Snap format

Contents

Introduction
Setup files
snap.yaml
Hooks
Interfaces
Desktop Files
Aliases

# 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, based on the default restricted environment plus any allowances granted to it via the interface system.

# 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).
• meta/gui/icon.svg - Icon for the snap.
• meta/gui/*.desktop - Desktop files for the snap (see below).

# snap.yaml

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

Most of its allowed content is optional. In fact, the simplest snap possible may have as little as this inside snap.yaml:

name: simplest
version: 1.0

A snap that offers an application to run is still very simple:

name: simple
version: 1.0
apps:
    hello:
        command: bin/hello --world

The following specification defines what is supported inside it:

# The suggested snap name, constrained to the [a-z0-9] charset and inner
# dashes. The final name when the snap is installed is defined by the
# snap-declaration assertion associated with the snap, if any.
name: <name>

# Version of the software packed inside the snap. Has no semantic value
# in the system (no greater/lower-than rules are ever applied to it).
version: <version>

# More details about what is contained in the snap.
summary: <line>
description: <text>

# Type of snap, defaults to "app".
type: app | core | gadget | kernel

# List of architectures the snap may run on. Defaults to [all].
architectures:
    - all | amd64 | i386 | armhf

# List of applications (commands, binaries, daemons) in the snap. 
apps:

  <app name>:

      # Path to executable (relative to snap base) and arguments to use
      # when this application is run.
      command: <command line>

      # Optional list of aliases that can be used to invoke the command 
      # when enabled.
      # (snapd 2.20+, snapcraft 2.24+)
      aliases:
          - <alias>

      # List of plug names this application is associated with.
      # When a plug is connected to one of these slots, this application
      # will be granted the permissions specified for that interface.
      # If attributes are required or the plug name does not match the
      # interface name, more details must be declared under the top-level
      # "plugs" field (see below).
      plugs:
          - <plug name>

      # List of slot names this application is associated with.
      # Same details as described above, but for slots.
      slots:
          - <slot name>

      # If daemon is set, the command is a daemon to run as specified.
      # See systemd documentation for details on those kinds.
      daemon: simple | forking | oneshot | notify

      # Optional command to use for stopping a daemon.
      stop-command: <command line>

      # Optional time to wait for daemon to stop.
      stop-timeout: <n>ns | <n>us | <n>ms | <n>s | <n>m

      # Optional command to run after daemon stops.
      post-stop-command: <command line>

      # Condition to restart the daemon under. Defaults to on-failure.
      # See the systemd.service manual on Restart for details.
      restart-condition: \
          on-failure | on-success | on-abnormal | on-abort | always | never

      # Whether the daemon is socket activated. Defaults to false, and
      # must be defined together with listen-stream if true.
      socket: true | false

# Hooks Hooks provide a mechanism for snapd to alert snaps that something has happened. See hooks for details. # 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 and Security for details.

# Desktop files

The meta/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.

The Exec= line must necessarily look like the following to be valid:

• Exec=<snap name>[.<app name>] [<argument> ...]

As in the executables contained under /snap, the .<app name> suffix is omitted if the application name and snap name are the same.

For example, assuming this content in snap.yaml:

name: http
version: 1.0
apps:
    get:
        command: bin/my-downloader

This desktop file is valid:

[Desktop Entry]
Name=My Downloader
Exec=http.get %U

Unsupported desktop keys

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

# Aliases (snapd 2.20+)

Applications can normally be invoked using their name as just <app name==snap name>, when the app name matches the snap name, or <snap name>.<app name> if distinct. An application can also list aliases (whose names must follow more relaxed rules than app names, for example . and _ are allowed). These for example can follow the original naming of the commands in the source project of the snap. The listing happens through the aliases field in the application stanza:

apps:
  pg-dump:
    command: ...
    aliases: [pg_dump, pg_dump9.5]

Aliases cannot be used until they are enabled. Enabling can be done by the user with snap alias (disabling is done with snap unalias or snap alias --reset):

snap alias pg pg_dump9.5

See also: https://insights.ubuntu.com/2017/01/28/ubuntu-core-how-to-enable-aliases-for-your-snaps-commands/