.. index:: group, file ownership
.. _groups1:

Understanding Ownership and Group Ownership
--------------------------------------------

:t:`Linux for Programmers and Users`, Sections 3.27, 3.28, 3.29

We have seen from the output of :option:`ls -l` (see :ref:`ls`), 
that each file has an owner and a group owner.

.. note::

    A :dfn:`group` is a collection of users that can potentially share
    files with each other that are not shared with everyone.  Groups are
    usually defined in the :file:`/etc/group` file.

File permissions are grouped by three classes of users:

#. Owner of the file
#. Group Owners
#. Others

In the output of :option:`ls -l`, the middle set of permissions for 
**r**\ ead, **w**\ rite and e\ **x**\ ecute are group permissions.


Entry for romeo in :file:`/etc/passwd`::

    romeo:x:1003:1003::/home/romeo:/usr/bin/ksh


Entry for romeo's group (1003) in /etc/group::

    romeo::1003:

Here romeo is in a group by himself.  This is a common practice, as it keeps
files with liberal group permission from being shared with others
unintentionally.  Our user romeo may also be in other groups so that select
files and directories may shared with colleagues.

.. _groups:

groups
^^^^^^^

.. index:: groups
.. program:: groups
.. describe:: groups

    Lists names of groups that the user is in.

SYNOPSIS

    :command:`groups` [USERNAME]...


::

    $ groups
    romeo tailgaters shakespeare
    $ getent group shakespeare
    shakespeare:x:3023:romeo,juliet 

.. note::

    How does :option:`ls -l` print the name of the owner and group owner? If
    a file `foo` is owned by `romeo` and group owned by `users`, the inode
    for foo stores 1003 and 101 as the UID and GID, respectively.
    :command:`ls` first looks up the inode for these two parameters and then
    translates them to their name representation using `/etc/passwd` and
    `/etc/group`.

    Why doesn't romeo's name appear in the entry in /etc/group?