Classic Chroots

WIth Ubuntu 16.04 onward, many of the snap tools required for building a snap appear to be readily available.

To avoid read-only filesystem issues (i.e., no useful package manager like Apt),you can create a chroot environment in which you develop your application. A chroot ignores the file path and some of the host system innards. Within the chrooted environment, you share the home/  directory between the container (your snap) and the ubuntu-core  snap (your snap’s OS). The next command lets you run a “classic” Ubuntu environment on a Snappy system:

$ snap install classic --edge --devmode
classic (edge) 16.04 from 'canonical' installed

When you run classic , it changes the prompt, after loads of output, by prepending (classic) , which is also a reminder to always run this from your home directory so snapd  knows what’s going on.

$ classic
(classic)root@chris:~#

Unlike my example, you should probably avoid using root and, if needed, sudo  from your user or use it directly.

You need to throw in some compilers and other build tools now, which, if your prompt has the classic  prefix, gets chucked into your chroot and not directly onto your system:

(classic)root@chris:~# apt install build-essential

If you did not install the snapcraft  package before this point, you should add that package to this command, as well. After hitting the Enter key, you’re faced with the usual (potentially insecure, if you leave it on a system unnecessarily) more than 200MB of compiler tools, such as the omnipotent gcc  package.

Make It Snappy

Unlike the init  command example in Figure 6, in this case, I’m running it inside my chroot.  For this example, I create and change to a directory called echo-chrisbinnie  before running the init  command. Pay close attention: If you’re not in a directory off your home directory, your chroot  might not work as expected and could potentially reference other system paths within the snap by accident.

In a slightly alarming oh-no-it’s-all-badly-broken sort of way, you might see a screen full of errors. Fret not, though, because on Ubuntu 16.04, I entered two suggested export  commands,

$ export LC_ALL=C.UTF-8; export LANG=C.UTF-8

and then ran the snapcraft init  command again with complete success. Thankfully, it’s happy days again when you see the friendly Edit the file to your liking  greeting that you saw in Figure 6. As you’ve probably guessed, the next thing to do is edit the template file under the snap/  directory.

When I first looked at snaps, I have to admit that I needed to tinker with paths, parts, and source file downloads, but ultimately, unless you’re building a massive file from source, it’s a relatively simple and rewarding experience to build your first snap. Once your YAML is correct, you witness a welcome screen of progress as your snap is being packaged (Figure 7). To compile your snapcraft.yaml  file, just make sure you’re in the snap/  directory within your chroot, and run the

(classic)root@chris:~/echo-chrisbinnie/snap# snapcraft

command a few times, adjusting your configuration according to any errors. Also, pay attention to the configflags  compile flags (Listing 5, last line). Would you believe even a simple echo  command needs tweaking with compile flags?!

Listing 5: Working Config for Echo Snap

$ snap install classic --edge --devmode
classic (edge) 16.04 from 'canonical' installed
 
$ classic
(classic)root@chris:~#
 
name: echo-chrisbinnie
version: '0.1'
summary: Echo a name
description: |
  Enter a name and I will echo it.
grade: devel
confinement: devmode
apps:
  echo-chrisbinnie:
    command: bin/echo
  bash:
    command: bash
parts:
  echo-chrisbinnie:
    source: https://launchpad.net/ubuntu/+archive/primary/+files/coreutils_8.25.orig.tar.xz
    plugin: autotools
    configflags: ["FORCE_UNSAFE_CONFIGURE=1"]

If you get stuck, you’ll find more docs at the Ubuntu developer site that are much easier to use if you spin up a fully supported (ephemeral) Ubuntu 16.04 cloud instance/droplet/VM somewhere. Note that I simply looked up Ubuntu’s 16.04 source files online for the coreutils  URL (which is the package that provides the echo  binary).

Once compilation and packaging was complete, I found an echo-chrisbinnie_0.1_amd64.snap  file inside my snap/  directory.  If you’re happy that your version worked like mine (Figure 7), you can change confinement  from devmode  to strict  and grade from devel  to stable  for publishing.

Figure 7: After loads of compiling and a bit of tweaking and re-running the snapcraft command in a chrooted environment, this output indicates everything is working as hoped (probably!).

Now, you need to run the packaging command one more time:

$ snapcraft

Assuming your last line says Snapped  with the snap name (and you’re not uploading the snap to the Snap Store but installing it locally), you’ll face the moment of truth when you try to execute your snap.

Oh No, It’s Snapped!

Because your hand-crafted snap hasn’t been signed by Canonical’s Snap Store at this stage, you’ll have to add --dangerous  to the end of the install  command. (By this stage, to publish your snap successfully, you have to have edited the config file and made the changes to confinement  and grade  as mentioned in the previous section). Even though confinement  is strict , your snap is still apparently dangerous. Don’t forget to run the snapcraft  command whenever you make a change to your config file.

A simple snap installation looks like this example:

(classic)root@chris:~/echo-chrisbinnie/snap# snap install echo-chrisbinnie_0.1_amd64.snap --dangerous
echo-chrisbinnie 0.1 installed