GidNumber (group identifier, often abbreviated to GID) is used in Unix Linux like and POSIX Operating Systems where multiple users can be put 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.

GidNumber (group identifier, often abbreviated to GID), is a Integer 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.

Reserved ranges#

Many Linux systems reserve the GID number range 0 to 99 for statically allocated groups, and either 100−499 or 100−999 for groups dynamically allocated by the system in post-installation scripts. These ranges are often specified in /etc/login.defs, for useradd, groupadd and similar tools.

On FreeBSD, porters who need a GID for their package can pick a free one from the range 50 to 999 and then register this static allocation in ports/GIDs.[2]

Special values#

  • 0: The superuser normally has a GID of zero (0).
  • −1: The value (gid_t) -1 is reserved by POSIX to identify an omitted argument.[3]
  • 65,534: The Linux Kernel defaults to 216−2 = 65,534 (which many Linux distributions map to the group name "nogroup") when a 32-bit GID does not fit into the return value of a 16-bit system call. The value is also returned by idmapd if a group name in an incoming NFSv4 packet does not match any known group on the system.

LDAP Attribute Definition#

The GidNumber AttributeTypes is defined as:

More Information#

There might be more information for this subject on one of the following: