Linux install instructions for Ruby applications often get complicated. To prevent modules from different Ruby applications clashing with each other, developer tools like rvm or rbenv must be used.
With snaps and snapcraft, it’s one command to produce a bundle that works anywhere.
Why are snaps good for Ruby projects?
- Snaps install and run the same across Linux They bundle the exact runtime versions required, along with all of your app’s dependencies.
- Simple creation of services Create a standard app or a daemon easily.
- Snaps are easy to discover and install Millions of users can browse and install snaps graphically in the Snap Store or from the command-line.
- Snaps automatically update to the latest version Four times a day, users’ systems will check for new versions and upgrade in the background.
- Upgrades are not disruptive Because upgrades are not in-place, users can keep your app open as it’s upgraded in the background.
- Upgrades are safe If your app fails to upgrade, users automatically roll back to the previous revision.
Ready to get started? By the end of this guide, you’ll understand how to make a snap of your Ruby app that can be published in the Snap Store, showcasing it to millions of Linux users.
For a brief overview of the snap creation process, including how to install snapcraft and how it’s used, see Snapcraft overview. For a more comprehensive breakdown of the steps involved, take a look at Creating a snap.
Getting started
Snaps are defined in a single YAML file placed in the root folder of your project. The following example shows the entire snapcraft.yaml file for an existing project, Markdown lint tool. Don’t worry, we’ll break this down.
Travis
Snaps are defined in a single yaml file placed in the root of your project. The Travis example shows the entire snapcraft.yaml for an existing project, leveraging the existing gemspec to satisfy runtime requirements. We’ll break this down.
name: test-mdl
version: "0.5.0"
summary: Markdown lint tool
description: |
Style checker/lint tool for markdown files
confinement: devmode
base: core18
parts:
test-mdl:
source: .
plugin: ruby
gems:
- rake
- bundler
override-build: |
snapcraftctl build
rake install
build-packages:
- git
apps:
test-mdl:
command: bin/mdl
Metadata
The snapcraft.yaml file starts with a small amount of human-readable metadata, which usually can be lifted from the GitHub description or project README.md. This data is used in the presentation of your app in the Snap Store.
name: test-mdl
version: "0.5.0"
summary: Markdown lint tool
description: |
Style checker/lint tool for markdown files
The name must be unique in the Snap Store. Valid snap names consist of lower-case alphanumeric characters and hyphens. They cannot be all numbers and they also cannot start or end with a hyphen.
Version is an arbitrary string or number.
The summary can not exceed 79 characters. You can use a chevron ‘>’ in the description key to declare a multi-line description.
Base
The base keyword declares which base snap to use with your project. A base snap is a special kind of snap that provides a run-time environment alongside a minimal set of libraries that are common to most applications:
base: core18
As used above, core18 is the current standard base for snap building and is based on Ubuntu 18.04 LTS.
See Base snaps for more details.
Security model
The next section describes the level of confinement applied to your app.
confinement: devmode
Snaps are containerised to ensure more predictable application behaviour and greater security. Unlike other container systems, the shape of this confinement can be changed through a set of interfaces. These are declarations that tell the system to give permission for a specific task, such as accessing a webcam or binding to a network port.
It’s best to start a snap with the confinement in warning mode, rather than strictly applied. This is indicated through the devmode keyword. When a snap is in devmode, runtime confinement violations will be allowed but reported. These can be reviewed by running journalctl -xe.
Because devmode is only intended for development, snaps must be set to strict confinement before they can be published as “stable” in the Snap Store. Once an app is working well in devmode, you can review confinement violations, add appropriate interfaces, and switch to strict confinement.
Parts
Parts define what sources are needed to assemble your app. Parts can be anything: programs, libraries, or other needed assets, but for now, we’re only going to use one part: the mdl source code.
parts:
test-mdl:
source: .
plugin: ruby
gems:
- rake
- bundler
override-build: |
snapcraftctl build
rake install
build-packages:
- git
For more details on Ruby-specific metadata, see The Ruby plugin.
Apps
Apps are the commands you want to expose to users and any background services your application provides. Each key under apps is the command name that should be made available on users’ systems.
The command specifies the full path to the binary to be run. This is resolved relative to the root of your snap contents.
apps:
test-mdl:
command: bin/mdl
If your application is intended to run as a service you simply add the line daemon: simple after the command keyword. This will automatically keep the service running on install, update and reboot.
If your command name matches the snap name, users will be able run the command directly. If the names differ, then apps are prefixed with the snap name (offlineimap.command-name, for example). This is to avoid conflicting with apps defined by other installed snaps.
You can request an alias on the Snapcraft forum if your command name and snap name do not match but you don’t want your command prefixed. These aliases are set up automatically when your snap is installed from the Snap Store.
Building the snap
You can download the example repository with the following command:
$ git clone https://github.com/snapcraft-docs/mdl
After you’ve created the snapcraft.yaml, you can build the snap by simply executing the snapcraft command in the project directory:
$ snapcraft
Using 'snapcraft.yaml': Project assets will be searched for from the 'snap' directory.
Launching a VM.
[...]
Snapped test-mdl_0.5.0_amd64.snap
The resulting snap can be installed locally. This requires the --dangerous flag because the snap is not signed by the Snap Store. The --devmode flag acknowledges that you are installing an unconfined application:
$ sudo snap install test-mdl_*.snap --devmode --dangerous
You can then try it out:
$ test-mdl -h
Removing the snap is simple too:
$ sudo snap remove test-mdl
You can also clean up the build environment, although this will slow down the next initial build:
$ snapcraft clean
By default, when you make a change to snapcraft.yaml, snapcraft only builds the parts that have changed. Cleaning a build, however, forces your snap to be rebuilt in a clean environment and will take longer.
Publishing your snap
To share your snaps you need to publish them in the Snap Store. First, create an account on the dashboard. Here you can customise how your snaps are presented, review your uploads and control publishing.
You’ll need to choose a unique “developer namespace” as part of the account creation process. This name will be visible by users and associated with your published snaps.
Make sure the snapcraft command is authenticated using the email address attached to your Snap Store account:
$ snapcraft login
Reserve a name for your snap
You can publish your own version of a snap, provided you do so under a name you have rights to. You can register a name on dashboard.snapcraft.io, or by running the following command:
$ snapcraft register mypythonsnap
Be sure to update the name: in your snapcraft.yaml to match this registered name, then run snapcraft again.
Upload your snap
Use snapcraft to push the snap to the Snap Store.
$ snapcraft upload --release=edge mynodesnap_*.snap
If you’re happy with the result, you can commit the snapcraft.yaml to your GitHub repo and turn on automatic builds so any further commits automatically get released to edge, without requiring you to manually build locally.
Congratulations! You’ve just built and published your first Go snap. For a more in-depth overview of the snap building process, see Creating a snap.