How to check if running in Cygwin, Mac or Linux?

Question

I have a shell script that is used both on Windows/Cygwin and Mac and Linux. It needs slightly different variables for each versions.

How can a shell/bash script de

Answer 1

Windows Subsystem for Linux did not exist when this question was asked. It gave these results in my test:

uname -s -> Linux
uname -o -> GNU/Linux
uname -r -> 4.4.0-17763-Microsoft

This means that you need uname -r to distinguish it from native Linux.