How to add a user to multiple primary groups on Linux?

Question

linux

And what system file will these added primary (primary) user groups be displayed by me?
Thanks in advance

Answer 1

1. There can be only one Primary group and nothing else
2. It's not a fact that all user groups can be found in files, you never know how pam is configured.
3. And so - in /etc/passwd primary group, in /etc/groups all other groups