Managing users in Linux

The data about users is stored in /etc/passwd. Editing it manually is usually a bad idea. You can read about its structure here.

/etc/group stores info about groups. Each user has a primary group, named the same as the user, and may be in supplementary groups (like sudo, fuse, etc.).

• who - show who is logged in
• whoami - current user name
• su - substitute user. Execute commands or log in as another user. Set to root by default.
  • su username - switch to a given user
  • su - username - switch to a given user and simulate a full login shell
• sudo <some_command> - execute command as root. Unlike su, will request the password of the current user, not root password. The user is required to be in sudo group. See also sudo.
• id - get user group (or other) id, e.g. id -g psharen

Modify users:

• adduser [_login_] - add a user
• deluser [_login_] - delete user
• usermod - modify user account
  • usermod -aG group1,group2 user - add user to groups
  • -a - append, used only with -G
  • -G - groups
• gpasswd -a username group - also add user to groups
• adduser [_login_] [_group_] - also add user to groups
• passwd [_user_] - set passwords. Defaults to current user, but can edit other users’ as well, if under root.

Groups:

• groups <username> - display the user’s groups, defaults to current user
• addgroup - create groups (only root can do that). There’s also groupadd, but it’s more for scripting purposes. On my machine, addgroup is a symlink to adduser
• delgroup - delete groups

• How to List Users in Linux warning: scripts are mostly not functional
• How To Edit the Sudoers File | DigitalOcean
• Groups and processes, how it works under the hood
• SystemGroups - Debian Wiki - has some info about system groups like cdrom, fuse, etc. Those are mostly obselete.
• GID, current, primary, supplementary, effective and real group IDs?
• How do groups work on Linux?
• addgroup vs groupadd

• Linux password policy