Reference: Check File Exists or Not[https://www.cyberciti.biz/faq/unix-linux-test-existence-of-file-in-bash/] Parameter expansion [http://wiki.bash-hackers.org/syntax/pe#search_and_replace] Calculate variable, and output it to another variable [https://unix.stackexchange.com/questions/50215/calculate-variable-and-output-it-to-another-variable]