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