If only a colon is given, then the user's default group is used.
如果只有给出冒号，那么就使用用户的默认组。
By default, he is only removed from the named group.
默认情况下，仅从指定的组中删除这名员工。
On Linux and UNIX platforms, a NULL value defaults to the primary group of the instance owner, which by default after an installation only contains the user ID of the instance owner.
在Linux和UNIX平台上，null值被缺省地赋给实例所有者的主组，完成安装后，缺省情况下这个组只包含用户ID和实例所有者。
On Linux and UNIX platforms, a NULL value defaults to the primary group of the instance owner, which by default after an installation only contains the user ID of the instance owner.
在Linux和UNIX平台上，null值被缺省地赋给实例所有者的主组，完成安装后，缺省情况下这个组只包含用户ID和实例所有者。
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