What is a user & group in Linux?

In Linux, the terms user & group refer to system identities and how access is managed. A user represents an individual identity, while a group defines a collection of users who share the same access rights. Ultimately, access control in Linux revolves around these two concepts, who is accessing (user) and what permissions they have through the group that they are associated with (group).

A Linux system without proper user and group management would be insecure, allowing anyone to access any data they want.

User

A user is an individual account on the system. Each user has:

  • A username
  • A password
  • Their own home directory (/home/<username>)
  • Their own set of permissions

Every time someone logs into the system, they do so as a “user”.

Group

A group is a collection of users. Instead of assigning permission to each other individually, Linux allows permissions to be assigned to a group. All users in a group inherit the group’s permissions.

How Permissions Work?

Every file and directory in Linux has three types of access control:

  1. Owner (user)
  2. Group (group)
  3. Others

Each of these can be given different permission levels:

  1. Read(r) - View the Contents
  2. Write (w) - Modify the Contens
  3. Execute (x) - Run/Execute the File (script/program)

Let take a look at a good example of how it’s going to look like on a file:

total 4
drwxr-xr-x. 1 iamyaash iamyaash  20 Jun 12 23:09 archetypes
-rw-r--r--. 1 iamyaash iamyaash 596 Jun 15 14:40 config.yaml
drwxr-xr-x. 1 iamyaash iamyaash  28 Jun 12 23:09 content
drwxr-xr-x. 1 iamyaash iamyaash 180 Jun 12 23:09 public
drwxr-xr-x. 1 iamyaash iamyaash  56 Jun 13 19:28 scripts
drwxr-xr-x. 1 iamyaash iamyaash  16 Jun 12 23:09 themes
  1. drwxr-xr-x - Means the
  • owner has full access (iamyaash)
  • group has execute & read access
  • others have execute access.
  1. rw-r-r - Means the
  • owner has read & write access
  • group and others only have read access
  1. iamyaash is the owner.
  2. iamyaash is the group (same name as the user, nvm :P)

How to Manage user & group?

Let’s learn how to manage both user & group using hands-on excercize to help understand and practice how users and groups work in Linux.

1. View Current User Information

whoami
id
grep <username> /etc/passwd #used to verify whether the user is created
groups #display groups associated with the current user
groups <username> #display groups associated with the specified username
id -Gn #same as the above
id -g -n  #this shows only your primary (current) group name.
  • whoami displays the current username
  • id display the group IDs which the current user is part of.

2. Create a New User

sudo useradd newUser
sudo passwd newUser
  • useradd create a user named “newUser”
  • passwd to set password for the user “newUser”

3. Create a New Group

sudo groupadd devGroup
  • create a group named devGroup

4. Add a User to a Group

sudo usermod -aG devGroup newUser
  • Add newUser (user) to devGroup (group)
  • a “a” stands of append used with G, it ensures the user is added to the specified group(s) without being removed from the existing groups.
  • G specifies the supplementary group(s) which the user should be added. (you can user multiple group separated by commas)

5. Delete a User

sudo userdel -r newUser
  • Executing the command with just userdel will only delete the user but not their home directory.
  • Executing the command with userdel -r flag will both the user and their home directory.

6. Delete a Group

sudo groupdel devGroup

Note: You cannot delete a group if it’s the primary group of an existing user. You must either delete the user first or change their group.

7. Create a File and Set Group Ownership

sudo su - newUser
cd -
touch demo.txt
ls -l demo.txt

In the above commands, we’re switching to newUser and creating a file with it. Listing it will show the file’s owner is newUser.

# Output from my device
iamyaash@fedora:~$ ll
total 0
-rw-r--r--. 1 iamyaash iamyaash   0 Jun 15 16:07  demo.txt

Let’s switch to root user and change the group permissions,

sudo chgrp dev /home/newUser/demo.txt
ls -l /home/newUser/demo.txt

# Output from my device
iamyaash@fedora:~$ sudo chgrp wheel demo.txt 
[sudo] password for iamyaash: 
iamyaash@fedora:~$ ll
total 0
-rw-r--r--. 1 iamyaash wheel      0 Jun 15 16:07  demo.txt

8. Set Group Permissions on a File

chmod 640 /home/newUser/demo.txt

Let’s understand the numeric permissions notations:

  1. 4 - r: Read
  2. 2 - w: Write
  3. 1 - x: Execute

This is the permission of the current file:

  1. 6 - user: read/write (4+2)
  2. 4 - group: read (4)
  3. 0 - others: no permission (0)

9. Test Access as Another Group Member

  1. Add yourself into customGroup (create a group for this to experiment)
sudo usermod -aG customGroup newUser
  1. Update your group access or log out and log back in.
newgrp customGroup
  1. Try accessing the /home/newUser/demo.txt
cat /home/newUser/demo.txt

Summary

To sum it up, user is an individual identity in Linux with its own username & password and set of permissions. A group is a collection of multiple users that allows shared access to files or resources based on group permissions. They both form the Foundation of Linux’s access control system.