We are happy to announce that a new version of Ubuntu Core, based on snapd 2.21, has been released to the stable snaps channel yesterday.
As with any stable release, your Ubuntu Core devices will update and reboot automatically. If you are using snaps on the desktop, the release will reach you through a snapd package update on Ubuntu 16.04, 16.10 and 17.04.
This release comes with several improvements you can read about in the changelog, but let’s focus on a big feature that will help people who snap very large software, especially software that comes with many commands (such as OpenStack, ImageMagick, most databases…) and their users.
When you launch a snap from the command line, you need to use the name of the snap, then the name of a command it contains. In most cases, you don’t notice it, because snapd simplifies the process by collapsing
<command-name>, when both are the same. This way, you don’t need to type
inkscape.inkscape, but simply
inkscape and get a familiar software experience.
But when a snap contains multiple commands, with various names, things can become less familiar. If we take the PostgreSQL snap as an example, we can see it comes with many commands:
createdb, etc. In this case, you have to run
The alias feature of snapd lets snaps declare their own aliases, for users to manually enable after install or for snap stores to declare as “auto-aliases” that will be enabled upon install.
To have an overview of all available aliases on a system, you can use the
snap aliases command. For this example, I’ve installed a modified version of the
postgresql96 snap with aliases for all commands.
$ snap aliases App Alias Notes postgresql96.clusterdb clusterdb - postgresql96.createdb createdb - postgresql96.createlang createlang - postgresql96.createuser createuser - postgresql96.dropdb dropdb - postgresql96.droplang droplang - postgresql96.dropuser dropuser - postgresql96.ecpg ecpg - postgresql96.initdb initdb - [...]
As you can see, the snap now exposes aliases to snapd. They are not enabled yet, but it’s simply a matter of running
snap alias <snap-name> <alias>. Note that you can pass multiple space-separated aliases to this command.
Let’s enable the
$ snap alias postgresql96 createuser $ snap aliases | grep createuser postgresql96.createuser createuser enabled
I can now ensure that there is indeed a
createuser command coming from a snap:
$ which createuser /snap/bin/createuser
And of course, run it:
$ createuser --help createuser creates a new PostgreSQL role.
To disable an alias you can use the
snap unalias command.
Declaring a new alias in your snap is as easy as adding an
aliases entry to your snapcraft.yaml
apps keys. Let’s look at the
createuser command in our snap:
[...] apps: [...] createuser: command: usr/bin/createuser plugs: [network] aliases: [createuser] [...]
And build your snap as usual with
snapcraft cleanbuild or let it flow through your CI.
See the full example for this snap in this pull request.
That’s it, heads-on to tutorials.ubuntu.com to make your own snap from scratch and give aliases a try!
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