Linux命令与shell脚本学习之字符串比较

 1.字符串比较的测试。和数字相反，数字使用eq等进行比较，字符串使用的是运算符进行比较。

=	等于
>	大于
<	下雨
!=	不等于
-n	字符是否非0，不为0成立
-z	字符是否为0，长度0成立

2.如何进行比较，比较的是什么。

 1）等于（=）：两个字符相等则成立，比较简单（注意这里是区分大小写，“=”两边留一个空格）

    1.1 “=”两边不留空格出错

 

val1=test val2=test val3=Test if [ $val1 = $val2 ] then    echo "$val1 = $val2" fi #未留空格 if [ $val1=$val3 ]   then    echo "$val1 = $val3" else    echo "$val1 != $val3" fi	val1=test val2=test val3=Test if [ $val1 = $val2 ] then    echo "$val1 = $val2" fi if [ $val1 = $val3 ] then    echo "$val1 = $val3" else    echo "$val1 != $val3" fi
[root@hadoop11 test]# ./test7.sh  test = test #不利空格的结果出错 test = Test	[root@hadoop11 test]# ./test7.sh  test = test test != Test

2）不等于号（!=）:与等于类似，不再赘述。

3）大于（<）小于（>）:比较的时字符的顺序，不是字符串的长短

    注意：1.使用过程中需要加转义斜线"\"。   2.排序过程小写大于大写字符，与sort命令排序相反。

	val1=abc val2=ac val3=Ac #不加转义字符，条件总是成立，执行then，并产生文件val2. if [ $val1 \> $val2 ] then    echo "$val1 is greater than $val2" else    echo "$val1 is less than $val2" fi if [ $val2 \> $val3 ] then    echo "$val2 is greater than $val3" else    echo "$val2 is less than $val3" fi
	[root@hadoop11 test]# ./test9.sh  #abc比ac长度长，但是比较的时字符顺序，第一个字符a相同，比较第二个b和c，c大于b，所以ac大于abc abc is less than ac #大写字符小于小写字符 ac is greater than Ac

3）为零（-z）和非零（-n）：注意-n的使用

    注意：当-n使用过程中，对于空字符判断，需要加双引号“”
 

 

#互换var之间值，结果和预期一样 var1='testing' var2=''  if [ -z $var2 ] then    echo "-z :the var2 = '$var2' is empty " else    echo "-z : the var2 = '$var2' is not empty" fi if [ -z $var1 ] then   echo "-z :the var1 = '$var1' is empty" else   echo "-z :the var1 = '$var1' is not empty" fi	var1=''        var2='testing' if [ -z $var2 ] then    echo "-z :the var2 = '$var2' is empty " else    echo "-z : the var2 = '$var2' is not empty" fi if [ -z $var1 ] then   echo "-z :the var1 = '$var1' is empty" else   echo "-z :the var1 = '$var1' is not empty" fi
[root@hadoop11 test]# ./test10.sh  -z :the var2 = '' is empty  -z :the var1 = 'testing' is not empty	[root@hadoop11 test]# ./test10.sh  -z : the var2 = 'testing' is not empty -z :the val1 = '' is empty

     2.-n,对于字符的判断和预期可能不同一样

        

未加双引号	加双引号
var1='testing' var2=''        if [ -n $var1 ]   then  echo "-n : the val1 = '$var1' is not empty" else  echo "-n : the val1 = '$var1' is empty" fi if [ -n $var2 ]   then    echo "-n :the val2 = '$var2' is not empty" else    echo "-n :the val2 = '$var2' is empty" fi	var1='testing' var2=''        if [ -n "$var1" ] then  echo "-n : the val1 = '$var1' is not empty" else  echo "-n : the val1 = '$var1' is empty" fi if [ -n "$var2" ] then    echo "-n :the val2 = '$var2' is not empty" else    echo "-n :the val2 = '$var2' is empty" fi
[root@hadoop11 test]# ./test10.sh  -n : the val1 = 'testing' is not empty #结果和预期不同 -n :the val2 = '' is not empty	[root@hadoop11 test]# ./test10.sh  -n : the val1 = 'testing' is not empty #结果符合预期 -n :the val2 = '' is empty