AppImage is a format for distributing portable software on Linux without needing superuser permissions to install the application. It tries also to allow Linux distribution-agnostic binary software deployment for application developers, also called Upstream packaging. Released first in 2004 under the name klik, it was continuously developed, then renamed in 2011 to PortableLinuxApps and later in 2013 to AppImage.
AppImage does not install the application in the traditional Linux sense. Instead of putting the application's various files in the distro's appropriate places in the file system, the AppImage file is just the application's compressed image. When it runs, the file is mounted with FUSE. This behavior is consistent with AppImage's predecessors, klik and PortableLinuxApps. It uses one file per application. Each file is self-contained: it includes all libraries the application depends on that are not already part of the targeted base-system. An AppImage of version 1.0 is an ISO 9660Rock Ridge file containing a minimal AppDir and a tiny runtime.. An AppImage application can be added to a live CD by adding only one file to the live CD. AppImage files are simpler than installing an application. No extraction tools are needed, nor is it necessary to modify the operating system or user environment. Regular users on the common Linux distributions can download it, make it executable, and run it. AppImage allows generation of embedded digital signature, which need to be verified with an external utility. The format does not offer form of self-check with package authenticity verification or runtime confinement by sandboxing.
History
klik
AppImage's predecessor klik was designed in 2004 by Simon Peter. The client-side software is GPL-licensed. klik integrated with web browsers on the user's computer. Users downloaded and installed software by typing a URL beginning with klik://. This downloaded a klik "recipe" file, which was used to generate a .cmg file. For main ingredients, usually pre-built .deb packages from Debian Stable repositories were fed into the recipe's .cmg generation process. In this way, one recipe could be used to supply packages to a wide variety of platforms. With klik, only eight programs could be run at once because of the limitation of mounting compressed images with the Linux kernel, unless FUSE was used. The file was remounted each time the program is run, meaning the user could remove the program by simply deleting the.cmg file. A next version, klik2, was in development; and would natively incorporate the FUSE kernel module, but it never reached past the beta stage. Around 2011, the klik project went dormant and the homepage went offline for some time.
PortableLinuxApps
Simon Peter started a successor project named PortableLinuxApps with similar goals around that time. The technology was adapted for instance by the "portablelinuxgames.org" repository, providing hundreds of mostly open-source video games.
AppImage
Around 2013, the software was renamed again from portableLinuxApps to AppImage; the license became the MIT license. AppImage is the format and AppImageKit is a concrete open source implementation. The development happens in a GitHub repository.
Reception and usage
Klik was in 2007 the inspiration for Alexander Larsson's glick project, the precursor of Flatpak which was released in 2016. Linus Torvalds' dive log application Subsurface started to use AppImage around 2015. Subsurface's traditional packaging was changed to a portable, self-contained, distro-agnostic AppImage, as binary software deployment for the Linux users of the various Linux distributions turned out to be problematic. MuseScore started in April 2016 to use AppImage builds for software deployment for all Linux flavors. Krita, a digital paintingfree and open-source software application, is also deployed using AppImage from version 3.0 in May 2016. digiKam, an open source photo management application, also includes an AppImage bundle from version 5.3.0, released in November 2016. Network World's Bryan Lunduke noted on March 31, 2017 positively the Linux distribution-agnostic packaging of OpenShot with AppImage. LMMS, an open source digital audio workstation, started distributing for Linux as an AppImage since version 1.2.0-RC5. KeePassXC is also using AppImage for Linux binary distributions. Neo4j uses AppImage for the Linux version of their "Neo4j Desktop" program. RPCS3, the most used PS3 emulator, for Linux is distributed through AppImage. GitHub desktop client app Linux version maintained by the community is distributed with an AppImage option. Iris, the computer-monitor eye-strain reduction and management software, offers a paid AppImage option. FreeCAD, an open-source parametric 3D modeler, uses AppImage as the primary Linux binary download. OpenSCAD, another open-source parametric 3D modeler, also uses AppImage as a Linux binary download option.
