User • Under Linux, every file and program must be owned by a user. • Each user has a unique identifier called a user ID (UID). • Each user must also belong to at least one group, a collection of users established by the system administrator. • Normal User----Root User
Group • Groups also have unique identifiers, called group IDs (GIDs). • Users may belong to multiple groups. • The accessibility of a file or program is based on its UIDs and GIDs.
The /etc/passwdFile • The file /etc/passwd contains all information regarding the user (login, passwords, etc.). Only the super user (root) must be able to change it. It is therefore necessary to change the rights of this file so that it can only be read by the other users.
The /etc/passwdFile • Username Field. • Password Field. • User-ID Field. • Group-ID Field. • GECOS • Directory • Startup Scripts.
The /etc/shadow File • Stores actual password in encrypted format for user's account with additional properties related to user password i.e. it stores secure user account information. All fields are separated by a colon (:) symbol. It contains one entry per line for each user listed in /etc/passwd file
The /etc/shadow File Information • Login name • Encrypted password • Days before password may be changed • Days after which password must be changed • Days before password is to expire that user is warned • Days after password expires that account is disabled • A reserved field
The /etc/group File • etc/group is a text file which defines the groups to which users belong under Linux and UNIX operating system. Under Unix / Linux multiple users can be categorized into groups. Unix file system permissions are organized into three classes, user, group, and others. • The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks, printers, and other peripherals.
The /etc/group File information • Group name The name of the group • Group password This is optional, but if set, it allows users who are not part of the group to join • Group ID (GID) The numerical equivalent of the group name • Group members A comma-separated list