Last time, I described how to write a simple Android app and get it talking to your code on Linux. So, of course, we need an example. Since I’ve been on something of a macropad kick lately, I decided to write a toolkit for building your own macropad using App Inventor and any sort of Linux tools you like.
I mentioned there is a server. I wrote some very basic code to exchange data with the Android device on the Linux side. The protocol is simple:
- All messages to the ordinary Linux start with >
- All messages to the Android device start with <
- All messages end with a carriage return
Security
You can build the server so that it can execute arbitrary commands. Since some people will doubtlessly be upset about that, the server can also have a restrictive set of numbered commands. You can also allow those commands to take arguments or disallow them, but you have to rebuild the server with your options set.
There is a handshake at the start of communications where Android sends “>.” and the server responds “<.” to allow synchronization and any resetting to occur. Sending “>#x” runs a numbered command (where x is an integer) which could have arguments like “>#20~/todo.txt” for example, or, with no arguments, “>#20” if you just want to run the command.
If the server allows it, you can also just send an entire command line using “>>” as in: “>>vi ~/todo.txt” to start a vi session.
