Consider using shellcheck to lint .sh files

[zcorpan]

See https://github.com/koalaman/shellcheck

This is installed on Travis CI already, the following command runs the check on all .sh files and gives an exit code indicating success or failure:

for file in $(find path/to/scripts -type f -name "*.sh"); do shellcheck --format=gcc $file; done;

https://github.com/koalaman/shellcheck/wiki/Recursiveness