you can
create a script from any editor, so you could start an emacs from a csh or run an instance of SciTE on a remote computer and still write a bash script. 'bash' is the most commonly used shell, but 'sh' is the only one you can guarantee to exist - bash is fully compatible with any vanilla bourne sh.
The first line of a shell script, or any script (including python, perl, ruby...) tells the system how to run it if it begins with '#!' this means that
tells the system that './script.sh' (if that's the script...) is equivalent to "/bin/sh ./script.sh" /