Changed test operator

Replace [[...]] with [...]
[[...]] doesn't add any value
Change condition

Directly test variable existence through variable expansion.
Replace echo with printf

For portability reason
Updated the README
Removing default failure getopts message

Placing a colon (:) in the option string as the first character
make getopts deactivate error printing. We can also access the
invalid option letter through OPTARG.
New script pathfind