Flatpak and Snap¶

Flatpak and Snap install applications together with their runtimes, separate from the distribution's packages. RHCSA 10 includes installing software with Flatpak; Snap is Canonical's equivalent and is preinstalled on Ubuntu.

Track: RHCSA · Interview weight: Low

Must-Know Facts¶

Fact	Value	Verify with
Flatpak remotes	Sources such as Flathub; system-wide or per user	`flatpak remotes`
Install, update, remove	`flatpak remote-add`, `install`, `update`, `uninstall`; apps pull shared runtimes	`flatpak list`
Scope	`--system` (default for root, `/var/lib/flatpak`) or `--user` (`~/.local/share/flatpak`)	`flatpak list --columns=installation`
Clean up	`flatpak uninstall --unused` removes orphaned runtimes	`du -sh /var/lib/flatpak`
Snap	Squashfs images mounted under `/snap`, managed by `snapd`	`snap list`
Snap updates	Automatic by default; `snap refresh --hold` pauses them	`snap refresh --list`

Flatpak¶

sudo flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
sudo flatpak install -y --noninteractive flathub org.gnome.Tetravex
flatpak remotes
flatpak list --columns=application,branch,installation
flatpak info org.gnome.Tetravex | grep -E 'ID:|Version:|Runtime:|Installation:|Installed:'
du -sh /var/lib/flatpak

Output:

# ... (trimmed)
Installing runtime/org.gnome.Platform/x86_64/50
Installing app/org.gnome.Tetravex/x86_64/stable
flathub system
# ... (trimmed)
org.gnome.Platform  50  system
org.gnome.Tetravex  stable  system
          ID: org.gnome.Tetravex
     Version: 3.38.3
Installation: system
   Installed: 1.7 MB
     Runtime: org.gnome.Platform/x86_64/50
2.4G    /var/lib/flatpak

Uninstalling an app leaves its runtime behind

The application is 1.7 MB; its runtime and graphics extensions brought the total to 2.4 GB. sudo flatpak uninstall <app> leaves the runtime behind, and sudo flatpak uninstall --unused removes it. A remote added with sudo exists only in the system installation, so flatpak install --user needs flatpak remote-add --user first.

Snap¶

snap list
findmnt -t squashfs -o TARGET,SOURCE | head -3
sudo snap remove hello-world

Output:

Name         Version             Rev    Tracking       Publisher    Notes
core         16-2.61.4-20260225  17292  latest/stable  canonical**  core
hello-world  6.4                 29     latest/stable  canonical**  -
snapd        2.76.3              27738  latest/stable  canonical**  snapd
TARGET               SOURCE
/snap/snapd/27738    /dev/loop0
/snap/core/17292     /dev/loop1
hello-world removed (snap data snapshot saved)

Each snap revision is a squashfs image on a loop device

The hello-world snap was installed with sudo snap install hello-world. Each snap revision is a squashfs image on a loop device, which is why lsblk and df on Ubuntu show /dev/loopN entries.

Common Errors¶

`error: No remote refs found for ‘flathub’`¶

Cause: flatpak install --user ran while the remote exists only in the system installation (or the reverse); Flatpak prints Looking for matches… first.

Fix: flatpak remotes --show-details to see the scope; add the remote to the scope being used.

Interview Checkpoints¶

L1: How do Flatpak and Snap differ from RPM or DEB packages?

Say first: they bundle an application with its runtime, run it sandboxed, and update independently of the distribution; RPM and DEB share system libraries.

Proof: flatpak info <app> names its own runtime; snap list shows each snap's revision.

Follow-up: What does that cost in disk space and in patching responsibility?

L2: Install an application from Flathub for all users on RHEL 10.

Say first: add the remote system-wide, then install.

Proof: sudo flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo && sudo flatpak install flathub <app-id>

Follow-up: How do you update it later? (flatpak update.)

L2: Free disk space used by old Flatpak runtimes.

Say first: remove runtimes no application uses.

Proof: sudo flatpak uninstall --unused; du -sh /var/lib/flatpak

Follow-up: Where do user installations keep their data?

L2: Explain the loop devices in lsblk on an Ubuntu server.

Say first: each snap revision is a squashfs image mounted from a loop device.

Proof: snap list; findmnt -t squashfs

Follow-up: How do you limit how many old revisions are kept? (snap set system refresh.retain=2.)

L3: A user's flatpak install fails with No remote refs found, although flathub is configured.

Say first: compare the scope of the remote with the scope of the install.

Proof: flatpak remotes --show-details shows system; the user ran --user.

Follow-up: Which scope suits a shared workstation?

L3: An Ubuntu server's application changed version overnight without an apt upgrade.

Say first: check whether it is a snap, which refreshes automatically.

Proof: snap list, snap changes; snap refresh --hold=<duration> <snap> pauses updates.

Follow-up: How do you schedule refreshes instead? (snap set system refresh.timer=....)

Packaging Concepts: what traditional packages contain; Repositories: remotes for RPM and DEB

Captured on Rocky Linux 10.2 and Ubuntu 24.04.4 LTS (iximiuz Labs microVMs, kernel 6.1.167), 2026-09.