Shell变量while循环内改变无法传递到循环外

转自: https://blog.csdn.net/shawhe/article/details/65631543

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