Unix & Docker on Dev Knowledgebase

🐧 Debian User Management Cheat Sheet

Sun, 31 May 2026 00:00:00 +0000

🐧 Debian User Management Cheat Sheet #

Complete guide for managing users and groups on Debian systems with best practices and security considerations.

📋 Table of Contents #

🎯 Core Concepts #

User Types #

System Users: UID < 1000, for services and daemons
Regular Users: UID ≥ 1000, for human users
Root User: UID 0, superuser with unlimited privileges

Key Files #

/etc/passwd - User account information
/etc/shadow - Encrypted passwords and aging info
/etc/group - Group definitions
/etc/gshadow - Group passwords and administrators

User Database Structure #

# /etc/passwd format:
username:x:UID:GID:GECOS:home_directory:shell

# /etc/shadow format:
username:encrypted_password:last_change:min_age:max_age:warn:inactive:expire:reserved

🛠️ Essential Tools #

Debian-Specific (Recommended) #

adduser - Interactive user creation (Debian frontend)
deluser - Safe user removal (Debian frontend)
addgroup - Group creation
delgroup - Group removal

Low-Level Tools (Universal) #

useradd - Create user accounts
userdel - Delete user accounts
usermod - Modify user accounts
groupadd - Create groups
groupdel - Delete groups
passwd - Change passwords
chsh - Change login shell
chfn - Change user information

👤 Creating Users #

Interactive User Creation (Recommended) #

# Create regular user with home directory
sudo adduser username

# Create system user
sudo adduser --system --group servicename

# Create user with specific shell
sudo adduser --shell /bin/zsh username

# Create user without home directory
sudo adduser --no-create-home username

# Create user with specific UID/GID
sudo adduser --uid 1500 --gid 1500 username

Low-Level User Creation #

# Basic user creation
sudo useradd username

# Create user with home directory and shell
sudo useradd -m -s /bin/bash username

# Create user with specific UID and groups
sudo useradd -u 1500 -g users -G sudo,audio username

# Create system user
sudo useradd --system --shell /bin/false servicename

# Create user with expiration date
sudo useradd -e 2024-12-31 tempuser

Common useradd Options #

-m, --create-home     # Create home directory
-d, --home DIR        # Specify home directory
-s, --shell SHELL     # Set login shell
-g, --gid GROUP       # Primary group
-G, --groups GROUPS   # Supplementary groups
-u, --uid UID         # User ID
-e, --expiredate DATE # Account expiration
-c, --comment GECOS   # User information
-k, --skel DIR        # Skeleton directory
-r, --system          # Create system account

✏️ Modifying Users #

Change User Properties #

# Change username
sudo usermod -l newname oldname

# Change user ID
sudo usermod -u 1600 username

# Change primary group
sudo usermod -g newgroup username

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

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

# Change login shell
sudo usermod -s /bin/zsh username
sudo chsh -s /bin/zsh username  # Alternative

# Lock/unlock user account
sudo usermod -L username  # Lock
sudo usermod -U username  # Unlock

# Set account expiration
sudo usermod -e 2024-12-31 username

# Change user information (GECOS)
sudo usermod -c "Full Name,Room,Work Phone,Home Phone" username
sudo chfn username  # Interactive alternative

Password Management #

# Change user password
sudo passwd username

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

# Set password aging
sudo passwd -n 7 -x 90 -w 7 username  # min 7, max 90, warn 7 days

# Lock/unlock password
sudo passwd -l username  # Lock
sudo passwd -u username  # Unlock

# Delete password (dangerous)
sudo passwd -d username

🗑️ Deleting Users #

Safe User Removal (Recommended) #

# Remove user (keep home directory)
sudo deluser username

# Remove user and home directory
sudo deluser --remove-home username

# Remove user and all files
sudo deluser --remove-all-files username

# Remove user from specific group only
sudo deluser username groupname

Low-Level User Removal #

# Remove user (keep home directory)
sudo userdel username

# Remove user and home directory
sudo userdel -r username

# Force removal even if user is logged in
sudo userdel -f username

👥 Group Management #

Creating Groups #

# Create group
sudo addgroup groupname
sudo groupadd groupname  # Alternative

# Create group with specific GID
sudo groupadd -g 2000 groupname

# Create system group
sudo groupadd -r systemgroup

Managing Group Membership #

# Add user to group
sudo adduser username groupname
sudo usermod -aG groupname username  # Alternative

# Remove user from group
sudo deluser username groupname
sudo gpasswd -d username groupname  # Alternative

# List user's groups
groups username
id username

# List group members
getent group groupname

Deleting Groups #

# Delete group
sudo delgroup groupname
sudo groupdel groupname  # Alternative

# Force delete group even if it's primary group
sudo groupdel -f groupname

🔐 Password Management #

Password Policies #

# Install password quality checking
sudo apt install libpam-pwquality

# Configure in /etc/pam.d/common-password
password requisite pam_pwquality.so retry=3 minlen=8 difok=3 ucredit=-1 lcredit=-1 dcredit=-1 ocredit=-1

