Extended Doug McIlroy's program from book "Classic Shell Scripting" to process a text file, and output a list of the n most-frequent words, with counts of their frequency of occurrence, sorted by descending count.

count-word-freq.sh

#! /bin/sh -
# Read a text stream on standard input, and output a list of
# the n (default: 25) most frequently occurring words and
# their frequency counts, in order of descending counts, on
# standard output.
#
# Usage:
#       wf [n]

tr -cs A-Za-z\' '\n' |                      # Replace nonletters with newlines
    tr A-Z a-z |                            # Map uppercase to lowercase
        sort |                              # Sort the words in ascending order
            uniq -c |                       # Eliminate duplicates, showing their counts
                sort -k1,1nr -k2 |          # Sort by descending count, and then by ascending word
                    sed ${1:-25}q           # Print only the first n (default: 25) lines