What is FUSE?

I was trying to run AppImage for Heynote.

But running heynote showed this error:

AppImages require FUSE to run.
You might still be able to extract the contents of this AppImage
if you run it with the --appimage-extract option.
See https://github.com/AppImage/AppImageKit/wiki/FUSE
for more information

I started to wonder what is FUSE and why do AppImages require FUSE to run?

What is AppImage?

AppImage packages an application and all its dependencies into a single executable file (like a .exe on Windows).
To run this self-contained app without extracting it, AppImage mounts itself as a virtual filesystem, and this is where FUSE comes in.

What FUSE Does for AppImage?

1. Mounting the AppImage internally:

AppImage is a SquashFS archive (a compressed read-only filesystem).

Instead of extracting the archive, AppImage uses FUSE to mount it on-the-fly when you run the .AppImage.

1. No root required:

Because FUSE works in userspace, AppImage can mount and run its contents without root permissions.

1. Transparent execution:

Once mounted, the internal files (like the real app binary, libraries, icons, etc.) are accessed as if they were in a normal folder.

Typical Flow When You Run an AppImage

1. You execute the AppImage.
2. AppImage uses FUSE to mount itself (the SquashFS inside).
3. The actual application binary is launched from the mounted image.

NOTE: In fact, some AppImages do not require FUSE.

• Type 1 AppImages: These extract themselves to /tmp and run from there — don’t require FUSE, but are slower and messier.
• Type 2 AppImages: These mount themselves using FUSE — preferred, faster, and cleaner. But they require FUSE to be installed. Many modern AppImages are Type 2. You can check their type by ./MyApp.AppImage --appimage-version //TODO: how to distinguish between Type 1 and Type 2?

Workarounds

• Manually extracting the AppImage: Extract the AppImage manually so that it does not require any mountings.

./Heynote_2.2.2_x86_64.AppImage --appimage-extract
cd squashfs-root
./AppRun

• Install FUSE

If you are on Ubuntu:

sudo apt install fuse

If you are on Arch:

sudo apt install fuse2 fuse3

Once installed, you can check if the FUSE kernel modules are loaded by:

NOTE: Some old applications might require fuse2 instead of fuse3.

✅ Summary

FUSE lets AppImage mount its own SquashFS archive in user space so that it can run without root permission and any extraction. You can extract manually by ./your_appimage.AppImage --appimage-extract if FUSE is not available.

Reference

[1] Heynote
[2] ArchLinux Wiki