Local groups linux:

Every user on a Linux system have their own primary group. It usually has the same name as their username
The user "debian" for example will have as its primary group the "debian" group.

Secondary groups:

Secondary groups are groups that users could belong to in addition to their primary group to have access to other folders.


If the user "debian" becomes a member of the group "group_1", he could have "read-write" access to the file "file_1". 

Adding a group:

The below command adds a group named "group_2" to the system:


Displaying the groups a user belongs to:

We use the below command to display the groups a user belongs to:


We see that the "debian" user primary group is "debian", the rest are his secondary groups.

Changing the primary group of a user:

To modify the primary group of a user we use:


Adding new groups to the secondary groups of a user:

To add a new secondary group for a user we use:


Important:

The above command will delete all the secondary groups the user is a member of, it will only keep the newly added group "newgroup".

To avoid the above scenario, we use "-a" option "append":



We can check whether the new group was added using the below command:


The primary group role:

The primary group is used when a user creates a new file "file_1" in our example below:


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