A user account is required for a user to log into any Linux system. An account typically includes identity information such as username, password, user id (UID), and group identifier (GIDs) so that the system can identify the user. An account will also include a set of resources such as accessible disk space that the user can work in, typically called a home directory and information about the default shell preference.

Username

Every account in a Linux system is associated with a unique username, which is typically a sequence of alphanumeric characters at least three characters in length. It is case sensitive; for example, Apple01 is a different username than apple01. A unique integer, the user id (UID), is assigned to each username. Linux uses the UID rather than the username to manage user accounts since it is more efficient to process numbers than strings. However, you don't necessarily need to know your UID.

Group

Linux also has the notion of a group of users who need to share files and processes. Each account is assigned a primary group with a numerical group id (GID) that corresponds to the particular group. A single account can belong to many groups, but you may have only one primary group. Groups can also be used to assign certain permissions to users on the system.

Password

Each username requires a password. The username is the account identifier, and the password is the authenticator. A password is required for operations such as logging into the system and accessing files. Password requirements differ between systems; a sample requirement is that the password must be a minimum of 8 characters with at least 3 of the following character classes:

• lower-case letters
• upper-case letters
• numerical digits
• punctuation

When you enter your password, the system encrypts it and compares it to a stored string. This ensures that even the Operating System does not know your plain text password. This method is frequently used on websites and servers, especially those that run Linux. Once you have the system username and password, you are ready to log into the remote system using a secure shell (ssh). See the remote connections page for details about SSH.