I use Emacs for shell scripts. I know I read somewhere there's some mode to make it easier (code completion, autocomplete, bracket matching, syntax highligting, check syntax) to write Shell scripts on Emacs but haven't been able to find it. In short, I would like to turn Emacs into a Bash IDE.
Also, is there any other tool/IDE to make scripting easier on the GNU/Linux Shell?
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.