Snap format

Contents

Introduction
Setup files
snap.yaml
Hooks

# Introduction

A snap is a squashfs file carrying content and a bit of metadata that tells the system how to manipulate it. When installed the squashfs file is mounted read-only under

• /snap/<snap name>/<revision>/

This means fast and extremely predictable installations, with no leftovers and no way for the content to be mutated over traditional means. Either it's all installed and available as originally built, or it is not available at all.

Applications declared in the snap become commands at

• /snap/bin/<snap name>[.<app name>]

The suffix is omitted if <app name> matches <snap name>. That file is not the actual application, though, but rather a command that will trigger the real application to be run under the proper isolation and confinement rules, as specified via the snap metadata and the choices made via the local system configuration.

# Setup files

The following files control the behavior of a snap:

• meta/snap.yaml - Basic snap details. See below.
• meta/hooks/ - Hooks called on specific events. See below.

# snap.yaml

The meta/snap.yaml file contains the basic metadata and is a requirement for all snaps.

The following specification defines what is supported inside it:

name: <name>

# Hooks

---

PENDING CONTENT TO ORGANIZE:

Package metadata

This document describes the meta data of a snappy package. All files are located under the meta/ directory.

The following files are supported:

snap.yaml

This file describes the snap package and is the most important file for a snap package. The following keys are mandatory:

• name: the name of the snap (only ^[a-z](?:-?[a-z0-9])*$)
• version: the version of the snap (only [a-zA-Z0-9.+~-] are allowed)

The following keys are optional:

• summary: A short summary

• description: A long description

• license-agreement: set to explicit if the user needs to accept a special meta/license.txt before the snap can be installed

• license-version: a string that, when it changes and license-agreement is explicit, prompts the user to accept the license again.

• type: (optional) the type of the snap, can be:

  • app - the default if empty
  • gadget - a special snap that Gadgets can use to customize snappy for their hardware
  • framework - a specialized snap that extends the system that other snaps may use

• architectures: (optional) a yaml list of supported architectures ["all"] if empty

• frameworks: a list of the frameworks the snap needs as dependencies

• apps: the map of apps (binaries and services) that a snap provides

  • command: (required) the command to start the service
  • daemon: (optional) [simple|forking|oneshot|dbus]
  • stop-command: (optional) the command to stop the service
  • stop-timeout: (optional) the time in seconds to wait for the service to stop
  • restart-condition: (optional) if specified, use the given restart condition. Can be one of on-failure (default), never, on-success, on-abnormal, on-abort, and always. See systemd.service(5) (search for Restart=) for details.
  • post-stop-command: (optional) a command that runs after the service has stopped
  • slots: a map of interfaces
  • ports: (optional) define what ports the service will work
    • internal: the ports the service is going to connect to
      • tagname: a free form name
        • port: (optional) number/protocol, e.g. 80/tcp
        • negotiable: (optional) Y if the app can use a different port
    • external: the ports the service offer to the world
      • tagname: a free form name, some names have meaning like "ui"
        • port: (optional) see above
        • negotiable: (optional) see above
  • socket: (optional) Set to "true" if the service is socket activated. Must be specified with listen-stream.
  • listen-stream: (optional) The full path of the stream socket or an abstract socket. When specifying an absolute path, it should normally be in one of the app-specific writable directories. When specifying an abstract socket, it must start with '@' and typically be followed by either the snap package name or the snap package name followed by '_' and any other characters (eg, '@name' or '@name_something').

• slots: a map of interfaces

Interfaces

Interfaces allow snaps to communicate or share resources according to the protocol established by the interface and play an important part in security policy configuration. See interfaces.md and security.md for details.

license.txt

A license text that the user must accept before the snap can be installed.

gui/ directory

The gui directory contains GUI releated files for the snap.

desktop files

The gui/ directory may contain .desktop files for the snap. Those desktop files may contain all valid desktop entries from the xdg Desktop Entry Specification version 1.1 with some exceptions listed below. If there is a line with an unknown key or an unofficial key that line is silently removed from the desktop file on install.

Only Exec= lines that start with Exec=$snap.$app are valid, but arguments may be passed. E.g. for a snap like:

name: http
version: 1.0
apps:
 GET:
  command: myhttpdownlaoder.get-stuff

With a desktop file:

[Desktop Entry]
Name=My downloader
Exec=http.GET %U

The Exec= line is valid because it starts with Exec=http.GET (the snap is called "http" and the app is called "GET").

Unsupported desktop keys

The DBusActivatable, TryExec and Implements keys are currently not supported and will be silently removed from the desktop file on install.

hooks/ directory

See config.md for details.

Examples

See https://github.com/snapcore/snapd-testdata for up-to-date examples.