Command-line QR code generation is enabled on Linux by:
apt install qrencode
This example is for 300 dpi with Medium error correction, making a PNG image.
qrencode -s10 -d 300 -lM https://example.com -o qr-example.png
The QtQR GUI program allowed easy generation of QR codes.
However, by Ubuntu 18.04 the QtQR application seemed broken, giving syntax errors that may be related to using too old Qt version.
apt install qtqr