The expr command

The expr command is used to evaluate an expression and output the corresponding value. It could be used for example to use the terminal as a calculator:

    $ expr 80 - 25
    55
    $ expr 80 / 25
    3
 
Note that there is a space between the different arguments, otherwise the expression might not be evaluated properly.
It can be used as well for string operations and boolean operations

    $ expr length "abc"
    3
    $ expr 1 "&" 0
    0

Working with multiple files in vim

There is a couple of useful commands that can be used to work with multiple files as tabs. I've mentioned them in a stackoverflow answer:
http://stackoverflow.com/a/14395583/612920

• Open a file in another tab
  
  :tabe filepath
 

• save the current session in order to be able to open the same set of files again
  
  :mksession session_name.vim
 

•  navigate between different tabs
  
  gt: next tab
  gT: previous tab
 

• close all tabs at once
  
  :qa
 

• open a saved session
  
  vim -S session_name.vim