How to modify user-account settings with usermod
There are quite a few changes you can make to user accounts on Linux systems: setting them up, deleting or disabling them, adding or removing users from secondary groups, changing usernames or UIDs, moving home directories, changing users’ shells, altering account expiration timing, and so on.
One command that can make nearly all of these changes easier is usermod. The only real constraints are 1) that the accounts you intend to change must already exist on the system (this command won’t set them up from scratch), and 2) that the affected users should probably not be logged in when you make these changes.
The basic syntax for the command is usermod [options] LOGIN but that options section has a lot more possibilities than you might anticipate. In addition, sudo permissions will be required for this command since superuser access is required to set up or change nearly all user account settings.
Source:: Network World – Linux