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。

举个例子:

  1. #!/bin/sh
  2. a=10
  3. b=20
  4. if [ $a == $b ]
  5. then
  6. echo "a is equal to b"
  7. fi
  8. if [ $a != $b ]
  9. then
  10. echo "a is not equal to b"
  11. 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:

  1. #!/bin/sh
  2. a=10
  3. b=20
  4. if [ $a == $b ]
  5. then
  6. echo "a is equal to b"
  7. else
  8. echo "a is not equal to b"
  9. fi

执行结果:

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:

  1. #!/bin/sh
  2. a=10
  3. b=20
  4. if [ $a == $b ]
  5. then
  6. echo "a is equal to b"
  7. elif [ $a -gt $b ]
  8. then
  9. echo "a is greater than b"
  10. elif [ $a -lt $b ]
  11. then
  12. echo "a is less than b"
  13. else
  14. echo "None of the condition met"
  15. 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:

  1. if test $[2*3] -eq $[1+5]; then echo 'The two numbers are equal!'; fi;


if ... else 语句也经常与 test 命令结合使用,如下所示:

  1. num1=$[2*3]
  2. num2=$[1+5]
  3. if test $[num1] -eq $[num2]
  4. then
  5. echo 'The two numbers are equal!'
  6. else
  7. echo 'The two numbers are not equal!'
  8. fi

输出:

The two numbers are equal!

test 命令用于检查某个条件是否成立,与方括号([ ])类似。

posted @ 2016-10-17 11:49  疯狂的tiger  阅读(35457)  评论(0编辑  收藏  举报