unix command to extract words from a file (Mac/Linux)

gistfile1.sh

tr -sc 'A-Za-z' '\n' < filename.txt

From the tr man page (http://unixhelp.ed.ac.uk/CGI/man-cgi?tr+1):

-C      Complement the set of characters in string1, that is ``-C ab''
          includes every character except for `a' and `b'.

-c      Same as -C but complement the set of values in string1.

-s      Squeeze multiple occurrences of the characters listed in the last
         operand (either string1 or string2) in the input into a single
         instance of the character.  This occurs after all deletion and
         translation is completed.

This command takes the complement of [A-Za-z] which is all non-alphabetic characters and converts them into newlines '\n' (-c parameter). It also squeezes all successive newlines into a single newline (-s parameter).