Linux - Logging on to another group

3.4.2.2. Logging on to another group

When you type id on the command line, you get a list of all the groups that you can possibly belong to, preceded by your user name and ID and the group name and ID that you are currently connected with. However, on many Linux systems you can only be actively logged in to one group at the time. By default, this active or primary group is the one that you get assigned from the /etc/passwd file. The fourth field of this file holds users' primary group ID, which is looked up in the /etc/group file. An example:

asim:~> id
uid=501(asim) gid=501(asim) groups=100(users),501(asim),3400(web)

asim:~> grep asim /etc/passwd
asim:x:501:501:Asim El Baraka:/home/asim:/bin/bash

asim:~> grep 501 /etc/group
asim:x:501:

The fourth field in the line from /etc/passwd contains the value "501", which represents the group asim in the above example. From /etc/group we can get the name matching this group ID. When initially connecting to the system, this is the group that asim will belong to.

	User private group scheme
	In order to allow more flexibility, most Linux systems follow the so-called user private group scheme, that assigns each user primarily to his or her own group. This group is a group that only contains this particular user, hence the name "private group". Usually this group has the same name as the user login name, which can be a bit confusing.

Apart from his own private group, user asim can also be in the groups users and web. Because these are secondary groups to this user, he will need to use the newgrp to log into any of these groups. In the example, asim needs to create files that are owned by the group web.

asim:/var/www/html> newgrp web

asim:/var/www/html> id
uid=501(asim) gid=3400(web) groups=100(users),501(asim),3400(web)

When asim creates new files now, they will be in group ownership of the group web instead of being owned by the group asim:

asim:/var/www/html> touch test

asim:/var/www/html> ls -l test
-rw-rw-r--  1 asim web   0 Jun 10 15:38 test

Logging in to a new group prevents you from having to use chown (see Section 3.4.2.4) or calling your system administrator to change ownerships for you.

See the manpage for newgrp for more information.