usermod -a -G参数a加与不加的区别

[云海幻梦]

usermod -a -G参数a加与不加的区别，查看man手册，usermod -G与usermod -a -G效果应该是不一样，建了3个用户组做测试我执行了一下怎么都是一样添加的效果？！
-a, --append
Add the user to the supplementary group(s). Use only with the -G
option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of.
Each group is separated from the next by a comma, with no
intervening whitespace. The groups are subject to the same
restrictions as the group given with the -g option.

If the user is currently a member of a group which is not listed,
the user will be removed from the group. This behaviour can be
changed via the -a option, which appends the user to the current
supplementary group list.

[onlylove]

If the user is currently a member of a group which is not listed,
the user will be removed from the group. This behaviour can be
changed via the -a option, which appends the user to the current
supplementary group list.


你只看了添加，没看移除？

[云海幻梦]

英文看得云里雾里，单对移除方面来讲，我理解为加了-a后的原user不移除？不加-a后原user移除？！

[云海幻梦]

从字面上翻译:(命令中)没有被列出的user
将被从原组中移除？

[astolia]

一个是设置用户只属于指定的组，一个是将用户添加到指定的组

[云海幻梦]

[云海幻梦]

以下这个翻译貌似还可以，但也生硬:
-a将用户添加到新组时，请始终使用(append) 选项。如果省略该-a选项，则用户将从-G选项后未列出的组中删除。