Debian stores user information in password files.
/etc/passwdThe first file is called /etc/passwd; it stores the mapping of the user to a group, their login, the shell to run (bash, for example), UID, etc. (not much), one line per user.
File format:
login:encrypted_password:UID:GID:full_name:home_directory:shell
Where:
UID : Unique ID of the user
GID : Unique ID of the user's primary group
The password field usually contains 'x', which means the encrypted password is not stored here.
/etc/shadowThe actual user passwords are stored in the /etc/shadow file, also one line per user. If the passwd file has an entry for a user with 'x' in place of the password, but the shadow file has no entry for that user, the user is considered invalid.
File format:
login:
encrypted_password:
days_since_last_login:
days_before_password_can_be_changed:
days_after_which_password_MUST_be_changed:
days_before_expiry_when_user_starts_being_warned:
days_after_expiry_until_lockout:
days_since_the_Unix_epoch_when_the_user_was_locked:
reserved
/etc/groupGroups are defined in the /etc/group file. Also - one line per group.
File format:
group_name:password:GID:list_of_users
Users are separated from each other by commas. The password is usually absent.
More details (man)
# man 5 passwd
# man 5 shadow
# man 5 group
These help pages relate specifically to the password and group files.
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