shell循环跳出的break/continue命令

break命令

当满足某个条件时，有时会希望终止循环，shell和大多数变成语言一样支持break/continue命令，其使用方法:

#!/bin/bash
for (( a=1; a<=10; a++ ))
do
    echo "value a $a"
    for((b=1; b<=10; b++ ))
    do
        echo "  value b $b"
        if [ $b -eq 4 ]; then
            echo "  break"
            break 2
        fi
    done
done

# break
# value 1
# value 2
# value 3
# value 4

值得注意的是break默认是跳出当前层的循环，而与其他大多数语言不同的是shell的break命令还支持break n参数，n表示跳出几层循环，默认为1。

#!/bin/bash
for (( a=1; a<=10; a++ ))
do
    echo "value a $a"
    for((b=1; b<=10; b++ ))
    do
        echo "  value b $b"
        if [ $b -eq 4 ]; then
            echo "  break"
            break 2
        fi
    done
done

# value a 1
#   value b 1
#   value b 2
#   value b 3
#   value b 4
#   break

如上，break会直接结束循环。在while和until循环中同样支持break命令。

continue命令

continue命令可以提前中止某次循环中的命令，但并不会完全终止整个循环。

#!/bin/bash
for (( v=1; v<=10; v++ ))
do
    if [ $v -gt 5 ] && [ $v -lt 8 ]; then
        continue
    fi
    echo "Iteration: $v"
done

# Iteration: 1
# Iteration: 2
# Iteration: 3
# Iteration: 4
# Iteration: 5
# Iteration: 8
# Iteration: 9
# Iteration: 10

也可以在 while 和 until 循环中使用 continue 命令，但要特别小心。记住，当shell执行continue 命令时，它会跳过剩余的命令。

和 break 命令一样， continue 命令也允许通过命令行参数指定要继续执行哪一级循环continue n。

#!/bin/bash
for (( a = 1; a <= 5; a++ ))
do
    echo "Iteration $a:"
    for (( b = 1; b < 3; b++ ))
    do
        if [ $a -gt 2 ] && [ $a -lt 4 ]; then
            continue 2
        fi
    var3=$[ $a * $b ]
    echo "    The result of of $a * $b is $var3"
    done
done

# Iteration 1:
#     The result of of 1 * 1 is 1
#     The result of of 1 * 2 is 2
# Iteration 2:
#     The result of of 2 * 1 is 2
#     The result of of 2 * 2 is 4
# Iteration 3:
# Iteration 4:
#     The result of of 4 * 1 is 4
#     The result of of 4 * 2 is 8
# Iteration 5:
#     The result of of 5 * 1 is 5
#     The result of of 5 * 2 is 10

trick

在上面的命令中使用重定向，可以将循环中命令的结果保存到文件中。

#!/bin/bash
for (( a = 1; a <= 5; a++ ))
do
    echo "Iteration $a:"
    for (( b = 1; b < 3; b++ ))
    do
        if [ $a -gt 2 ] && [ $a -lt 4 ]; then
            continue 2
        fi
    var3=$[ $a * $b ]
    echo "    The result of of $a * $b is $var3"
    done
done > out.txt

还可以使用管道命令将循环的结果传递给别的命令，

#!/bin/bash

for (( a = 1; a <= 5; a++ ))
do
    echo "Iteration $a:"
    for (( b = 1; b < 3; b++ ))
    do
        if [ $a -gt 2 ] && [ $a -lt 4 ]; then
            continue 2
        fi
    var3=$[ $a * $b ]
    echo "    The result of of $a * $b is $var3"
    done
done | sort

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