existing group. Syntax groupmod [options] GROUP
groupadd command in linux is used to create a new group. groupadd [Options] group. The list of existing groups in the system is available in /etc/group file
usermod command in linux is used to modify an existing user’s account information. Only the super user can use this command.
userdel command in linux is used to delete an existing user and related files. Only the super user can use this command.
useradd command is used to add a new user in the system. In other words it is used to create a new login account for a different user. useradd command can be used only by the root user.