Shell脚本之：if-else

Shell 有三种 if ... else 语句：

1.if ... fi 语句；

2.if ... else ... fi 语句；

3.if ... elif ... else ... fi 语句。

if ... fi 语句

if ... else 语句的语法：

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

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

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

if ... elif ... else ... fi 语句

if ... elif ... else ... fi 语句的语法

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

配合test的if-else语句

if ... else 语句也经常与 test 命令结合使用，如下所示：

num1=1
num2=2
if test ${num1} -eq ${num2}
then
    echo 'The two numbers are equal!'
else
    echo 'The two numbers are not equal!'
fi