Triggerhappy is a hotkey daemon that operates on a system wide scale. It watches all configured input devices for key, switch or button events and can launch arbitrary commands specified by the administrator. In contrast to hotkey services provided by desktop environments, Triggerhappy is especially suited to hardware related switches like volume or wifi control; it works independently from a specific user being logged in and is also suitable for embedded systems that do not a graphical user interface.
The source can be found on https://github.com/wertarbyte/triggerhappy

Found on Tony Finch's link log https://dotat.at/:/
The pamtester program used there might be useful to learn more about pam.

See also
https://askubuntu.com/questions/458427/who-reads-etc-default-keyboard
https://askubuntu.com/questions/1184746/how-to-make-caps-lock-an-additional-ctrl
https://askubuntu.com/questions/769462/request-for-a-guide-on-setting-up-apple-aluminium-keyboard-on-ubuntu-16-04

Different ways to swap CapsLock and Control.
See also https://askubuntu.com/questions/976996/add-custom-keyboard-layout?r=SearchResults

Welcome! Funtoo Linux (distrowatch) is a community-developed Linux meta-distribution based upon Gentoo Linux. Funtoo Linux is optimized for the best possible performance on the latest Intel and AMD hardware. Funtoo is led by Daniel Robbins, the creator of Gentoo Linux, and actively developed by the Funtoo community.
Funtoo does not use systemd, but OpenRC https://www.funtoo.org/FAQ:Do_You_Support_Systemd

Gqrx is an open source software defined radio receiver (SDR) powered by the GNU Radio and the Qt graphical toolkit.

How to use the RTC alarm function.

Overview of current (2018-10) compression algorithms.

Coccinelle is a program matching and transformation engine which provides the language SmPL (Semantic Patch Language) for specifying desired matches and transformations in C code. Coccinelle was initially targeted towards performing collateral evolutions in Linux. Such evolutions comprise the changes that are needed in client code in response to evolutions in library APIs, and may include modifications such as renaming a function, adding a function argument whose value is somehow context-dependent, and reorganizing a data structure. Beyond collateral evolutions, Coccinelle is successfully used (by us and others) for finding and fixing bugs in systems code.

Minifree provides secure, privacy-respecting computers with 100% free (as in freedom) software, including the Libreboot BIOS replacement and GNU+Linux operating system, certified by FSF under their Respects Your Freedom certification program.