qrencode
--------

This might be old news to some, but I discovered it only yesterday.
qrencode is a handy little program, installable on Debian with a
simple `apt install qrencode`, for creating QR codes.  By default
it will produce a PNG image, but with the options `-o - -t UTF8`
it will actually produce a "textual" QR code right in your terminal
using Unicode block drawing characters (assuming you are using a
Unicode-aware terminal emulator and have the appropriate fonts
installed).  You can just pipe arbitrary text content into it,
up to 4,000 characters.  This is a super quick and convenient way
to transfer small snippets of data (smaller configuration files,
for example, or passwords, or URLs, or email addresses) from an
offline/air-gapped machine to just about any modern device, where
"modern" means "has a camera, but lacks basic practical computing
peripherals like USB or SD slots".

Of course, in conjunction with base64 or similar encodings, one
could send binary data in this way, too.  Not a lot of it in one
QR code.  Presumably if one actually wanted to write software for
one of these "modern devices" (which I sure don't), it wouldn't be
hard to come up with some kind of protocol where larger files are
split into chunks and the consecutive QR codes flashed sequential
on screen at a fixed rate, decoded and stitched back together on the
other end.  This reminds of a Timex watch that the brother of one of
my high-school friends had in the late 90s which could ingest data
(probably phone numbers?) from a PC if you held it up against the
monitor and ran the right software (I presume this was a very simple
setup with an LDR that could differentiate being held up against
a block of solid white pixels from a block of solid black pixels).
Something like this might actually be genuinely useful (as opposed
to just a nerdy gimmick) in some retrocomputing contexts, perhaps?
To copy files off a machine which has a graphics framebuffer but
no modern I/O?  Just an idle thought...