Linux is a multi-user operating system, designed from the ground up to allow multiple users to work simultaneously. Understanding how to manage users and groups is essential for system administration, security, and effective collaboration.

In this part, we'll explore how to create and manage users, work with groups, configure authentication, and set up user environments.

Users and Groups Fundamentals

Users

A user account represents an entity (a person or service) that can access the system. Each user has:

A unique username
A numeric user ID (UID)
A primary group
An optional set of secondary groups
A home directory
A login shell

Groups

Groups organize users with similar needs. Each group has:

A unique group name
A numeric group ID (GID)
A list of member users

Groups simplify permission management by allowing you to assign permissions to multiple users at once.

Before diving into commands, let's understand the important files that store user and group information:

/etc/passwd

This file contains basic information about each user:

cat /etc/passwd | head -5

Each line follows this format:

username:x:UID:GID:comment:home_directory:shell

For example:

alice:x:1000:1000:Alice Smith:/home/alice:/bin/bash

The x indicates that the password is stored in /etc/shadow.

/etc/shadow

This file contains encrypted password information and is only readable by root:

sudo cat /etc/shadow | head -5

Each line follows this format:

username:encrypted_password:last_change:min_age:max_age:warn_period:inactive_period:expiration_date:reserved

The encrypted password field might be ! or * if the account is locked.

/etc/group

This file defines the groups on the system:

cat /etc/group | head -5

Each line follows this format:

group_name:x:GID:user_list

For example:

developers:x:1001:alice,bob,charlie

/etc/gshadow

This file contains encrypted group passwords (if any) and is only readable by root:

sudo cat /etc/gshadow | head -5

User Management Commands

Viewing User Information

Get information about the current user:

id

Get information about a specific user:

id username

Show who is currently logged in:

who

Show detailed login information:

Display last login times for users:

last

Creating Users

Create a new user:

sudo useradd username

This creates a basic user with minimal settings. Typically, you'll want more options:

sudo useradd -m -d /home/username -s /bin/bash -c "Full Name" username

Options:

-m: Create the home directory
-d: Specify the home directory path
-s: Specify the login shell
-c: Add a comment (usually the user's full name)
-G: Add to supplementary groups

A more user-friendly command is adduser (on Debian-based systems), which is interactive:

sudo adduser username

Set or change a user's password:

sudo passwd username

Modifying Users

Modify an existing user:

sudo usermod [options] username

Common options:

-c: Change the comment field
-d: Change the home directory
-g: Change the primary group
-G: Set supplementary groups
-s: Change the login shell
-L: Lock the account
-U: Unlock the account

Examples:

# Change a user's shell
sudo usermod -s /bin/bash username

# Add a user to supplementary groups
sudo usermod -aG sudo,docker username

The -a option with -G adds to existing groups rather than replacing them.

Deleting Users

Delete a user:

sudo userdel username

Delete a user and their home directory:

sudo userdel -r username

User Account Expiration

Set account expiration:

sudo usermod -e YYYY-MM-DD username

Force password change on next login:

sudo chage -d 0 username

View account aging information:

sudo chage -l username

Group Management Commands

Creating Groups

Create a new group:

sudo groupadd groupname

With a specific GID:

sudo groupadd -g 1005 groupname

Modifying Groups

Modify a group:

sudo groupmod [options] groupname

Common options:

-g: Change the GID
-n: Change the group name

Example:

# Rename a group
sudo groupmod -n new_name old_name

Deleting Groups

Delete a group:

sudo groupdel groupname

Managing Group Membership

Add a user to a group:

sudo usermod -aG groupname username

Remove a user from a group:

sudo gpasswd -d username groupname

List groups a user belongs to:

groups username

Change a user's primary group:

sudo usermod -g groupname username

The sudo Command

The sudo command allows regular users to execute commands with elevated privileges.

Understanding sudo

When a user uses sudo, they're temporarily granted the permissions of another user (typically root) to execute a specific command. This provides an audit trail and limits the time spent with elevated privileges.

Basic usage:

sudo command_to_run

Run a command as a specific user:

sudo -u username command_to_run

Open a root shell:

sudo -i

Configuring sudo Access

The sudo configuration is in /etc/sudoers and files in /etc/sudoers.d/. Always edit these files with visudo to prevent syntax errors:

sudo visudo

A basic sudo entry:

username ALL=(ALL:ALL) ALL

This allows username to run any command as any user on any host.

