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  • Shell if else语句

    if 语句通过关系运算符判断表达式的真假来决定执行哪个分支。Shell 有三种 if ... else 语句:

    • if ... fi 语句;
    • if ... else ... fi 语句;
    • if ... elif ... else ... fi 语句。

    1) if ... else 语句

    if ... else 语句的语法:

    if [ expression ]
    then
       Statement(s) to be executed if expression is true
    fi

    如果 expression 返回 true,then 后边的语句将会被执行;如果返回 false,不会执行任何语句。

    最后必须以 fi 来结尾闭合 if,fi 就是 if 倒过来拼写,后面也会遇见。

    注意:expression 和方括号([ ])之间必须有空格,否则会有语法错误。

    #!/bin/sh
    a=10
    b=20
    
    
    #下面两种写法都是正确的,除此之外的写法都是错误的
    if [ $a == $b ]
    then
        echo 'a is equal to b'
    fi
    
    if [ $a != $b ] ; then
        echo 'a is not equal to b'
    fi

    运行结果:

    a is not equal to b

    2) if ... else ... fi 语句

    if ... else ... fi 语句的语法:

    if [ expression ]
    then
       Statement(s) to be executed if expression is true
    else
       Statement(s) to be executed if expression is not true
    fi

    如果 expression 返回 true,那么 then 后边的语句将会被执行;否则,执行 else 后边的语句。

    举个例子:

    #!/bin/sh
    
    a=10
    b=20
    
    if [ $a == $b ] ;then
       echo "a is equal to b"
    else
       echo "a is not equal to b"
    fi
    
    if [ $a == $b ]
    then
        echo "a is equal to b"
    else
        echo "a is not equal to b"
    fi
    
    :<<EOF 下面的写法是错误的,请注意
    if [ $a == $b ] then
       echo "a is equal to b"
    else 
        echo "a is not equal to b"
    fi
    EOF

    执行结果:

    a is not equal to b
    a is not equal to b

    3) if ... elif ... fi 语句

    if ... elif ... fi 语句可以对多个条件进行判断,语法为:

    if [ expression 1 ]
    then
       Statement(s) to be executed if expression 1 is true
    elif [ expression 2 ]
    then
       Statement(s) to be executed if expression 2 is true
    elif [ expression 3 ]
    then
       Statement(s) to be executed if expression 3 is true
    else
       Statement(s) to be executed if no expression is true
    fi

    哪一个 expression 的值为 true,就执行哪个 expression 后面的语句;如果都为 false,那么不执行任何语句。

    举个例子:

    #!/bin/sh
    
    a=10
    b=20
    
    if [ $a == $b ] ; then
       echo "a is equal to b"
    elif [ $a -gt $b ] ; then
        echo "a is greater than b"
    elif [ $a -lt $b ] ; then
        echo "a is less than b"
    else
        echo "None of the condition met"
    fi

    运行结果:

    a is less than b

    if ... else 语句也可以写成一行,以命令的方式来运行,像这样:

    #!/bin/bash
    
    if test $[2*3] -eq $[1+5]; then echo 'The two numbers are euqal!'; fi;
    #!/bin/bash
    
    if test $(2*3) -eq $(1+5); then echo 'The two numbers are euqal!'; fi;

    if ... else 语句也经常与 test 命令结合使用,如下所示:

    #!/bin/bash
    
    num1=$[2*3]
    num1_1=$[2*6]
    num2=$[1+5]
    num2_1=$[1+5]
    if test $[num1] -eq $[num2] ; then
        echo "The two numbers are euqal!"
    else
        echo 'The tow numbers are not equal!'
    fi
    
    
    
    if test $num1_1 -eq $num2_1 ; then
        echo "The two numbers are euqal!"
    else
        echo 'The tow numbers are not equal!'
    fi

    输出:

    The two numbers are euqal!
    The tow numbers are not equal!

    test 命令用于检查某个条件是否成立,与方括号([ ])类似。

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  • 原文地址:https://www.cnblogs.com/ghgyj/p/4023429.html
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