Group identifier (Unix)

In Unix-like systems, multiple users can be into groups. POSIX and conventional 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. This method, among others, also enables the superuser to delegate some administrative tasks to normal users, similar to the Administrators group on Microsoft Windows NT and its derivatives.

A group identifier, often abbreviated to GID, is a numeric value used to represent a specific group. The range of values for a GID varies amongst different systems; at the very least, a GID can be between 0 and 32,767, with one restriction: the login group for the superuser must have GID 0. This numeric value is used to refer to groups in the /etc/passwd and /etc/group files or their equivalents. Shadow password files and Network Information Service also refer to numeric GIDs. The group identifier is a necessary component of Unix file systems and processes.

In Unix systems, every user must be a member of at least one group, the primary group which is identified by the numeric GID of the user's entry in the group database, which can be viewed with the command getent passwd (usually stored in /etc/passwd or LDAP). This group is referred to as the primary group ID. A user may be listed as member of additional groups in the relevant entries in the group database, which can be viewed with getent group (usually stored in /etc/group or LDAP); the IDs of these groups are referred to as supplementary group IDs.

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