A bash debugger - http://bashdb.sourceforge.net/

Really really good tips - http://fahdshariff.blogspot.com.au/2013/10/shell-scripting-best-practices.html

Style guide - http://www.kfirlavi.com/blog/2012/11/14/defensive-bash-programming

Script checker - https://www.shellcheck.net/

How to write a shell - https://indradhanush.github.io/blog/writing-a-unix-shell-part-1 https://indradhanush.github.io/blog/writing-a-unix-shell-part-2