To grant access without requiring a password:

username ALL=(ALL:ALL) NOPASSWD: ALL

To limit to specific commands:

username ALL=(ALL:ALL) /usr/bin/apt, /usr/bin/systemctl restart apache2

Grant sudo access to all members of a group:

%sudo ALL=(ALL:ALL) ALL

Understanding Sudoers Syntax

The syntax is:

user_list host_list=(user_list:group_list) command_list

user_list: Users or groups (with % prefix) to whom the rule applies
host_list: Hosts where the rule applies
user_list:group_list: Users/groups the command can be run as
command_list: Commands that can be executed

User Limits and Resource Control

Linux provides ways to limit user resources:

/etc/security/limits.conf

This file controls resource limits for users and groups:

cat /etc/security/limits.conf

Example entries:

# Limit number of processes
username hard nproc 100

# Limit file size
@developers soft fsize 1000000

Types of limits:

soft: User-adjustable up to the hard limit
hard: Maximum value, only adjustable by root

Resources that can be limited:

nproc: Number of processes
nofile: Number of open files
fsize: Maximum file size
cpu: CPU time
as: Address space (memory)

Process Accounting

To track user activity, install process accounting:

# On Debian/Ubuntu
sudo apt install acct

# On Fedora/RHEL/CentOS
sudo dnf install psacct

Start the service:

sudo systemctl enable --now acct

View command usage summary:

sudo sa

View user activity:

sudo lastcomm username

Authentication System

Linux uses Pluggable Authentication Modules (PAM) for flexible authentication:

PAM Configuration

PAM configuration files are in /etc/pam.d/:

ls -l /etc/pam.d/

Each file configures authentication for a different service.

Password Policies

Set password policies in /etc/security/pwquality.conf or /etc/pam.d/common-password:

# View password quality configuration
cat /etc/security/pwquality.conf

Common settings:

minlen: Minimum password length
minclass: Minimum character classes (lowercase, uppercase, digits, special)
maxrepeat: Maximum repeated characters
reject_username: Reject passwords containing the username
enforce_for_root: Apply policies to root user

Configure login restrictions in /etc/security/access.conf:

sudo nano /etc/security/access.conf

Example rules:

- : ALL EXCEPT root : cron
+ : admins : ALL
- : ALL : ALL

This denies cron access to all except root, allows the admins group from anywhere, and denies all other access.

SSH Key Authentication

SSH keys provide secure authentication without passwords:

Generating SSH Keys

Generate a key pair:

ssh-keygen -t ed25519 -C "your_email@example.com"

Or with RSA:

ssh-keygen -t rsa -b 4096 -C "your_email@example.com"

Adding Keys to Servers

Copy your public key to a remote server:

ssh-copy-id username@remote_host

Or manually:

cat ~/.ssh/id_ed25519.pub | ssh username@remote_host "mkdir -p ~/.ssh && chmod 700 ~/.ssh && cat >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys"

SSH Server Configuration

Configure SSH settings in /etc/ssh/sshd_config:

sudo nano /etc/ssh/sshd_config

Common security settings:

PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys

After changes, restart the SSH service:

sudo systemctl restart sshd

User Environment Configuration

System-Wide Environment

System-wide environment variables are set in several files:

/etc/environment: Simple KEY=VALUE pairs
/etc/profile: Executed for login shells
/etc/profile.d/*.sh: Scripts executed by profile
/etc/bash.bashrc: Executed for interactive non-login bash shells

User Environment

User-specific environment configuration:

~/.profile: Executed for login shells
~/.bashrc: Executed for interactive non-login bash shells
~/.bash_profile or ~/.bash_login: Alternative to .profile for bash

Add personal environment variables in ~/.bashrc:

echo 'export PATH="$HOME/bin:$PATH"' >> ~/.bashrc
source ~/.bashrc

Default User Settings

The /etc/skel/ directory contains files copied to new users' home directories:

ls -la /etc/skel/

Customize these files to provide default configurations for new users.

User Account Defaults

The /etc/login.defs file controls defaults for user accounts:

cat /etc/login.defs

Important settings:

PASS_MAX_DAYS: Maximum password age
PASS_MIN_DAYS: Minimum password age
PASS_WARN_AGE: Password expiration warning days
UID_MIN, UID_MAX: Range for regular user IDs
GID_MIN, GID_MAX: Range for regular group IDs
CREATE_HOME: Whether to create home directories
UMASK: Default file creation permissions

Default settings for useradd are in /etc/default/useradd:

cat /etc/default/useradd

Practical Examples

Let's walk through some real-world user management scenarios:

Setting Up a Development Team

Create a development team with shared access to a project directory:

# Create a developers group
sudo groupadd developers

# Create the project directory
sudo mkdir -p /opt/projects/web-app

# Set ownership and permissions
sudo chown root:developers /opt/projects/web-app
sudo chmod 2775 /opt/projects/web-app

# Create a user and add to the group
sudo useradd -m -s /bin/bash -c "Sarah Developer" sarah
sudo usermod -aG developers sarah

