<varname>pkgs.appimageTools</varname> is a set of functions for extracting and wrapping <linkxlink:href="https://appimage.org/">AppImage</link> files. They are meant to be used if traditional packaging from source is infeasible, or it would take too long. To quickly run an AppImage file, <literal>pkgs.appimage-run</literal> can be used as well.
There are different formats for AppImages, see <linkxlink:href="https://github.com/AppImage/AppImageSpec/blob/74ad9ca2f94bf864a4a0dac1f369dd4f00bd1c28/draft.md#image-format">the specification</link> for details.
type1.AppImage: ELF 64-bit LSB executable, x86-64, version 1 (SYSV) ISO 9660 CD-ROM filesystem data 'AppImage' (Lepton 3.x), scale 0-0,
spot sensor temperature 0.000000, unit celsius, color scheme 0, calibration: offset 0.000000, slope 0.000000, dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.18, BuildID[sha1]=d629f6099d2344ad82818172add1d38c5e11bc6d, stripped\012- data
<prompt>$ </prompt>file -k type2.AppImage
type2.AppImage: ELF 64-bit LSB executable, x86-64, version 1 (SYSV) (Lepton 3.x), scale 232-60668, spot sensor temperature -4.187500, color scheme 15, show scale bar, calibration: offset -0.000000, slope 0.000000 (Lepton 2.x), scale 4111-45000, spot sensor temperature 412442.250000, color scheme 3, minimum point enabled, calibration: offset -75402534979642766821519867692934234112.000000, slope 5815371847733706829839455140374904832.000000, dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.18, BuildID[sha1]=79dcc4e55a61c293c5e19edbd8d65b202842579f, stripped\012- data
<varname>extraPkgs</varname> allows you to pass a function to include additional packages inside the FHS environment your AppImage is going to run in. There are a few ways to learn which dependencies an application needs:
Looking through the extracted AppImage files, reading its scripts and running <command>patchelf</command> and <command>ldd</command> on its executables. This can also be done in <command>appimage-run</command>, by setting <command>APPIMAGE_DEBUG_EXEC=bash</command>.