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 map
declare -A map=()

# iterator lines in file
#cat words.txt | while read line
while read line
do
    for word in $line
    do  
        echo $word
        if [ -z ${map[$word]} ];then
            map[$word]=1
        else
            let map[$word]++
        fi  
    done
done < words.txt

for 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