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<h3 class="section">29.1 User and Group IDs</h3>

<p><a name="index-login-name-3290"></a><a name="index-user-name-3291"></a><a name="index-user-ID-3292"></a>Each user account on a computer system is identified by a <dfn>user
name</dfn> (or <dfn>login name</dfn>) and <dfn>user ID</dfn>.  Normally, each user name
has a unique user ID, but it is possible for several login names to have
the same user ID.  The user names and corresponding user IDs are stored
in a data base which you can access as described in <a href="User-Database.html#User-Database">User Database</a>.

   <p><a name="index-group-name-3293"></a><a name="index-group-ID-3294"></a>Users are classified in <dfn>groups</dfn>.  Each user name belongs to one
<dfn>default group</dfn> and may also belong to any number of
<dfn>supplementary groups</dfn>. Users who are members of the same group can
share resources (such as files) that are not accessible to users who are
not a member of that group.  Each group has a <dfn>group name</dfn> and
<dfn>group ID</dfn>.  See <a href="Group-Database.html#Group-Database">Group Database</a>, for how to find information
about a group ID or group name.

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