format of the group permissions
The file /etc/group consists of newline
separated ASCII records, one per group, containing four colon
:’) separated fields. These fields
are as follows:
- Name of the group.
- Group's encrypted password.
- The group's decimal ID.
- Group members.
The group field is the group name used for
granting file access to users who are members of the group. The
gid field is the number associated with the group
name. They should both be unique across the system (and often across a group
of systems) since they control file access. The passwd
field is an optional encrypted password. This field is rarely used and an
asterisk is normally placed in it rather than leaving it blank. The
member field contains the names of users granted the
privileges of group. The member names are separated by
commas without spaces or newlines. A user is automatically in a group if
that group was specified in their
passwd(5) entry and does not need to be added to that group in the
If YP is active, the
group file also
supports YP exclusions and inclusions.
Lines beginning with a ‘
(minus sign) are entries marked as being excluded from any following
inclusions, which are marked with a `+' (plus sign).
Lines of the format
cause the specified group to be included from the
group.byname YP map. If no group name is specified,
or the ‘
+’ (plus sign) appears alone
on a line, all groups are included from the YP map.
YP references may appear anywhere in the file, but the single
+’ form should be on the last line,
for historical reasons. Only the first group with a specific name
encountered, whether in the
group file itself, or
included via YP, will be used.
Proper YP group support requires consistent group.byname, group.bygid and netid.byname YP maps. See getgrent(3) and getgrouplist(3) for details.
When YP is enabled but temporarily unavailable, login becomes impossible for all users except those having an entry in the netid(5) file.
passwd(1), setgroups(2), crypt(3), initgroups(3), netid(5), passwd(5), yp(8)
group file format first appeared in
Version 6 AT&T UNIX.
The YP file format first appeared in SunOS.
The passwd(1) command does not change the
Lines in /etc/group are limited to 1024 characters. YP groups are not affected by this limit.
Groups are limited to a maximum of 200 members per group.