in order to avoid errors in linux machines.
This will avoid `[[` conditional errors not present in all OSs.
From this gitter comment:
> Hi (Sorry to disturb, but I don't have the time to clone, fix and send back a pull request, so I broadcast it here for anyone interested). In besu-quickstart, consider changing "#/bin/bash". This will fix many weird errors in Debian 9 (and possible Alpine and other light-systems). They all default to simplest shells that will fail with any script containing "[[" conditionals.