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# Enables userdel(8) to remove user groups if no members exist.
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USERGROUPS_ENAB yes

$ man useradd
<...>
       By default, a group will also be created for the new user (see -g, -N,
       -U, and USERGROUPS_ENAB).
<...>
       -g, --gid GROUP
           The name or the number of the user's primary group. The group name
           must exist. A group number must refer to an already existing group.
           If not specified, the behavior of useradd will depend on the
           USERGROUPS_ENAB variable in /etc/login.defs. If this variable is set
           to yes (or -U/--user-group is specified on the command line), a
           group will be created for the user, with the same name as her
           loginname. If the variable is set to no (or -N/--no-user-group is
           specified on the command line), useradd will set the primary group
           of the new user to the value specified by the GROUP variable in
           /etc/default/useradd, or 1000 by default.
<...>
       -N, --no-user-group
           Do not create a group with the same name as the user, but add the
           user to the group specified by the -g option or by the GROUP
           variable in /etc/default/useradd.
           The default behavior (if the -g, -N, and -U options are not
           specified) is defined by the USERGROUPS_ENAB variable in
           /etc/login.defs.
<...>
       -U, --user-group
           Create a group with the same name as the user, and add the user to
           this group.
           The default behavior (if the -g, -N, and -U options are not
           specified) is defined by the USERGROUPS_ENAB variable in
           /etc/login.defs.
<...>
       USERGROUPS_ENAB (boolean)
           If set to yes, userdel will remove the user's group if it contains
           no more members, and useradd will create by default a group with the
           name of the user.