Re: Changing groups

[Dan Jones <ddjones@riddlemaster.org>]

On Thu, 2003-08-14 at 16:21, Alan Shutko wrote:
> Dan Jones <ddjones@riddlemaster.org> writes:
> 
> > For example, say I supervise several teams working on a project.  Each
> > person should have access to only the project directories of their
> > teams.  However, some people are on more than one team.  And as
> > supervisor, I'm a member of them all. 
> 
> For the most part, there's no reason to change groups.  When you log
> in, your session is considered a member of all the groups it is
> defined to be in (up to some number I can't remember).  The common
> usage method is "I just added you to the group, Bob.  Log out and log
> back in."

Uh, no.  That's the way I thought it worked.  That's the way BSD works. 
That's not the way Linux works.  I found this out the hard way when I
created groups and could not access files owned by them, even though I
was clearly in the group.  From "Linux Unleashed" by Tim Parker:

<Quote>
You are a member of one group at all times. When you log in you are
placed in your default group, which is set when root creates your user
account. You can belong to many different groups, but you can be logged
in to only one group at a time. To change the group you are a member of,
use the newgrp command. For example, if you are a member of a group
called users and also a group called programmers, and you need to change
to the programmers group because they have special access to a compiler,
issue this command:

newgrp programmers

Linux does not tell you which group youʼre in. Usually the only way to
find out which group is active is to save a file and then look at the
permissions.
</Quote>

> Why is newgrp there?  Two reasons.  One, to change to a group you
> aren't defined as a member of, but have the password (who ever
> thought THAT was a good idea!).  Two, to get a shell in that group
> without logging everything out.

Try it yourself.  Create a new group and add yourself to it.  Then
create a file, change it's owner to whoever and its group to the new
group.  Set permissions to 770 and try to read the file.

Then use newgrp to change your group and try to read it.