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

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=10
b=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

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!

test 命令用于检查某个条件是否成立，与方括号([ ])类似。

例子：是否有NULL值或没有NULL值

让我们详细看看test命令的语法和例子。if命令的语法如下。

my_var="nixCraft"
if [ -z "$my_var" ]
then
      echo "\$my_var is NULL"
else
      echo "\$my_var is NOT NULL"
fi

或

my_var=""
if test -z "$my_var" 
then
      echo "\$my_var is NULL"
else
      echo "\$my_var is NOT NULL"
fi
Another option to check if bash shell variable is NULL or not
[ -z "$my_var" ] && echo "NULL"
[ -z "$my_var" ] && echo "NULL" || echo "Not NULL"

或

[[ -z "$my_var" ]] && echo "NULL"
[[ -z "$my_var" ]] && echo "NULL" || echo "Not NULL"

或

## Check if $my_var is NULL using ! i.e. check if expr is false ##
[ ! -z "$my_var" ] || echo "NULL"
[ ! -z "$my_var" ] && echo "Not NULL" || echo "NULL"

[[ ! -z "$my_var" ]] || echo "NULL"
[[ ! -z "$my_var" ]] && echo "Not NULL" || echo "NULL"