# Set password aging in /etc/login.defs
PASS_MAX_DAYS 90
PASS_MIN_DAYS 7
PASS_WARN_AGE 7

Password Commands #

# Generate secure password
openssl rand -base64 32
pwgen -s 16 1

# Check password expiration
sudo chage -l username

# Set password expiration
sudo chage -M 90 -m 7 -W 7 username

# Interactive password aging setup
sudo chage username

🔑 Privilege Escalation #

sudo Configuration #

# Edit sudoers file (always use visudo)
sudo visudo

# Add user to sudo group
sudo usermod -aG sudo username

# Check sudo access
sudo -l

# Run command as another user
sudo -u username command

# Preserve environment
sudo -E command

# Run shell as root
sudo -i
sudo su -

Common sudoers Entries #

# Allow user to run all commands
username ALL=(ALL:ALL) ALL

# Allow group to run specific commands
%groupname ALL=(ALL) /usr/bin/systemctl, /usr/bin/service

# Allow without password
username ALL=(ALL) NOPASSWD: ALL

# Allow specific commands on specific hosts
username hostname=(root) /usr/bin/apt, /usr/bin/dpkg

su Command #

# Switch to root
su -
su root

# Switch to another user
su - username

# Run single command as another user
su -c "command" username

📁 Configuration Files #

/etc/passwd #

# View user database
cat /etc/passwd
getent passwd

# Find specific user
getent passwd username

# List all users with UID >= 1000
awk -F: '$3 >= 1000 {print $1}' /etc/passwd

/etc/shadow #

# View password database (root only)
sudo cat /etc/shadow

# Check password status
sudo passwd -S username

/etc/group #

# View group database
cat /etc/group
getent group

# Find groups for user
groups username
id -Gn username

/etc/login.defs #

# Key settings for user creation
UID_MIN 1000
UID_MAX 60000
GID_MIN 1000
GID_MAX 60000
CREATE_HOME yes
UMASK 022
USERGROUPS_ENAB yes
ENCRYPT_METHOD SHA512

🔒 Security Best Practices #

Account Security #

# Disable unused accounts
sudo usermod -L -e 1 username

# Set strong password policies
# Edit /etc/pam.d/common-password
password requisite pam_pwquality.so minlen=12 dcredit=-1 ucredit=-1 lcredit=-1 ocredit=-1

# Enable account lockout
# Edit /etc/pam.d/common-auth
auth required pam_tally2.so deny=5 unlock_time=900

# Monitor failed logins
sudo pam_tally2 --user username

User Limits #

# Configure resource limits in /etc/security/limits.conf
* soft nproc 1000
* hard nproc 2000
* soft nofile 1024
* hard nofile 2048
@users hard core 0

Auditing Users #

# List all users
cut -d: -f1 /etc/passwd

# Find users with shell access
awk -F: '$7 !~ /nologin|false/ {print $1}' /etc/passwd

# Check for users with UID 0
awk -F: '$3 == 0 {print $1}' /etc/passwd

# Find users without passwords
sudo awk -F: '$2 == "" {print $1}' /etc/shadow

# Check for duplicate UIDs
awk -F: '{print $3}' /etc/passwd | sort | uniq -d

# List recently created users
sudo find /home -maxdepth 1 -type d -newerct "1 week ago"

🔧 Advanced Topics #

PAM Configuration #

# PAM configuration directory
ls /etc/pam.d/

# Common PAM modules
/lib/x86_64-linux-gnu/security/

# Key PAM files
/etc/pam.d/common-auth      # Authentication
/etc/pam.d/common-account   # Account management
/etc/pam.d/common-password  # Password management
/etc/pam.d/common-session   # Session management

User Environment #

# Skeleton directory for new users
ls -la /etc/skel/

# Default shell configuration
/etc/bash.bashrc
/etc/profile

# User-specific configurations
~/.bashrc
~/.profile
~/.bash_profile

Batch Operations #

# Create multiple users from file
# Format: username:password:uid:gid:gecos:home:shell
sudo newusers userlist.txt

# Change passwords in batch
# Format: username:password
sudo chpasswd < passwordlist.txt

# Mass user creation script
#!/bin/bash
for user in user1 user2 user3; do
    sudo adduser --disabled-password --gecos "" $user
    echo "$user:defaultpass" | sudo chpasswd
    sudo passwd -e $user  # Force password change
done

