You are in luck, because Linux (actually Xorg) has the most versatile and powerful keyboard configuration system of all major OSes. You can choose from a number of APL symbol dispositions and add them to your own national keyboard layout. You can even choose which key to use to switch between regular letters and APL, and apply this choice all over the system.

If you open your distribution's internationalization or keyboard layout settings (it could be in a control panel-like GUI, or it could be a settings file, for example /etc/default/keyboard in Debian) you will be able to add the apl keyboard layout to your existing national layout. The APL layout includes the basic APL symbols in a somewhat standard position and provides a number of variants that select additional symbols or different dispositions, including those of commercially available hardware keyboards: unified , sax , apl2 , aplplusII , aplx , dyalog .

Please note that the APL layout is meant to be added to your existing national layout, alongside a choice of key to switch or toggle between the two. If you replace your national layout with APL, you won't be able to type standard letters anymore!

To test a combination of layouts, variants, and options, you can use a graphical control panel-thingy if you have it, or you can use the setxkbmap command.

For example, this selects the us layout, adds the apl one, in its default variant, and designates the Windows key as the switch between the two, so that Win + A gives ⍺, etc.

setxkbmap us,apl -option grp:win_switch

Protip: you might want to keep another terminal open with some default command ready, for example setxkbmap us , so that you can easily press Enter inside it and reset your layout, if you render your keyboard unusable. If you can't even press Enter anymore, you can copy+paste a line break to execute the command.

After you are happy with your combination of layouts, variants, and options, you will have to put them into the global configuration file, or use the control panel button that makes them a permanent choice.

If you are having trouble with some specific combination of options, you might want to look over the XKB documentation wiki, chat on one of the many Linux IRC channels, or post a more specific question here on StackExchange, although I would suggest using Super User, Unix & Linux, or Ask Ubuntu, instead of StackOverflow.