IDE / Emacs mode for Shell scripting in Bash/Sh, etc
Well, there's a built-in shell mode that does syntax highlighting, if it's not already turned on, M-x shell-script-mode will do it for you.
Regarding a tool to make scripting easier. There's a bash shell debugger, so if you program in bash, that would help quite a bit.
Of course there's eshell, a shell written in Emacs lisp, so you can get all the syntax highlighting you'd expect from lisp, the auto-completion, and the emacs lisp debugger.
I don't think there is any mode for code completion for shell-scripts, at least not in Emacs. (I've actually never felt the need.) However, there is support in bash for completion when running interactively (the package is called "bash-completion", or something similar). This provides tab-completion for many shell-commands.