Thanks to @bkueang 's review comment I switched to an explicit check for
an empty sting instead of a condition that could be theoretically true
in other cases and is less readable.

Type "man test" on your terminal to read up what -n stands for.

The shell script which checks the style relies on greping for the
keyword "Formatted" in the output of astyle. But the program has
localization support and will output in other languages e.g. german.
This leads to all style checks always succeeding. I only tested this
on Windows in Cygwin but I can imagine the problem also exists in
non-english Ubuntu installations.

Solution is the parameter --formatted of astyle which only produces
any output if there was something to fix. This allows for a display
language independent condition for an empty string inside the shell
script.