Shell變量while循環內改變無法傳遞到循環外

今天刷Leecode（192 Word frequency）時，遇到一個shell語法問題，記錄下來。首先將題目描述和代碼呈上

#!/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

 題目的意思是統計一個文件中單詞重複的次數，開始寫法如L24，while循環結束後，map依然爲空，後來才知道是使用了管道的緣故
 當啓用管道時，會生成一個subshell，while循環的代碼在subshell中執行，那麼變量map也是在subshell中被修改，
 while循環結束後，回到主shell，map沒有被修改，也就是說，兩個map不是同一個map，while中修改的map是外層map的副本
 修改代碼，將讀取文件的格式改成L36，程序運行正常