Shell变量while循环内改变无法传递到循环外

今天刷Leecode（192 Word frequency）时，遇到一个shell语法问题，记录下来。首先将题目描述和代码呈上

#!/bin/bash# Write a bash script to calculate the frequency of each word in a text file words.txt.## For simplicity sake, you may assume:#  words.txt contains only lowercase characters and space ' ' characters.#  Each word must consist of lowercase characters only.#  Words are separated by one or more whitespace characters.## For example, assume that words.txt has the following content:# the day is sunny the the# the sunny is is## Your script should output the following, sorted by descending frequency:# the 4# is 3# sunny 2# day 1# define a mapdeclare -A map=()# iterator lines in file#cat words.txt | while read linewhile read linedo    for word in $line    do          echo $word        if [ -z ${map[$word]} ];then            map[$word]=1        else            let map[$word]++        fi      donedone < words.txtfor key in ${!map[@]}do    echo $key ${map[$key]}done

 题目的意思是统计一个文件中单词重复的次数，开始写法如L24，while循环结束后，map依然为空，后来才知道是使用了管道的缘故
 当启用管道时，会生成一个subshell，while循环的代码在subshell中执行，那么变量map也是在subshell中被修改，
 while循环结束后，回到主shell，map没有被修改，也就是说，两个map不是同一个map，while中修改的map是外层map的副本
 修改代码，将读取文件的格式改成L36，程序运行正常