# Set password for the new user
sudo passwd sarah

The directory will have the following permissions:

group ownership by developers
group write access
setgid bit (2) to ensure new files in the directory inherit the developers group

Setting Up a Restricted User

Create a user with limited access:

# Create restricted shell directory
sudo mkdir /home/restricted
sudo cp /bin/bash /home/restricted/rbash
sudo chmod 755 /home/restricted/rbash

# Create a user with restricted shell
sudo useradd -m -d /home/guest -s /home/restricted/rbash guest

# Create a bin directory with allowed commands
sudo mkdir /home/guest/bin
sudo ln -s /bin/ls /home/guest/bin/
sudo ln -s /bin/cat /home/guest/bin/

# Set ownership
sudo chown -R guest:guest /home/guest/bin

# Add to .bashrc to restrict PATH
echo 'PATH=$HOME/bin' | sudo tee -a /home/guest/.bashrc
echo 'export PATH' | sudo tee -a /home/guest/.bashrc
sudo chown guest:guest /home/guest/.bashrc

This creates a restricted user who can only run the ls and cat commands.

Setting Up a Service User

Create a non-login user for running a service:

# Create user without home directory or login shell
sudo useradd --system --no-create-home --shell /usr/sbin/nologin appservice

# Create application directory
sudo mkdir -p /var/www/app
sudo chown appservice:appservice /var/www/app

# Create systemd service file
sudo tee /etc/systemd/system/app.service > /dev/null << 'EOF'
[Unit]
Description=My Application Service
After=network.target

[Service]
User=appservice
Group=appservice
WorkingDirectory=/var/www/app
ExecStart=/usr/bin/python3 /var/www/app/app.py
Restart=on-failure

[Install]
WantedBy=multi-user.target
EOF

sudo systemctl daemon-reload

This creates a system user that can run a service but cannot log in interactively.

Setting Up Disk Quotas

Limit how much disk space users can use:

# Install quota tools
sudo apt install quota

# Enable quotas in /etc/fstab for the user partition
# Add usrquota to the mount options, e.g.:
# UUID=xxxx / ext4 defaults,usrquota 0 1

# Remount the file system
sudo mount -o remount /

# Create quota files and turn on quotas
sudo quotacheck -cum /
sudo quotaon /

# Set quota for a user (soft limit: 500MB, hard limit: 550MB)
sudo setquota -u username 500000 550000 0 0 /

Check quota usage:

sudo quota -u username

Cloud Environment User Management

When working with cloud servers, you'll often need to set up users. Here's how to do it on a DigitalOcean Droplet:

Create a new Droplet (use this link for $200 in credits)
Connect as root
Create a non-root user with sudo privileges:

# Create user
adduser newuser

# Add to sudo group on Ubuntu
usermod -aG sudo newuser

# Add to wheel group on CentOS/RHEL
# usermod -aG wheel newuser

# Create SSH directory and set permissions
mkdir -p /home/newuser/.ssh
chmod 700 /home/newuser/.ssh

# Add your public key
echo "your_public_key_here" > /home/newuser/.ssh/authorized_keys
chmod 600 /home/newuser/.ssh/authorized_keys
chown -R newuser:newuser /home/newuser/.ssh

# Test that you can log in as the new user
# ssh newuser@your_server_ip

This process creates a secure, non-root user that can execute administrative commands using sudo.

Moving Forward

You now understand how to create and manage users and groups, control access with permissions, configure authentication, and set up user environments. These skills are essential for maintaining secure and well-organized Linux systems.

In the next part, we'll explore process management, learning how to monitor, control, and optimize processes running on your Linux system.

User and group management is fundamental to Linux's multi-user design. Whether you're administering a personal system or an enterprise server, these concepts and commands will help you maintain security, organization, and efficiency.

Users and Groups Fundamentals

Users

Groups

Key User-Related Files

/etc/passwd

/etc/shadow

/etc/group

/etc/gshadow

User Management Commands

Viewing User Information

Creating Users

Modifying Users

Deleting Users

User Account Expiration

Group Management Commands

Creating Groups

Modifying Groups

Deleting Groups

Managing Group Membership

The sudo Command

Understanding sudo

Configuring sudo Access

Understanding Sudoers Syntax

User Limits and Resource Control

/etc/security/limits.conf

Process Accounting

Authentication System

PAM Configuration

Password Policies

Login Access Control

SSH Key Authentication

Generating SSH Keys

Adding Keys to Servers

SSH Server Configuration

User Environment Configuration

System-Wide Environment

User Environment

Default User Settings

User Account Defaults

Practical Examples

Setting Up a Development Team

Setting Up a Restricted User

Setting Up a Service User

Setting Up Disk Quotas

Cloud Environment User Management

Moving Forward

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