🚨 Troubleshooting #

Common Issues #

# User cannot login
sudo passwd -S username  # Check password status
sudo usermod -U username  # Unlock if locked

# Permission denied errors
ls -la /home/username/    # Check home directory permissions
sudo chown -R username:username /home/username/

# Group membership not taking effect
# User needs to log out and back in, or:
newgrp groupname

# Cannot delete user
sudo pkill -u username    # Kill user processes
sudo userdel -f username  # Force deletion

# Home directory not created
sudo mkhomedir_helper username

Verification Commands #

# Verify user creation
id username
getent passwd username
ls -la /home/username/

# Check group membership
groups username
id -Gn username

# Verify sudo access
sudo -l -U username

# Test user login
sudo -i -u username

Recovery Procedures #

# Reset forgotten root password (single user mode)
# 1. Boot to GRUB menu
# 2. Edit kernel parameters, add: init=/bin/bash
# 3. Mount filesystem: mount -o remount,rw /
# 4. Change password: passwd root
# 5. Reboot: exec /sbin/init

# Restore from backup
sudo cp /etc/passwd.backup /etc/passwd
sudo cp /etc/shadow.backup /etc/shadow
sudo cp /etc/group.backup /etc/group

📚 Useful Commands Reference #

Information Gathering #

whoami                    # Current user
id                       # Current user ID and groups
who                      # Logged in users
w                        # Detailed user activity
last                     # Login history
lastlog                  # Last login times
finger username          # User information (if installed)

Quick Operations #

# One-liner user creation with sudo access
sudo adduser newuser && sudo usermod -aG sudo newuser

# Create temporary user (expires in 30 days)
sudo useradd -m -e $(date -d "+30 days" +%Y-%m-%d) tempuser

# Bulk password reset
echo "username:newpassword" | sudo chpasswd

# Find all users with bash shell
grep "/bin/bash" /etc/passwd | cut -d: -f1

🎯 Best Practices Summary #

Always use adduser instead of useradd for interactive user creation
Use visudo to edit sudoers file to prevent syntax errors
Implement strong password policies with PAM modules
Regular audit user accounts and remove unused ones
Use groups for permission management instead of individual user permissions
Set appropriate resource limits to prevent abuse
Monitor user activities with logging and auditing tools
Backup user databases before making bulk changes
Test user accounts after creation or modification
Follow principle of least privilege for sudo access

man adduser - Debian user creation tool
man useradd - Low-level user creation
man usermod - User modification
man sudo - Privilege escalation
man passwd - Password management
man pam - Pluggable Authentication Modules
/usr/share/doc/adduser/ - Debian adduser documentation

This cheat sheet covers comprehensive Debian user management. Always test commands in a safe environment before applying to production systems.

🐧 Linux Shells Cheat Sheet

Sun, 31 May 2026 00:00:00 +0000

🐧 Linux Shells Cheat Sheet #

An overview of common and specialized Linux shells, from the standard Bash to the user-friendly Fish, and security-focused options like rbash and chroot.

📋 Shell Paths at a Glance #

Path	Description
`/bin/sh`	The standard command language shell. Often a symbolic link to `bash` or `dash` for POSIX compatibility.
`/bin/bash`	The Bourne Again Shell, a common interactive and scripting shell. Located in `/bin` for essential access.
`/usr/bin/bash`	Another location for Bash. On many modern systems, `/bin` is a symlink to `/usr/bin`, making them identical.
`/bin/rbash`	Restricted Bash. A version of `bash` with limited features for security.
`/usr/bin/rbash`	Another location for Restricted Bash.
`/bin/dash`	Debian Almquist Shell. A lightweight, fast, POSIX-compliant shell, often used as the default `/bin/sh`.
`/usr/bin/dash`	Another location for the Dash shell.
`/bin/bash (chrooted)`	Not a path, but `bash` running in a `chroot` jail, isolated from the main system’s file structure.

🌐 Interactive Shells #

These are the shells you typically use for day-to-day command-line work.

🐧 systemd Cheat Sheet

Sun, 31 May 2026 00:00:00 +0000

🐧 systemd Cheat Sheet #

A comprehensive guide to systemd, the init system and service manager for Linux. This sheet covers core concepts, essential commands, unit file structure, and best practices for Debian-based systems.

📜 Core Concepts #

Unit: The fundamental object systemd manages. Units represent services, mount points, devices, sockets, etc.
Unit File: A plain text file that describes a unit and its behavior. Common types include .service, .socket, .target, and .timer.
Target: A synchronization point for other units. Targets group units together and are used to bring the system to specific states (e.g., multi-user.target, graphical.target).
journald: The systemd service that handles logging. Logs are accessed via the journalctl command.

⚙️ `systemctl`: The Master Command #

The primary tool for interacting with systemd.

🐳 Docker Cheat Sheet (v28.x)

Sun, 31 May 2026 00:00:00 +0000

🐳 Docker Cheat Sheet (v28.x) #

A condensed guide to essential Docker commands, Dockerfile best practices, and Docker Compose.

📜 Core Concepts #

Image: A lightweight, standalone, executable package that includes everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and config files.
Container: A runnable instance of an image. You can create, start, stop, move, or delete a container using the Docker API or CLI.
Volume: A mechanism for persisting data generated by and used by Docker containers. Volumes are managed by Docker and are the preferred way to handle persistent storage.
Network: Provides a way to isolate containers from each other and the host machine, but also allows them to communicate with each other, the host, or external networks.
Dockerfile: A text document that contains all the commands a user could call on the command line to assemble an image.
Docker Compose: A tool for defining and running multi-container Docker applications. Uses a YAML file to configure the application’s services.

⚙️ Essential Docker CLI Commands #

General & Info #

docker version: Show Docker version information.
docker system info: Display system-wide information.
docker help: Display a list of all Docker commands.
docker <command> --help: Show help for a specific command.

Image Management #

docker build .: Build an image from a Dockerfile in the current directory.
docker build -t my-app:1.0 .: Build and tag an image.
docker images: List all local images.
docker rmi my-app:1.0: Remove an image.
docker rmi $(docker images -f "dangling=true" -q): Remove all dangling (unused) images.
docker pull nginx:latest: Pull an image from a registry (Docker Hub by default).
docker push my-repo/my-app:1.0: Push an image to a registry.
docker tag my-app:1.0 my-repo/my-app:1.0: Add a new tag to an existing image.

Container Lifecycle #

docker run -d -p 8080:80 --name my-nginx nginx: Run a container from an image in detached mode with port mapping and a name.
docker ps: List all running containers.
docker ps -a: List all containers (running and stopped).
docker stop my-nginx: Stop a running container gracefully (sends SIGTERM).
docker kill my-nginx: Stop a running container immediately (sends SIGKILL).
docker start my-nginx: Start a stopped container.
docker restart my-nginx: Restart a container.
docker rm my-nginx: Remove a stopped container.
docker rm -f my-nginx: Force-remove a running container.
docker logs my-nginx: Fetch the logs of a container.
docker logs -f my-nginx: Follow the log output in real-time.
docker exec -it my-nginx /bin/bash: Execute a command in a running container (interactive shell).

Inspection & Resource Management #

docker inspect my-nginx: Display low-level information on Docker objects (containers, images, etc.).
docker stats: Display a live stream of container(s) resource usage statistics.
docker system df: Show Docker disk usage.
docker system prune: Remove all unused data (dangling images, stopped containers, unused networks, and build cache).
docker system prune -a --volumes: Remove all unused images (not just dangling ones) and volumes.

Network Management #

docker network ls: List networks.
docker network create my-network: Create a new bridge network.
docker network inspect my-network: Display detailed information on a network.
docker network rm my-network: Remove a network.

Volume Management #

docker volume ls: List volumes.
docker volume create my-volume: Create a volume.
docker volume inspect my-volume: Display detailed information on a volume.
docker volume rm my-volume: Remove a volume.
docker volume prune: Remove all unused local volumes.

✍️ Dockerfile Best Practices #

Use .dockerignore: Exclude files and directories not needed for the build to keep the build context small and avoid leaking secrets.
Use Multi-Stage Builds: Separate build-time dependencies from runtime dependencies to create smaller, more secure final images.
Order Matters for Caching: Place instructions that change less frequently (like installing dependencies) before those that change more frequently (like copying source code).
Combine RUN instructions: Chain commands using && to reduce the number of image layers. For apt-get, combine update, install, and clean-up in one line.
```
RUN apt-get update && apt-get install -y --no-install-recommends curl && rm -rf /var/lib/apt/lists/*
```
Use Specific Base Images: Avoid latest. Use specific tags (e.g., node:18-alpine) for reproducibility and stability.
Use Minimal Base Images: Prefer smaller base images like alpine, slim, or distroless to reduce the attack surface and image size.
COPY over ADD: COPY is more transparent. Use ADD only when you need to auto-extract a tar file or fetch a remote URL.
Run as a Non-Root User: Create a dedicated user and group, and use the USER instruction to avoid running containers with root privileges.
Lint Your Dockerfile: Use tools like hadolint to check for common mistakes and apply best practices automatically.

