Day 06 — Users, Groups & System-Level Commands in Linux

🎯 Goal

Understand how Linux manages:

• Users and permissions
• Access control
• System-level operations

This is critical for:

• Security
• Multi-user systems
• Production server management

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🧠 Mental Model

Linux is a multi-user operating system.

Every action happens in this chain:

User → Group → Permissions → System Access

You are never just running commands —
you are running them as a user with privileges.

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1️⃣ System-Level Commands (Core OS Interaction)

These commands help you understand who you are, where you are, and system state.

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uname — System Information

uname -a

What it does:
Shows kernel, OS, architecture.

Why it matters:

• Identify system after SSH login
• Debug compatibility issues

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uptime — System Running Time

uptime

What it shows:

• How long system is running
• Load average (CPU usage trend)

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date — Current Time

date

Used for:

• Debugging logs
• Checking timezone issues

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who — Logged-in Users

who

Shows:

• Active sessions
• TTY
• Login time

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whoami — Current User

whoami

Very important in scripts and debugging.

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which — Command Location

which python

What it does:
Shows path of executable.

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id — User Identity

id

Output example:

uid=1000 gid=1000 groups=1000,27

Why it matters:

• Shows user ID and group memberships

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sudo — Run as Superuser

sudo apt update

What it does:
Executes command with elevated privileges.

Why it matters:

• Required for admin operations

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shutdown — Power Off System

sudo shutdown now
sudo shutdown -h +10

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reboot — Restart System

sudo reboot

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2️⃣ Package Management Commands

Used to install and manage software.

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Debian/Ubuntu (apt)

sudo apt update
sudo apt install nginx
sudo apt upgrade

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RHEL/CentOS (yum / dnf)

sudo yum install nginx
sudo dnf update

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Arch (pacman)

sudo pacman -S nginx

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Gentoo (portage)

emerge nginx

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3️⃣ Users & Groups — Core Concepts

Every file/process has:

• Owner (user)
• Group
• Permissions

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User Types

Type	Description
root	Superuser
normal user	login user
system user	service accounts

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4️⃣ User Management Commands

useradd — Create User

sudo useradd username
sudo useradd -m username

Creates home directory with -m.

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passwd — Set Password

sudo passwd username

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su — Switch User

su username
sudo su

Switch to another user or become root.

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userdel — Delete User

sudo userdel username
sudo userdel -r username

-r removes home directory.

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5️⃣ Group Management Commands

groupadd — Create Group

sudo groupadd devops

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groupdel — Delete Group

sudo groupdel devops

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gpasswd — Manage Groups

Add user to group

sudo gpasswd -a user group

Add multiple users

sudo gpasswd -M user1,user2 group

Check groups

groups username

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6️⃣ File Ownership (Important)

Even though detailed permissions come later, basic idea:

ls -l

Example output:

-rw-r--r-- 1 user group file.txt

Meaning:

• user owns file
• group owns file

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🧪 Day-6 Lab

# system info
uname -a
uptime
date
who
whoami
id
 
# command location
which python
 
# user creation
sudo useradd -m devuser
sudo passwd devuser
 
# switch user
su devuser
whoami
 
# create group
sudo groupadd devops
 
# add user to group
sudo gpasswd -a devuser devops
 
# check groups
groups devuser
 
# delete user
sudo userdel -r devuser

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🧠 Key Takeaways

• Linux is multi-user by design
• Permissions define system security
• sudo controls privilege escalation
• Users and groups control access
• Package managers manage software lifecycle

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✅ Outcome

You can now:

• Understand user identity and permissions
• Create and manage users
• Manage groups
• Use sudo correctly
• Install and manage software
• Perform system-level operations

Next → File Management & Permissions 🚀