Lesson 8: User & Group Management

Goal: Learn to create, modify, and delete users and groups on a Linux system.

Table of Contents

1. Current User Info
2. User Management
3. Password Management
4. Group Management
5. Important Files
6. Exercises

Current User Info

# Who am I?
whoami

# Full user ID info
id

# Example output:
# uid=1000(benjamin) gid=1000(benjamin) groups=1000(benjamin),27(sudo),999(docker)

# Which groups do I belong to?
groups

# Info about another user
id alice
groups alice

# Currently logged-in users
who
w

User Management

Create a user

# Create user with home directory
sudo useradd -m alice

# Create user with home dir, default shell, and comment
sudo useradd -m -s /bin/bash -c "Alice Smith" alice

# Create user and set password immediately
sudo useradd -m -s /bin/bash alice && sudo passwd alice

Flags:

Modify a user

# Change the user's shell
sudo usermod -s /bin/zsh alice

# Add user to a group (append — IMPORTANT: use -a)
sudo usermod -aG docker alice
sudo usermod -aG sudo alice

# Change username
sudo usermod -l alice-new alice

# Lock a user account
sudo usermod -L alice

# Unlock a user account
sudo usermod -U alice

# Change home directory
sudo usermod -d /new/home -m alice

Delete a user

# Delete user (keep home directory)
sudo userdel alice

# Delete user AND their home directory
sudo userdel -r alice

Switch users

# Switch to another user
su - alice

# Run a single command as another user
su - alice -c "whoami"

# Switch to root
sudo -i
# or
su -

Password Management

# Set or change password for a user
sudo passwd alice

# Change your own password
passwd

# Force password change on next login
sudo passwd -e alice

# Lock/unlock password
sudo passwd -l alice    # lock
sudo passwd -u alice    # unlock

# Check password status
sudo passwd -S alice
# Output: alice P 03/22/2026 0 99999 7 -1
# P = has password, L = locked, NP = no password

# Set password expiry policy
sudo chage -M 90 alice      # max 90 days
sudo chage -m 7 alice       # min 7 days between changes
sudo chage -W 14 alice      # warn 14 days before expiry
sudo chage -l alice          # list policy

Group Management

Create a group

sudo groupadd developers
sudo groupadd -g 1500 webteam    # with specific GID

Add users to groups

# Add existing user to a group
sudo usermod -aG developers alice
sudo usermod -aG developers bob

# Verify
groups alice
# Output: alice : alice developers

Remove user from a group

sudo gpasswd -d alice developers

Delete a group

sudo groupdel developers

List all groups

# All groups on the system
cat /etc/group

# Just group names
cut -d: -f1 /etc/group

# Groups with their members
getent group developers
# Output: developers:x:1001:alice,bob

Practical: Shared project directory

# 1. Create a group
sudo groupadd project-x

# 2. Add users to the group
sudo usermod -aG project-x alice
sudo usermod -aG project-x bob

# 3. Create shared directory
sudo mkdir /opt/project-x

# 4. Set ownership and permissions
sudo chown root:project-x /opt/project-x
sudo chmod 2775 /opt/project-x
# 2 = SetGID (new files inherit group)
# 775 = owner+group: full, others: read+execute

# 5. Now alice and bob can both create files here,
#    and all files will belong to the project-x group

Important Files

/etc/passwd — User accounts

cat /etc/passwd | head -3

Format:

benjamin:x:1000:1000:Benjamin Tietz:/home/benjamin:/bin/bash
│        │ │    │    │               │               └── Login shell
│        │ │    │    │               └── Home directory
│        │ │    │    └── Comment (full name)
│        │ │    └── Primary GID
│        │ └── UID
│        └── Password placeholder (actual in /etc/shadow)
└── Username

/etc/shadow — Encrypted passwords

sudo cat /etc/shadow | head -3

This file is only readable by root. It stores encrypted passwords.

/etc/group — Group definitions

cat /etc/group | head -5

Format:

developers:x:1001:alice,bob
│          │ │    └── Members
│          │ └── GID
│          └── Password placeholder
└── Group name

Exercises

Exercise 1: User information

# 1. Display your username
whoami

# 2. Show your UID, GID, and groups
id

# 3. List all groups you belong to
groups

# 4. Check how many users exist on the system
wc -l /etc/passwd

# 5. List only real users (UID >= 1000)
awk -F: '$3 >= 1000 {print $1}' /etc/passwd

Exercise 2: Create a user (requires sudo)

# 1. Create a test user
sudo useradd -m -s /bin/bash -c "Test User" testuser

# 2. Set a password
sudo passwd testuser

# 3. Verify
id testuser
ls -la /home/testuser/

# 4. Switch to the user
su - testuser
whoami
exit

# 5. Clean up
sudo userdel -r testuser

Exercise 3: Group management (requires sudo)

# 1. Create a group
sudo groupadd testgroup

# 2. Verify
getent group testgroup

# 3. Add your user to the group
sudo usermod -aG testgroup $USER

# 4. Check (may need to log out and back in)
groups

# 5. Clean up
sudo gpasswd -d $USER testgroup
sudo groupdel testgroup

Key Takeaways

• whoami, id, groups — check your identity
• useradd -m -s /bin/bash — create users properly
• usermod -aG group user — add to group (always use -a)
• userdel -r — delete user and home directory
• groupadd / groupdel — manage groups
• passwd — manage passwords
• /etc/passwd, /etc/shadow, /etc/group — the key files

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