What is an Identity? Users, Groups & Permissions Explained

Every computer on earth has a gatekeeper. You’ve met it — it’s the login screen. But have you ever wondered how the computer actually decides what you can and can’t do once you’re in?

Whether you’re on your home laptop, SSHing into a Linux server, or configuring AWS — there is one mental model powering all of it. Learn it once. Use it everywhere for the rest of your career.

What is an Identity?

An identity is anything that can take an action on a computer system. It’s the answer to the question: “Who is doing this?”

An identity isn’t just a person. It can be:

Type	Example	What It Does
Human User	shekhar, root, alice	You — the person typing commands
Service Account	www-data, mysql, nginx	Programs that run as their own “user”
Process	Your browser, a cron job	Every running program inherits an identity
Machine Identity	An EC2 instance, a Kubernetes pod	Cloud resources that need to call APIs

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The Three Building Blocks

1. Users — The Individual Identity

A user is a unique named identity, represented by two things:

• A username (human-readable): shekhar, root, www-data
• A UID (User ID): a unique number the OS actually uses. shekhar might be UID 1001. root is always UID 0.

shekhar = the name YOU see
1001    = the number the COMPUTER sees

2. Groups — The Team Identity

A group is a collection of users. Instead of setting permissions for every user individually, you set them for a group — and every user in the group automatically inherits those permissions.

Real-world example:

• Group developers → has read/write access to /var/www/html
• You add alice, bob, and charlie to the developers group
• All three can now edit the website — without touching individual permissions

This is exactly how companies manage access: “Add them to the marketing group” means they instantly get all marketing system permissions.

3. Permissions — The Rules

A permission is a rule that says: “What can this identity DO with this resource?”

The classic Linux permission model answers three questions:

• Who? — User (owner), Group, or Others (everyone else)
• What? — Read (r), Write (w), Execute (x)
• To what? — A file, directory, or device

-rwxr-xr--
 │││└─ Others: read only
 │││
 ││└── Group: read and execute
 ││
 │└─── Owner: read, write, execute
 │
 └──── File type (- = regular file)

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This Model Is Everywhere

Here’s the important realization: you will see this same model in every IT environment you ever work in — just with different names:

Concept	Linux	Windows / AD	AWS IAM
User	Linux user (useradd)	AD User Account	IAM User
Group	Linux group (groupadd)	AD Security Group	IAM Group
Permission	chmod, file permissions	NTFS permissions, GPO	IAM Policy (JSON)
Superuser	root (UID 0)	Administrator / Domain Admin	Root Account + Admin Role
User Database	/etc/passwd + /etc/shadow	Active Directory (LDAP)	IAM User List
Service Account	www-data, mysql user	Service Account	IAM Role (for EC2, Lambda)

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Hands-On: Discover Your Own Identity

Right now, on your Linux or macOS machine, you have an identity. Let’s look at it.

Step 1: See who you are

Step 2: See your full identity (UID, GID, all groups)

Step 3: See what groups a user belongs to

Step 4: View all users on the system

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Hands-On Challenge

Try this sequence on your own Linux system:

1. Run whoami — note your username
2. Run id — find your UID and list your groups
3. Run cat /etc/passwd | grep your-username — see your full user entry
4. Run ls -la /home/ — notice how each home directory is owned by a different user
5. Bonus: Run sudo cat /etc/shadow — this is where hashed passwords are stored. Only root can read it. This is the permission system protecting your passwords.

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Key Takeaways

1. An identity is anything that takes an action — humans, services, programs, cloud resources.
2. Every user has a UID — the computer uses numbers, not names.
3. Groups let you manage permissions at scale — set it once, it applies to everyone in the group.
4. The model is universal — Linux users/groups, Windows AD, AWS IAM — same foundation, different syntax.
5. Root (UID 0) is the master key — treat it like live electricity. Use when needed, exit immediately after.
6. Every running process inherits an identity — nginx isn’t “just running”, it runs AS www-data.

Next Steps in This Series

• Linux Users & Groups: Complete Guide — Create and manage users/groups with useradd, groupadd, usermod
• File Permissions: Visual Guide — Master chmod, chown, and the rwx permission bits in depth
• sudo & The Sudoers File — Learn controlled privilege escalation and the sudoers configuration

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