Group identifier

In Unix-like systems, 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.
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 codice_1 and codice_2 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.
Supplementary groups.
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 passwd database, which can be viewed with the command codice_3 (usually stored in codice_1 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 codice_5 (usually stored in codice_2 or LDAP); the IDs of these groups are referred to as "supplementary group IDs".
Effective vs. real.
Unix processes have an effective (EUID, EGID), a real (UID, GID) and a saved (SUID, SGID) ID. Normally these are identical, but in codice_7 and codice_8 processes they are different.
Conventions.
Type.
Originally, a signed 16-bit integer was used. Since the sign was not necessary – negative numbers do not make valid group IDs – an unsigned integer is now used instead, allowing group IDs between 0 and 65,535. Modern operating systems usually use unsigned 32-bit integers, which allow for group IDs between 0 and 4,294,967,295.
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 codice_9, for codice_10, codice_11 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.
Personal groups.
Many system administrators allocate for each user also a personal primary group that has the same name as the user's login name, and often also has the same numeric GID as the user's UID. Such personal groups have no other members and make collaboration with other users in shared directories easier, by allowing users to habitually work with codice_13. This way, newly created files can have by default write permissions enabled for group members, because this will normally only enable write access for members of the personal group, that is only for the file's owner. However, if a file is created in a shared directory that belongs to another group and has the setgid bit set, then the created file will automatically become writable to members of that directory's group as well.
On many Linux systems, the codice_14 variable in codice_9 controls whether commands like codice_10 or codice_17 automatically add or delete an associated personal group.