【Linux】结构化命令：嵌套循环

1、for-for嵌套

#!/bin/bash

for (( a = 1; a <= 5; a++ ))
do
  echo "外层循环，此时a=$a:"
  for (( b =1; b <=3; b++ ))
  do
    echo "   内层循环，此时b=$b；"
  done
done

2、while-for嵌套

#!/bin/bash

var1=7

while [ $var1 -ge 0 ]
do
  echo "外层循环，此时var1=$var1"
  for (( var2 = 1; var2 < 4; var2++ ))
  do
    var3=$[ $var1 * $var2 ]
    echo "   内层循环，var3=$var3"
  done
  var1=$[ $var1 - 1 ]
done

3、until-while嵌套

#!/bin/bash
var1=4

until [ $var1 -eq 0 ]
do
  echo "外层循环，此时var1=$var1"
  var2=1
  while [ $var2 -lt 5 ]
  do
    var3=$(echo "scale=3; $var1 / $var2" | bc)
    echo "   内层循环，var3=$var3"
    var2=$[ $var2 + 1 ]
  done
  var1=$[ $var1 - 1 ]
done

4、循环处理文件数据

在处理文件数据时，可以综合运用“嵌套循环”和“修改IFS环境变量”这两种手段。

#!/bin/bash
IFS.OLD=$IFS #将IFS的当前数据保存好
IFS=$"\n" #指定换行符作为字段分隔符
for item in $(cat /etc/passwd)
do
  echo "values in $item is:"
  IFS=: #指定冒号作为分隔符
  for value in $item
  do
    echo "   $value."
  done
done