🎼 Docker Compose (v2 Syntax) #

File: compose.yml or docker-compose.yml

📦 NPX (Node Package eXecute) on Debian Cheat Sheet

Sun, 31 May 2026 00:00:00 +0000

📦 NPX (Node Package eXecute) on Debian Cheat Sheet #

Complete guide for using NPX package runner on Debian systems with best practices and security considerations.

📋 Table of Contents #

🎯 What is NPX #

NPX (Node Package eXecute) is a package runner tool that comes bundled with npm 5.2+ and allows you to execute npm packages without installing them globally.

🖥️ tmux Cheat Sheet

Sun, 31 May 2026 00:00:00 +0000

🖥️ tmux Cheat Sheet #

📋 What is tmux? #

tmux (Terminal Multiplexer) is a powerful command-line tool that allows you to:

Create multiple terminal sessions within a single window
Detach and reattach to sessions (survive SSH disconnections)
Split windows into panes for multitasking
Share sessions between multiple users
Keep processes running in the background

🚀 Installation #

Ubuntu/Debian #

sudo apt update
sudo apt install tmux

CentOS/RHEL/Fedora #

# CentOS/RHEL 7
sudo yum install tmux

# CentOS/RHEL 8+ / Fedora
sudo dnf install tmux

macOS #

brew install tmux

Verify Installation #

tmux -V

🏗️ Core Concepts #

Hierarchy #

Session: Collection of windows (like a workspace)
Window: Collection of panes (like browser tabs)
Pane: Individual terminal instance (split screen)

Prefix Key #

Default prefix: Ctrl-b (press before any tmux command) All shortcuts below assume you press Ctrl-b first

Unix & Docker on Dev Knowledgebase

🐧 Debian User Management Cheat Sheet

🐧 Debian User Management Cheat Sheet #

📋 Table of Contents #

🎯 Core Concepts #

User Types #

Key Files #

User Database Structure #

🛠️ Essential Tools #

Debian-Specific (Recommended) #

Low-Level Tools (Universal) #

👤 Creating Users #

Interactive User Creation (Recommended) #

Low-Level User Creation #

Common useradd Options #

✏️ Modifying Users #

Change User Properties #

Password Management #

🗑️ Deleting Users #

Safe User Removal (Recommended) #

Low-Level User Removal #

👥 Group Management #

Creating Groups #

Managing Group Membership #

Deleting Groups #

🔐 Password Management #

Password Policies #

Password Commands #

🔑 Privilege Escalation #

sudo Configuration #

Common sudoers Entries #

su Command #

📁 Configuration Files #

/etc/passwd #

/etc/shadow #

/etc/group #

/etc/login.defs #

🔒 Security Best Practices #

Account Security #

User Limits #

Auditing Users #

🔧 Advanced Topics #

PAM Configuration #

User Environment #

Batch Operations #

🚨 Troubleshooting #

Common Issues #

Verification Commands #

Recovery Procedures #

📚 Useful Commands Reference #

Information Gathering #

Quick Operations #

🎯 Best Practices Summary #

🔗 Related Documentation #

🐧 Linux Shells Cheat Sheet

🐧 Linux Shells Cheat Sheet #

📋 Shell Paths at a Glance #

🌐 Interactive Shells #

🐧 systemd Cheat Sheet

🐧 systemd Cheat Sheet #

📜 Core Concepts #

⚙️ systemctl: The Master Command #

🐳 Docker Cheat Sheet (v28.x)

🐳 Docker Cheat Sheet (v28.x) #

📜 Core Concepts #

⚙️ Essential Docker CLI Commands #

General & Info #

Image Management #

Container Lifecycle #

Inspection & Resource Management #

Network Management #

Volume Management #

✍️ Dockerfile Best Practices #

🎼 Docker Compose (v2 Syntax) #

📦 NPX (Node Package eXecute) on Debian Cheat Sheet

📦 NPX (Node Package eXecute) on Debian Cheat Sheet #

📋 Table of Contents #

🎯 What is NPX #

🖥️ tmux Cheat Sheet

🖥️ tmux Cheat Sheet #

⚙️ `systemctl`: The Master Command #