Installing GNU APL

Two parts: the keyboard layout and the program itself.  This is on Ubuntu GNOME 17.04.

Install GNU APL.

cd ~/Downloads
wget ftp://ftp.gnu.org/gnu/apl/apl-1.7.tar.gz
tar xzf apl-1.7.tar.gz
cd apl-1.7
./configure
make 
sudo make install

Set up the keyboard (for all the weird symbols). On Ubuntu GNOME 17.04, go to:

settings -> region and language -> [+] input source -> English (region) -> APL (dyalog)

Then set a good fixed-width font so the symbols show up correctly.  Go here and download the recommended font, open and install it.  Then open the GNOME Tweak Tool:

fonts -> monospace -> APL385 Unicode Regular -> Select

And you're done.  Use

win + space

to switch between the fonts.