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