What is the difference between primary group and secondary group in Ubuntu? - Ask Ubuntu

What is the difference between primary group and secondary group in Ubuntu? - Ask Ubuntu

If you are talking about file system groups, they are pretty well explained here cyberciti article. The primary group is used by default when creating a new file. You can test this

touch foo  ls -la foo  

The file will be owned by you and be in your primary group. Users who are also in your primary group will have group level permissions on those files.

You can check your secondary groups with

groups $(whoami)  

It also possible to share files with people who are not in your primary group by setting Set Group ID on a directory. This is explained here: shared folder with SetGID.

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