Word-frequency filter

摘自 Robbins A., Beebe N. - Classic Shell Scripting - 2005

Chapter 5.

Problem:

Given a text file and an integer n, you are to print the words (and their frequencies of occurrence) whose frequencies of occurrence are among the n largest in order of decreasing frequency.(找到一個文檔中出現次數最多的n哥單詞，並顯示他們的出現次數)

McIlroy’s program illustrates the power of the Unix tools approach: break a complex problem into simpler parts that you already know how to handle. To solve the
word-frequency problem, McIlroy converted the text file to a list of words, one per
line (tr does the job), mapped words to a single lettercase (tr again), sorted the list
(sort), reduced it to a list of unique words with counts (uniq), sorted that list by
descending counts (sort), and finally, printed the first several entries in the list (sed,
though head would work too).

Example 5-5. Word-frequency filter#! /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; see Chapter 3