What language should I learn to create command line scripts and GUIs?

For scripting, I'd recommend going through a good resource on bash scripting. Here's a super-quick introduction to the subject. http://www.linuxconfig.org/Bash_scripting_Tutorial

For GUI programming, I personally prefer Python with wxPython. A Python tutorial is here: http://www.diveintopython.net/ and a guide to GUI programming with wxPython is here: http://wiki.wxpython.org/AnotherTutorial#The_wxPython_Linux_Tutorial

If you want Mac-specific scripting AppleScript and Automator are useful in addition to shell scripting.

The Advanced Bash-Scripting Guide is, in contrast to its name, actually a perfectly good introduction to Bash scripting, especially if you already have a programming background.

wxPython is good for GUI work.

Python, and one of it's portable graphics GUI kits.