I've been running Ubuntu 20.04 since it came out. Today the Ubuntu Software app is telling me that there's an update to Core 18 ("Runtime environment based on Ubuntu 18.04"). I don't have a update source that appears to cause this. Is software from 18.04 part of 20.04? Should I perform the update?
3 Answers
The Software app in 20.04 has a bug that notifies the user of an update to a package that isn't installed. If you ignore this notification it disappears the next time you open the Software app and it never comes back.
I have core and core18 snap packages installed in 20.04. core18 was automatically installed as a dependency of another snap package. The software app in 20.04 has a bug in it that it shows an available update for a snap package that isn't installed, for example core16. When you click the Update button a notification pops up that says Unable to update core: snap has no updates available
as shown in the screenshot, even though the results of snap list
show that the core16 snapd-runtime environment is not installed. Not only that but core16 does not have a stable channel, only beta and edge candidates, so I ignored the green number next to Updates on the toolbar. The next time I opened the Software app the Update notification for core16 was gone, and it has never reappeared since then.

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My search engine brought me here, running Ubuntu 22.04 LTS.
Solution: click the refresh arrow button in the upper left corner. Afterwards the application shows everything is up to date.
My guess it, that Ubuntu Software
is not properly checking for pending updates
$ snap changes
ID Status Spawn Ready Summary
117 Done today at 08:41 CET today at 08:41 CET Auto-refresh snap "core18"
118 Done today at 13:37 CET today at 13:37 CET Refresh all snaps: no updates
119 Done today at 13:41 CET today at 13:41 CET Refresh all snaps: no updates
The erroneous pending update was presented to me at 13:xy, and according to snap
the core18
was updated in the morning at 8:41 (13:37 and :41 is me trying snap refresh
manually)

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I have found the same on my system & continue trying to update, then ignore it until the next time the update message appears.
snap search core18
on my groovy box). It's talking about the Snap core18 I believe; my groovy box having both core18 and core20 installed. Sorry I upgrade packages by terminal so have little experience of how it shows in apps. – guiverc Jul 14 '20 at 00:54snap refresh
) but my knowledge in this area is incomplete sorry. – guiverc Jul 14 '20 at 01:24core
(based on 16.04),core18
(based on 18.04) andcore20
(based on 20.04). The core snaps are runtime environments, other snaps are built on them. Which of the core snaps are installed in your system depends on other snaps installed, the core snaps are pulled in as dependencies. Since the snapsnap-store
should be installed in your system,core18
should also be installed. This question might be interesting for you. – mook765 Jul 14 '20 at 06:35