Shell跳出循环

在循环过程中，有时候需要在未达到循环结束条件时强制跳出循环，像大多数编程语言一样，Shell也使用 break 和 continue 来跳出循环

break命令

break命令允许跳出所有循环（终止执行后面的所有循环）。

示例1：
脚本进入死循环直至用户输入数字大于5。要跳出这个循环，返回到shell提示符下，就要使用break命令。

#!/bin/bash

while :
do
  echo -n "Input a number between 1 to 5:"
  read num
  case $num in
   1|2|3|4|5) echo "Your number is $num!"
   ;;
   *) echo "You do not select a number between 1 to 5,game is over!"
    break
   ;;
  esac
done

脚本运行结果示例：

Input a number between 1 to 5:1
Your number is 1!
Input a number between 1 to 5:10
You do not select a number between 1 to 5,game is over!

在嵌套循环中，break 命令后面还可以跟一个整数，表示跳出第几层循环。

break n

示例2：
如果 var1 等于 2，并且 var2 等于 0，就跳出循环。

#!/bin/bash

for var1 in 1 2 3
do
  for var2 in  0 5
  do
    if [ $var1 -eq 2 -a $var2 -eq 0 ]
    then
      break 2
    else
      echo "$var1 $var2"
    fi
  done
done

如上，break 2 表示直接跳出外层循环。脚本运行结果如下：

1 0
1 5

continue命令

continue命令与break命令类似，只有一点差别，它不会跳出所有循环，仅仅跳出当前循环。
针对上述示例进行改造。

示例1：

#!/bin/bash

while :
do
  echo -n "Input a number between 1 to 5:"
  read num
  case $num in
   1|2|3|4|5) echo "Your number is $num!"
   ;;
   *) echo "You do not select a number between 1 to 5,game is over!"
    continue
    echo "Game is over!"
   ;;
  esac
done

运行代码发现，当输入大于5的数字时，该例中的循环不会结束，语句

echo "Game is over!"

永远不会被执行。
脚本运行结果如下：

Input a number between 1 to 5:1
Your number is 1!
Input a number between 1 to 5:10
You do not select a number between 1 to 5,game is over!
Input a number between 1 to 5:3
Your number is 3!
Input a number between 1 to 5:

continue 后面也可以跟一个数字，表示跳出第几层循环。

示例2：

#!/bin/bash

NUMS="1 2 3 4 5 6 7"
for NUM in $NUMS
do
  Q=`expr $NUM % 2`
  if [ $Q -eq 0 ]
  then
    echo "Number is an even number!"
    continue
  fi
  echo "Found odd number"
done

脚本运行结果如下：

Found odd number
Number is an even number!
Found odd number
Number is an even number!
Found odd number
Number is an even number!
Found odd number