Trying to build a Snap for the first time

firstsnapexperience.md

#My Notes on My First Snap Building Experience

Hi, I wanted to try building a Snap package, so from my experiment thought on leave what my wandering left me, lot's of this content is from the Ubuntu Desktop Developer website . The website has a nice getting started too! .

I advise using Ubuntu 16.04 LTS as a building environment. I made a clean Virtual Box machine (my main PC is 14.04, but I also prefer a clean environment to work). In the machine, snaps should be already available, so you only need snapcraft. For completeness, the minimum requirement is below. I used a 15 GB drive, but you can sure go smaller if you want.

in command line:

sudo apt install snapd snapcraft

Building a snap app from already existent source is easy. Create an empty dir somewhere.

in command line:

mkdir ~/my1snap
cd ~/my1snap

The most important file in building a snap is the snapcraft YAML file descriptor. I didn't knew YAML before, the most easy docs on it is this ANSIBLE docs , the important thing here is how to list things:

[ apple, banana, coconut]

or

- apple
- banana
- coconut

Now, snapcraft.yaml syntax is available here but the best way to dive into it, is really getting started. Since we are already with terminal opened in that empty my1snapdir, let's start.

in command line:

snapcraft init

This will create an almost empty snapcraft.yaml file. It's content must be something like below.

name: # the name of the snap
version: # the version of the snap
summary: # 79 char long summary
description: # A longer description for the snap

Ok, so open this file in your preferred editor and fill the name, version, summary and description. Just write something at the right of the :.

Next, let's look at this tutorial Your First Snap.

name: webcam-webui
version: 1
summary: Webcam web UI
description: Exposes your webcam over a web UI
icon: icon.png

apps:
  webcam-webui:
    command: bin/webcam-webui
    daemon: simple

parts:
  golang-static-http:
    plugin: go
    source: git://github.com/mikix/golang-static-http
    stage-packages:
     - fswebcam
  glue:
    plugin: copy
    files:
      webcam-webui: bin/webcam-webui

From the snapcraft.yaml file here in this tutorial it's possible to understand that there is a main part called apps, that let's you specify a daemon to start and a command to call - for this app. And also there is the parts, which contain the pieces that build the app itself.

You can see that there plugins associated to each piece in the parts, these plugins are really a nicely crafted part of snaps. Let's learn about some plugins.

in command line:

snapcraft help python3
snapcraft help go
snapcraft help copy

So in the python3, for example, you can add an additional line called python-packages that will list which python packages you want to install. You can use source to point to a source code that will benefit from those packages.

python-packages: [pack1, pack2]

python-packages:
- pack1
- pack2

You can also add to this piece inside the parts the stage-packages which are binaries straight from the apt or more recommended build-packages, which will build from source - it's important to make your snapcraft.yaml file platform agnostic when possible. For example, the package pillow in Python requires libjpeg. To build it from source, add in your piece.

build-packages: [libjpeg-dev]

For building a package that includes a source, it's important that your source has an adequate setup.py file, and follows all the recommended practices of distributing packages in python. If you don't, it's harder to make it work with snaps. This will be important for other languages too. The documentation at the moment is a little small, but I recommend reading and using snapcraft --help to learn more. Also any problems in the official docs, fill bug reports, the developers will answer.

If you are having problems, and everything fails (google, stackoverflow, and others), I would recommend trying the #snappy channel on irc.freenode.net .