Donald Knuth and Doug McIlroy both wrote a program that would read a textfile, 
determine the n most frequently used words, and print out a sorted list of those 
words along with their frequencies. Knuth's was 10 pages of (very tightly/well 
written/literately documented) Pascal. McIlroy's was tweetable:

   tr -cs A-Za-z '\n' |
   tr A-Z a-z |
   sort |
   uniq -c |
   sort -rn |
   sed ${1}q

If you are not a UNIX adept, you may need a little explanation, but not much, to 
understand this pipeline of processes. The plan is easy:

1. Make one-word lines by transliterating the complement (-c) of the alphabet 
into newlines (note the quoted newline), and squeezing out (-s) multiple 
newlines.

2. Transliterate upper case to lower case.

3. Sort to bring identical words together.

4. Replace each run of duplicate words with a single representative and include 
a count (-c).

5. Sort in reverse (-r) numeric (-n) order.

6. Pass through a stream editor; quit (q) after printing the number of lines 
designated by the script's first parameter (${1}).