Groupd and User 's Group Changing 2003-03-15 - By Cameron Simpson
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On 11:26 14 Mar 2003, Nestor Waldyd <nwaldyd@(protected)> wrote:
| I really don't have a good definition of what a group is in Linux... Would
you please give me one? What i know
| is that a group is a set of users, but how can i change an user's group?
A group is a set of users, but the sets need not be disjoint. A user may
be a member of several groups for access to various things.
The mapping is mostly in the file /etc/group, whihc is a list of group
names, group ids (the internal numeric values used to represent each
group) and the users in each.
At any given time a process has
- a user id (uid)
- a primary group id
- a list of secondary groups ids
The uid and primary gid come from the /etc/passwd file.
The secondary gids come from the /etc/group file.
There's nothing special about the primary versus secondary gids; the
primary gid dates from an early revision of UNIX where there was only the
primary gid and no secondaries; the eatra groups were a BSD innovation,
now universal.
When accessing a file exactly _one_ of the three permission groups on the
file are considered:
- if the user id of the file matches the user id of file
then the first (user) set of permissions are used
- otherwise, if the gid of the file matches one of the gids of the
process, the second set is used
- otherwise, the third ("other") set is used
Cheers,
--
Cameron Simpson, DoD#743 cs@(protected) http://www.zip.com.au/~cs/
Success in software development depends on making a carefully planned
series of small mistakes in order to avoid making unplanned large
mistakes. - Steve McConnell, _Software Project Survival Guide_
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