=== Modify users ===

== Add user to group ==

  usermod -aG group user
  
== To change just the username: ==

  usermod -l new_username old_username
  
== To change the username and home directory name: ==

  usermod -l new_username -m -d /new/home/dir old_username

== You may also want to change the name of the group associated with the user: ==

  groupmod -n new_username old_username