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=
b=
 
#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=
b=
 
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  ]
then
   Statement(s) to be executed if expression  is true
elif [ expression  ]
then
   Statement(s) to be executed if expression  is true
elif [ expression  ]
then
   Statement(s) to be executed if expression  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=
b=
 
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 $[*] -eq $[+]; then echo 'The two numbers are euqal!'; fi;

#!/bin/bash
 
if test $(*) -eq $(+); 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=$[*]
num1_1=$[*]
num2=$[+]
num2_1=$[+]
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。