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Shell if else语句

2024-07-27 01:37:54   218人阅读

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 truefi

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/sha=10b=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‘fiif [ $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 trueelse   Statement(s) to be executed if expression is not truefi

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/sha=10b=20if [ $a == $b ] ;then   echo "a is equal to b"else   echo "a is not equal to b"fiif [ $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"fiEOF

执行结果：

a is not equal to ba 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 trueelif [ expression 2 ]then   Statement(s) to be executed if expression 2 is trueelif [ expression 3 ]then   Statement(s) to be executed if expression 3 is trueelse   Statement(s) to be executed if no expression is truefi

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/sha=10b=20if [ $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/bashif test $[2*3] -eq $[1+5]; then echo ‘The two numbers are euqal!‘; fi;
#!/bin/bashif 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/bashnum1=$[2*3]num1_1=$[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!‘fiif 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。

Shell if else语句


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