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。