Solved Difference between "usermod -aG" and "usermod -G" options in linux?

[Brief-Wishbone9091]

I'm on centos 7. I'm wondering what could be the differences between these two?
I've read this but could not grasp what they're trying to say. Can you give me an example scenario to work with?

Difference between "usermod -aG" and "usermod -G" options

I'm preparing for the LPIC-1, exam 102, and was wondering what is the difference between these two commands with respect to options -G and -aG: usermod -G projectA, projectB jsmith usermod -aG pro...

unix.stackexchange.com

 

[f33dm3bits]

The -a stands for append, if you only use -G it will remove all the other supplementary groups that the user was a member of before.

[root@localhost ~]# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser)
[root@localhost ~]# usermod -G group1 testuser
[root@localhost ~]# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser),1002(group1)
[root@localhost ~]# usermod -G group2 testuser
[root@localhost ~]# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser),1003(group2)
[root@localhost ~]# usermod -aG group1 testuser
[root@localhost ~]# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser),1002(group1),1003(group2)

 

[Brief-Wishbone9091]

The -a stands for append, if you only use -G it will remove all the other supplementary groups that the user was a member of before.

[root@localhost ~]# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser)
[root@localhost ~]# usermod -G group1 testuser
[root@localhost ~]# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser),1002(group1)
[root@localhost ~]# usermod -G group2 testuser
[root@localhost ~]# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser),1003(group2)
[root@localhost ~]# usermod -aG group1 testuser
[root@localhost ~]# id testuser
uid=1001(testuser) gid=1001(testuser) groups=1001(testuser),1002(group1),1003(group2)

Thanks for the crisp example. I think I've got it.