Add / Remove user to an existing Group in Linux

Users & Groups in Linux : ( Add & Remove )

All Groups of unix system are listed in the file /etc/group
To see the list of all groups of system

$ cut -d: -f1 /etc/group

See all Groups of a user :

$ id -nG username

Suppose I want to see all Groups of user Thomas whose username is thomas :

$ id -nG thomas

The output is name of groups of thomas.

user1 root admin sambashare vboxusers usbuser mysql apache2

Here user1 is the Primary Group (First Group) & others are secondary Groups.
You can use following command too to see groups of a user.

$ groups username

The first group after colon is Primary Group.

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Add an existing Group to user (Other groups should not be removed) :

$ usermod -a -G groupname username

Suppose I want to add a new group to user thomas, other groups should not be removed.

$ usermod -a -G newgroup thomas

Now an existing group newgroup has been added to user thomas. To see the changes, Run

$ id -nG thomas

The output is

user1 root admin sambashare vboxusers usbuser mysql apache2 newgroup

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Add an existing Group to user (other groups should be removed, only Primary Group and currently added Group should be available) :

$ usermod -G groupname username

Suppose I want to add new group to user thomas, other groups should be removed. Only Primary Group and currently added group should be assigned to user.

$ usermod -G grp thomas

Now an existing group grp has been added to user thomas. All other secondary groups have been removed from thomas
To find current groups of user thomas

$ id -nG thomas

The output is

user1 grp

All other secondary groups are replaced with grp. Now there is only one secondary group for thomas.

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Remove Group of a user :
You can remove only secondary groups of a user. Primary Group can't be removed by this command. If you want to remove primary group, you have to assign a new primary group for user. A user can't be modified without primary group.

$ gpasswd -d username groupname

Suppose I want to remove a secondary group apache2 from user thomas.
Right now there are following Primary Group & Secondary Groups of user thomas.

$ id -nG thomas

The output is

user1 root admin sambashare vboxusers usbuser mysql apache2 newgroup

Now I delete apache2

$ gpasswd -d thomas apache2

See the groups of user thomas

$ id -nG thomas

The output is

user1 root admin sambashare vboxusers usbuser mysql newgroup

apache2 has been removed for thomas.

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Change Primary Group of a user :

$ usermod -g groupname username

It removes the current Primary Group of the user and replaces with new added primary group. Secondary groups are as it is. There is no change in Secondary Groups.
Suppose I want to change primary group of user thomas.
To see the groups of user thomas

$ id -nG thomas

The output is

user1 root admin sambashare vboxusers usbuser mysql apache2 newgroup

Now I run following command to change Primary Group

$ usermod -g newgroup2 thomas

Now Primary Group user1 has been removed from thomas and it is replaced with newgroup2. Now newgroup2 is primary group of thomas.
To verify the changes, I see the groups list.

$ id -nG thomas

The output is

newgroup2 root admin sambashare vboxusers usbuser mysql apache2 newgroup

Now the Primary Group is newgroup2.