Snap Package Format

Snap is a universal package management system developed by Canonical for Linux distributions. Snap packages bundle applications with their dependencies, providing secure, isolated, and easily distributable software packages that work across different Linux distributions.

Overview

Snap was introduced by Canonical in 2014 as part of Ubuntu Core and later extended to desktop Ubuntu and other Linux distributions. It addresses the challenges of dependency management, security isolation, and cross-distribution compatibility in Linux software distribution.

File Characteristics

• File Extension: .snap
• MIME Type: application/vnd.snap
• Format: SquashFS compressed filesystem image
• Distribution: Through Snap Store or manual installation
• Architecture: Multiple architecture support (amd64, arm64, armhf, etc.)

Package Structure

Snap Package Contents

snap-package.snap (SquashFS filesystem)
├── meta/
│   ├── snap.yaml          # Package metadata
│   ├── hooks/             # Lifecycle hooks
│   └── gui/               # Desktop integration files
├── bin/                   # Executable binaries
├── lib/                   # Shared libraries
├── usr/                   # Application files
└── snap/                  # Snap-specific directories

snap.yaml Metadata

name: hello-world
version: '6.4'
summary: The 'hello-world' of snaps
description: |
  This is a simple hello world example.
grade: stable
confinement: strict

apps:
  hello-world:
    command: bin/hello
    plugs: [home, network]

parts:
  hello:
    plugin: dump
    source: .

Confinement Levels

Strict Confinement

• Maximum security isolation
• Applications run in sandbox
• Limited system access through interfaces
• Recommended for most applications

Classic Confinement

• Full system access like traditional packages
• No sandboxing restrictions
• Required for development tools and system utilities
• Less secure but more compatible

Devmode Confinement

• Development mode with relaxed restrictions
• Used during development and testing
• Equivalent to classic but with additional logging
• Not suitable for production

Snap Commands

Installation and Management

# Install snap from store
sudo snap install hello-world

# Install from local file
sudo snap install ./my-app.snap --dangerous

# Install specific channel
sudo snap install code --classic
sudo snap install kubectl --channel=1.20/stable

# List installed snaps
snap list

# Search for snaps
snap search text-editor

Updates and Channels

# Refresh all snaps
sudo snap refresh

# Refresh specific snap
sudo snap refresh firefox

# Switch channels
sudo snap refresh firefox --channel=beta

# Revert to previous version
sudo snap revert firefox

Information and Management

# Show snap information
snap info firefox

# View snap connections
snap connections

# Disable/enable snap
sudo snap disable firefox
sudo snap enable firefox

# Remove snap
sudo snap remove firefox

Channels and Tracks

Channel Structure

track/risk/branch

Risk Levels

• stable: Production-ready releases
• candidate: Release candidates, pre-release testing
• beta: Beta versions for testing
• edge: Development snapshots, latest code

Track Examples

# Install from specific track/risk
snap install microk8s --channel=1.21/stable
snap install code --channel=insiders/stable
snap install lxd --channel=4.0/stable

Interfaces and Plugs

Common Interfaces

apps:
  my-app:
    command: bin/my-app
    plugs:
      - home              # Access to user home directory
      - network           # Network access
      - network-bind      # Bind to network ports
      - removable-media   # Access USB drives, etc.
      - camera            # Camera access
      - audio-playback    # Audio output
      - desktop           # Desktop integration

Interface Management

# List available interfaces
snap interface

# Connect interface manually
sudo snap connect my-app:camera

# Disconnect interface
sudo snap disconnect my-app:camera

# View interface details
snap interface home

Snap Development

snapcraft.yaml Configuration

name: my-application
version: '1.0'
summary: My application summary
description: |
  Detailed description of my application
  across multiple lines.

base: core20
confinement: strict
grade: stable

architectures:
  - build-on: amd64
    run-on: amd64

apps:
  my-app:
    command: bin/my-app
    plugs:
      - home
      - network

parts:
  my-app:
    plugin: python
    source: .
    requirements:
      - requests
      - flask

Build Process

# Install snapcraft
sudo snap install snapcraft

# Build snap package
snapcraft

# Clean build environment
snapcraft clean

# Build for specific architecture
snapcraft --target-arch=arm64

Testing and Validation

# Install built snap locally
sudo snap install --dangerous my-app_1.0_amd64.snap

# Test snap functionality
my-app

# Run in devmode for debugging
sudo snap install --devmode my-app_1.0_amd64.snap

Plugins and Build Systems

Common Plugins

parts:
  python-app:
    plugin: python
    source: .
    requirements:
      - flask
      - gunicorn
  
  nodejs-app:
    plugin: npm
    source: .
    npm-node-version: "14.17.0"
  
  go-app:
    plugin: go
    source: .
    go-channel: latest/stable
  
  cmake-app:
    plugin: cmake
    source: .
    cmake-parameters:
      - -DCMAKE_BUILD_TYPE=Release

Custom Build Steps

parts:
  my-part:
    plugin: nil
    override-build: |
      # Custom build commands
      make clean
      make all
      make install DESTDIR=$SNAPCRAFT_PART_INSTALL

Store and Distribution

Snap Store

# Login to store
snapcraft login

# Register snap name
snapcraft register my-app-name

# Upload to store
snapcraft upload my-app_1.0_amd64.snap

# Release to channel
snapcraft release my-app 1 stable

Publishing Workflow

1. Build: Create snap package with snapcraft
2. Test: Install and test locally
3. Upload: Upload to Snap Store
4. Review: Automated and manual review process
5. Release: Release to appropriate channels

Enterprise Features

Brand Stores

• Custom app stores for organizations
• Curated application collections
• Enterprise-specific software distribution
• White-label store branding

Snap Management

# Enterprise snap management
sudo snap ack my-assertion.assert
sudo snap install my-enterprise-app.snap

# Proxy configuration
sudo snap set system proxy.http=http://proxy:8080
sudo snap set system proxy.https=https://proxy:8080

Security Features

Application Isolation

• Each snap runs in its own mount namespace
• Separate filesystem view
• Limited access to system resources
• Mediated access through interfaces

Digital Signatures

• All snaps are digitally signed
• Cryptographic verification during installation
• Store-provided signatures for security
• Custom signing for enterprise deployments

Performance Considerations

Advantages

• Self-contained dependencies
• Atomic updates and rollbacks
• Cross-distribution compatibility
• Automatic security updates

Trade-offs

• Larger package sizes due to bundled dependencies
• Slower startup times for some applications
• Increased disk usage
• Potential for dependency duplication

Best Practices

Package Development

1. Minimal Base: Choose appropriate base snap
2. Proper Confinement: Use strict confinement when possible
3. Interface Selection: Request only necessary interfaces
4. Testing: Test across different environments
5. Documentation: Provide clear installation instructions

System Administration

1. Channel Management: Choose appropriate channels for environment
2. Update Strategy: Plan for automatic updates
3. Interface Auditing: Regularly review interface connections
4. Resource Monitoring: Monitor disk usage and performance
5. Security Policy: Establish snap installation policies

Common Use Cases

Application Distribution

• Cross-distribution software deployment
• Desktop application packaging
• Server application containers
• IoT device software updates

Development Environments

• Isolated development tools
• Version-specific runtime environments
• Containerized applications
• Testing and staging deployments

Snap represents a modern approach to Linux package management, addressing traditional challenges while introducing new capabilities for secure, maintainable software distribution across diverse Linux environments.