SHELL (4) —— 变量的数值计算实践

摘自:Oldboy Linux运维——SHELL编程实战

利用(())双括号进行比较及判断:

[root@yeebian ~]# echo $((3<8))
1                                   #1表示真。
[root@yeebian ~]# echo $((8<3))
0                                   #0表示假。
[root@yeebian ~]# echo $((8==8))
1
[root@yeebian ~]# if ((8>7&&5==5))
> then
> echo yes
> fi
yes

上面涉及的数字及变量必须为整数(整型),不能是小数(浮点数)或字符串。

在变量前后使用--和++特殊运算符的表达式。

[root@yeebian ~]# a=10
[root@yeebian ~]# echo $((a++))
10
[root@yeebian ~]# echo $a
11
[root@yeebian ~]# a=11
[root@yeebian ~]# echo $((a--))
11
[root@yeebian ~]# echo $a
10
[root@yeebian ~]# a=10
[root@yeebian ~]# echo $a
10
[root@yeebian ~]# echo $((--a))
9
[root@yeebian ~]# echo $a
9
[root@yeebian ~]# echo $((++a))
10
[root@yeebian ~]# echo $a
10

有关++、--运算的记忆方法:

变量a在运算符之前,输出表达式的值为a,然后a自增或自减;变量a在运算符之后,输出表达式会先自增或自减,表达式的值就是自增或自减后a的值。

各种(())运算的Shell脚本示例。

[root@yeebian ~]# vim test.sh 
testchars
#!/bin/bash
a=6
b=2
echo "a-b=$(($a-$b))"
echo "a+b=$(($a+$b))"
echo "a*b=$(($a*$b))"
echo "a/b=$(($a/$b))"
echo "a**b=$(($a**$b))"
echo "a%b=$(($a%$b))"                                                                                                                                                                     
[root@yeebian ~]# bash test.sh 
a-b=4
a+b=8
a*b=12
a/b=3
a**b=36
a%b=0

由此延伸的脚本:

[root@yeebian ~]# vim test.sh 
#!/bin/bash
a=$1
b=$2
echo "a-b=$(($a-$b))"
echo "a+b=$(($a+$b))"
echo "a*b=$(($a*$b))"
echo "a/b=$(($a/$b))"
echo "a**b=$(($a**$b))"
echo "a%b=$(($a%$b))"
[root@yeebian ~]# bash test.sh 10 5
a-b=5
a+b=15
a*b=50
a/b=2
a**b=100000
a%b=0

计算字符串长度:

[root@yeebian ~]# char="I am studying Shell."
[root@yeebian ~]# expr length "$char"
20 
[root@yeebian tmp]# echo ${#char}
20
[root@yeebian tmp]# echo $char | wc -L
20
[root@yeebian tmp]# echo $char | awk '{print length($0)}'
20

其中echo ${#char}这种方式是最快的。

expr实践

判断参数个数是否为2,若不是,则给出提示终止运行;

判断传入的参数是否为整数,若不是,则给出提示终止运行;

做运算。

[root@yeebian vurtne]# vim shell.sh
#!/bin/bash
[ $# -ne 2 ]&&{
   echo $"USAGE $0 NUM1 NUM2"
   exit 1
}
a=$1
b=$2
expr $a + $b + 110 &>/dev/null
if [ $? -ne 0 ]
then
   echo "You must input two nums."
   exit 2
fi
echo "a-b=$(($a-$b))"
echo "a+b=$(($a+$b))"
echo "a*b=$(($a*$b))"
echo "a/b=$(($a/$b))"
echo "a%b=$(($a%$b))"
echo "a**b=$(($a**$b))"
[root@yeebian vurtne]# bash shell.sh 1 q
You must input two nums.
[root@yeebian vurtne]# bash shell.sh 0
USAGE shell.sh NUM1 NUM2
[root@yeebian vurtne]# bash shell.sh 8 2
a-b=6
a+b=10
a*b=16
a/b=4
a%b=0
a**b=64

通过一条命令计算输出1+2+3+...+10的表达式,并计算结果:

[root@yeebian ~]# echo `seq -s '+' 10` = `seq -s "+" 10 | bc`
1+2+3+4+5+6+7+8+9+10 = 55
[root@yeebian ~]# echo "`seq -s '+' 10` = "$((`seq -s "+" 10`))
1+2+3+4+5+6+7+8+9+10 = 55
[root@yeebian ~]# echo `seq -s '+' 10` = `seq -s " + " 10 | xargs expr`
1+2+3+4+5+6+7+8+9+10 = 55
[root@yeebian ~]# echo `seq -s "+" 10` = $(echo $[`seq -s "+" 10`])
1+2+3+4+5+6+7+8+9+10 = 55

awk计算

[root@yeebian ~]# echo "7.7 3.8" | awk '{print ($1-$2)}'
3.9
[root@yeebian ~]# echo "358 113" | awk '{print ($1-3)/$2}'
3.14159
[root@yeebian ~]# echo "3 9" | awk '{print ($1+3)*$2}'
54

read命令实践

[root@yeebian vurtne]# vim shell.sh 
#!/bin/bash
read -p "Please input two num:" a b
[ ${#a} -le 0 ]&&{
   echo "The first num is null"
   exit 1
}
[ ${#b} -le 0 ]&&{
   echo "The second num is null"
   exit 1
}
expr $a + 1 &>/dev/null
RETVAL_A=$?
expr $b + 1 &>/dev/null
RETVAL_B=$?
if [ $RETVAL_A -ne 0 -o $RETVAL_B -ne 0 ]
then
   echo "One of the num is not num,please input again."
   exit 1
fi
echo "a-b=$(($a-$b))"
echo "a+b=$(($a+$b))"
echo "a*b=$(($a*$b))"
echo "a/b=$(($a/$b))"
echo "a%b=$(($a%$b))"
echo "a**b=$(($a**$b))"
[root@yeebian vurtne]# bash shell.sh 
Please input two num:1 qq       
One of the num is not num,please input again.
[root@yeebian vurtne]# bash shell.sh 
Please input two num:qq
The second num is null
[root@yeebian vurtne]# bash shell.sh 
Please input two num:5
The second num is null
[root@yeebian vurtne]# bash shell.sh
Please input two num:5 6 7
One of the num is not num,please input again.
[root@yeebian vurtne]# bash shell.sh
Please input two num:5 7
a-b=-2
a+b=12
a*b=35
a/b=0
a%b=5
a**b=78125

 

posted @ 2017-06-13 15:33  yancy.lu  阅读(437)  评论(0编辑  收藏  举报