#!/bin/sh
## Output words in order of frequency (with counts) in merge of given files 

## SYS V /usr/bin/tr and BSD /usr/ucb/tr treat strings differently.
## You could use just plain tr instead of /usr/ucb/tr by uncommenting:
# PATH=/usr/ucb:/usr/bin

cat "$@" |                     ## concatenate all given input files
/usr/ucb/tr 'A-Z' 'a-z' |      ## lower case words
/usr/ucb/tr -cs 'a-z' '\012' | ## -c: complement string 1
                               ## -s: suppress repeated characters of string 2
## In Sys V,  string 2 is not automatically padded out unless use [x*] notation
## and ranges are specified as [a-z] although a-z also works.  Thus:
##      /usr/bin/tr -cs '[a-z]' '[\n*]'
## Many versions of tr now allow specifications of type
## [:alpha:] for a-zA-Z, [:upper:] for A-Z, [:lower:] for a-z etc.

sort |
uniq -c |            ## replace n copies of 'line' with:  n  line
sort -nr             ## -n : numerical sort on 1st column  -r: reverse order