title: Using first 4 characters of file name to create unique list
body: <p>I primarily use Linux Mint 17.1 and I like to use command line to get things done.
  At the moment, I am working on organising a whole lot of family pictures and making them easy to view via a browser.
  I have a directory with lots of images.
  As I was filling the directory I made sure to keep the first four letters of the filename unique to a specific topic, eg, car_, hse_, chl_ etc
  The rest of the filename keeps it unique.
  There are some 120 different prefixes and I would like to create a list of the unique prefix.
  I have tried 'ls i | uniq -d -w 4' and it works but it gives me the first filename of each prefix.
  I just want the prefixes.
  Fyi, I will use this list to generate an HTML page as a kind of catalogue.
  Summary,
  Convert car_001,car_002,car_003,dog_001,dog_002
  to
  car_,dog_</p>