Linux RHCSA Part 30 - Users - Groups

Now that we have users and passwords, it is time to create groups. Creating groups is similar to creating users. Let's learn the commands for creating and managing groups.

Let's use the "groupadd" command to create a group. 

groupadd command syntax: # groupadd [options] group

groupadd command options:

• -r  - Create a system group 
• -g GID - Set the GID number for the group

# groupadd Sales

Just as users are contained in the /etc/passwd file, groups are contained in the /etc/group file.

GROUP FILE 

Group file structure : 

<group name>:<password placeholder>:<GID>:<members>

In this file, you can verify if new groups have been created. 

Step 1 - Adding users to a group : 

# usermod -G Sales user02

Step 2 - Verifying the addition : 

Step 3 - Let's add another user to the Sales group : 

# usermod -G Sales user03

Step 4 - and check what changes have occurred : 

Another way to verify is to use the "id" command. 

id command syntax: # id [options] username

id command options: 

• -G - Show GID
• -n - Show the name for the given ID
• -u - Show UID

If you run the "id" command without flags, you can see the UID and GID for the user.

Let's also check another user : 

If a group is no longer needed, it should be removed. Let's check the existing groups on the system : 

And we can easily delete the unnecessary group : 

# groupdel sales

If we want to add a user to an additional group without replacing the ones they are already in, we use the command to add a user to groups by adding the "-a" flag, e.g.: 

# usermod -a -G groupname username