shell--[重要]字符串是否相等, 字符串是否为空

      str1=
      
        ""
      
      
        

str2
      
      =
      
        

str3
      
      =
      
        "
      
      
        hello
      
      
        "
      
      
        

str4
      
      =
      
        "
      
      
        world
      
      
        "
      
      

      # true


      
        if
      
       [[ 
      
        "
      
      
        ${str1}
      
      
        "
      
       == 
      
        "
      
      
        ${str2}
      
      
        "
      
       ]]; 
      
        then
      
      
        echo
      
      
        "
      
      
        12 true
      
      
        "
      
      
        else
      
      
        echo
      
      
        "
      
      
        12 false
      
      
        "
      
      
        fi
      
      

      # false


      
        if
      
       [[ 
      
        "
      
      
        ${str1}
      
      
        "
      
       == 
      
        "
      
      
        ${str3}
      
      
        "
      
       ]]; 
      
        then
      
      
        echo
      
      
        "
      
      
        13 true
      
      
        "
      
      
        else
      
      
        echo
      
      
        "
      
      
        13 false
      
      
        "
      
      
        fi
      
      

      # false


      
        if
      
       [[ 
      
        "
      
      
        ${str3}
      
      
        "
      
       == 
      
        "
      
      
        ${str4}
      
      
        "
      
       ]]; 
      
        then
      
      
        echo
      
      
        "
      
      
        34 true
      
      
        "
      
      
        else
      
      
        echo
      
      
        "
      
      
        34 false
      
      
        "
      
      
        fi
      
      
        

        # false

# str5未声明


      
      
        if
      
       [[ 
      
        "
      
      
        ${str3}
      
      
        "
      
       == 
      
        "
      
      
        ${str5}
      
      
        "
      
       ]]; 
      
        then
      
      
        echo
      
      
        "
      
      
        35 true
      
      
        "
      
      
        else
      
      
        echo
      
      
        "
      
      
        35 false
      
      
        "
      
      
        fi
      
    

      str1=
      
        ""
      
      
        

str2
      
      =
      
        

str3
      
      =
      
        "
      
      
        hello
      
      
        "
      
      
        

str4
      
      =
      
        "
      
      
        world
      
      
        "
      
      
        



# 
      
      
        true
      
      
        if
      
       [ 
      
        "
      
      
        ${str1}
      
      
        "
      
       = 
      
        "
      
      
        ${str2}
      
      
        "
      
       ]; 
      
        then
      
      
        echo
      
      
        "
      
      
        12 true
      
      
        "
      
      
        else
      
      
        echo
      
      
        "
      
      
        12 false
      
      
        "
      
      
        fi
      
      
        



# 
      
      
        false
      
      
        if
      
       [ 
      
        "
      
      
        ${str1}
      
      
        "
      
       = 
      
        "
      
      
        ${str3}
      
      
        "
      
       ]; 
      
        then
      
      
        echo
      
      
        "
      
      
        13 true
      
      
        "
      
      
        else
      
      
        echo
      
      
        "
      
      
        13 false
      
      
        "
      
      
        fi
      
      
        



# 
      
      
        false
      
      
        if
      
       [ 
      
        "
      
      
        ${str3}
      
      
        "
      
       = 
      
        "
      
      
        ${str4}
      
      
        "
      
       ]; 
      
        then
      
      
        echo
      
      
        "
      
      
        34 true
      
      
        "
      
      
        else
      
      
        echo
      
      
        "
      
      
        34 false
      
      
        "
      
      
        fi
      
      
        



# 
      
      
        false
      
      
        

# str5未声明


      
      
        if
      
       [ 
      
        "
      
      
        ${str3}
      
      
        "
      
       = 
      
        "
      
      
        ${str5}
      
      
        "
      
       ]; 
      
        then
      
      
        echo
      
      
        "
      
      
        35 true
      
      
        "
      
      
        else
      
      
        echo
      
      
        "
      
      
        35 false
      
      
        "
      
      
        fi
      
    

      
        if
      
       [ 
      
        "
      
      
        $str
      
      
        "
      
       =  
      
        ""
      
      
         ] 


      
      
        if
      
       [ x
      
        "
      
      
        $str
      
      
        "
      
       =
      
         x ]


      
      
        if
      
       [ -z 
      
        "
      
      
        $str
      
      
        "
      
       ] （-
      
        n 为非空）

注意：都要代双引号，否则有些命令会报错



建议用:
      
    

      if [ "$str" =  "" ] 
      

      

      或
    

      if [[ "${str1}" == "" ]]
    

      str1=
      
        ""
      
      
        

str2
      
      =
      
        

str3
      
      =
      
        "
      
      
        hello
      
      
        "
      
      
        

str4
      
      =
      
        "
      
      
        world
      
      
        "
      
      
        



# 以下都输出empty

# $str5不存在, 也是empty




      
      
        if
      
       [ 
      
        "
      
      
        $str1
      
      
        "
      
       = 
      
        ""
      
       ]; 
      
        then
      
      
        echo
      
      
        "
      
      
        empty
      
      
        "
      
      
        fi
      
      
        if
      
       [ 
      
        "
      
      
        $str2
      
      
        "
      
       = 
      
        ""
      
       ]; 
      
        then
      
      
        echo
      
      
        "
      
      
        empty
      
      
        "
      
      
        fi
      
      
        if
      
       [ 
      
        "
      
      
        $str5
      
      
        "
      
       == 
      
        ""
      
       ]; 
      
        then
      
      
        echo
      
      
        "
      
      
        empty
      
      
        "
      
      
        fi
      
      
        if
      
       [ -z 
      
        "
      
      
        $str5
      
      
        "
      
       ]; 
      
        then
      
      
        echo
      
      
        "
      
      
        empty
      
      
        "
      
      
        fi
      
      
        if
      
       [[ $str5 == 
      
        ""
      
       ]]; 
      
        then
      
      
        echo
      
      
        "
      
      
        empty
      
      
        "
      
      
        fi
      
      
        if
      
       [[ 
      
        "
      
      
        $str5
      
      
        "
      
       == 
      
        ""
      
       ]]; 
      
        then
      
      
        echo
      
      
        "
      
      
        empty
      
      
        "
      
      
        fi
      
      
        if
      
       [[ 
      
        "
      
      
        $str1
      
      
        "
      
       == 
      
        ""
      
       ]]; 
      
        then
      
      
        echo
      
      
        "
      
      
        empty
      
      
        "
      
      
        fi