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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