This is an aside from the actual project, but I thought I would go ahead and mention a tool which has become one of my favorites: platformio.
If you are the sort of person who is reading about this project, chances are good that you are using the Arduino IDE to do a lot of your programming. The Arduino IDE has a lot going for it. It is available on Linux, Windows and Mac, and it works the same pretty much everywhere. In the most recent upgrade, it supports a fair number of architectures, and has a library manager as well. For many of you, it will be a comfortable and familiar download, and requires only the installation of a single application.
But I'm kind of old school.
I cut my teeth on Unix boxes. I have a Macbook that I use for a lot of my programming, but I spend the vast majority of my time in Terminal windows, using familiar utilities like ls, cat and vi. And, I mostly don't like to use a mouse all that much. I like to use utilities like make.
But what I don't enjoy is downloading and installing all the necessary cross compiling tools to compile code for the Arduino. The Arduino IDE is really just a wrapper around the GNU C compiler. You could use make and avr-gcc without the IDE environment, but setting that up can be a pain, and its hard to keep it up to date.
Until, that is, until platformio was created.
It is a system written in Python that allows you to compile code for the Arduino and many other systems (like the Teensy, the ESP8266, and mbed processors) all with just a simple set of consistent commands. It will go off and find all the command line tools you need for the board you've selected, and then install them so that you can compile code with just a simple set of commands. Since many boards like the ESP8266 have some compatibility with Arduino code, often you can easily recompile the same code to be executed on different boards, all at once.
When I eventually make my code available on github, I will include the necessary files to compile with platformio. You can still compile any of the Arduino sketches using the IDE if you like, but I urge you to give platformio a try. In the end, I'll show how you can use it to compile barebones C code for processors like the ATtiny13, as well as some other tricks like disassembling C code to learn more about how the machine language code generated by gcc can be modified to be more compact and to understand other optimizations.