Changed scripts to bash (closes #29) AND removed duplicate code (closes #28)

[afonsofrancof]

This PR closes #29. As stated by Meliketoaste /bin/sh is linked to the default system shell, and can be any shell, not just POSIX ones.
Check the comment bellow that explains the other changes

[afonsofrancof]

I added more code on the main branch, which was automatically added to this PR.
I wanted to do it on another branch so that I could create a new PR. (Will pay more attention to this in the future)

The code I added closes #28 . It makes all code depend on common-script.sh , by having the repetitive code there.
checkEnv has been split into multiple functions which are then called by the other files when needed.

[ChrisTitusTech]

Sadly you can't add or source other bash files from inside the linutil. We are working on a common library that gets passed to the script. I also want to stay away from using bash right now and might do some error checking while specifying sh or checking POSIX compliance before launch.