The command-not-found package in Ubuntu provides some slick functionality where if you type a command that can't be resolved to an executable (or bash function or whatever) it will query your apt sources and find a package that contains the binary you tried to execute. So, in this case, I typed ab at the command prompt:
% ab
The program 'ab' is currently not installed. You can install it by typing:
sudo apt-get install apache2-utils
bash: ab: command not found
Another way to search for missing files, e.g. if you use zsh, want to disable command-not-found (slows things down when you misstype commandnames), or are looking for a file that is not an executable: