I often find myself alt-tabbing to google How to do X in Linux. Is there a way to search (say, man pages) in Linux to find tools that might be of use in a particular situation? For example, earlier I needed the command to see how much space a folder was taking up. (Excuse my Windows terminology I'm new to this!) Is there any way I could have found the du -h command without leaving the terminal? 
What command(s) can I use in a terminal to find a program that does a particular thing?
command line;search;documentation