流程控制 -- if判断
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:
#!/bin/sh
a=10
b=20
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
合并
[ 条件 ] && 命令
[ 条件 ] || 命令
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=10b=20
if [ $a == $b ]
then
echo "a is equal to b"
else
echo "a is not equal to b"
fi
执行结果:
a is not equal to b
合并
if [ 条件 ];then 命令1;else 命令2;fi
[ 条件 ] && (命令1;命令2;命令3;命令4) || (命令a;命令b;命令c)
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:
if test $[2*3] -eq $[1+5]; then echo 'The two numbers are equal!'; fi;
if ... else 语句也经常与 test 命令结合使用,如下所示:
num1=$[2*3]
num2=$[1+5]
if test $[num1] -eq $[num2]
then
echo 'The two numbers are equal!'
else
echo 'The two numbers are not equal!'
fi
输出:
The two numbers are equal!