.APPIMAGE

Introduction

An AppImage is a Linux application packed into one file, with the libraries it needs, so that it runs on most distributions without being installed. The user downloads it, marks it as executable and runs it. Nothing is copied into the system, and deleting the file removes the application. The extension is usually written .AppImage.

Structure

An AppImage is two things joined together. It starts with a small ELF program, the runtime, followed by a filesystem image, usually SquashFS, holding the application. When run, the runtime mounts the image with FUSE and starts the program inside it. Current AppImages (type 2) mark themselves with the bytes 41 49 02 (AI and the type) at offset 8, in a part of the ELF header that is otherwise unused.

Inside the image is an AppDir: an AppRun entry point that starts the application, a .desktop file with its name and category, an icon, and the application’s files. The runtime can also extract the image with --appimage-extract.

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