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:
# the4
# is3
# sunny2
# day1
# define a map
declare -A map=()
# iterator lines in file
L24 #cat words.txt |whileread line
whileread line
do
forword in $line
do
echo $word
if[ -z ${map[$word]} ];then
map[$word]=1
else
let map[$word]++
fi
done
L36done < words.txt
forkey 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
