shell基础--变量的数值计算

Posted 2020-09-09

变量的数值计算

1.$((表达式))

(1).实验1

[[email protected]~_~ day4]# cat test.sh

#!/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))"

[[email protected]~_~ day4]# sh test.sh

a-b=4

a+b=8

a*b=12

a/b=3

a%b=0

a**b=36

 

(2).实验2

[[email protected]~_~ day4]# cat test2.sh

#!/bin/bash

echo $(($1$2$3))

[[email protected]~_~ day4]# sh test2.sh 1 + 2

3

注意：”1 + 2”有空格，如无则只传给$1

2. $[表达式]

[[email protected]~_~ day4]# echo $[3+4]

7

3.let赋值表达式

等同与(()),但后者效率高

[[email protected]~_~ day4]# i=10

[[email protected]~_~ day4]# let i=i+1

[[email protected]~_~ day4]# echo $i

11

4.expr表达式

注意：运算符及计算的数值左右均有空格

(1).四则运算

[[email protected]~_~ day4]# cat expr.sh

#!/bin/bash

a=$1

b=$2

echo "a*b=`expr $a + $b`"

echo "a-b=`expr $a \* $b`"

[[email protected]~_~ day4]# sh expr.sh 2 3

a*b=5

a-b=6

(2).判断文件拓展名

运用：ssh-copy-id文件中;(vim `which ssh-copy-id`)

用法：

[[email protected]~_~ ~]# cat exprfile.sh

#!/bin/bash

if expr "$1" : ".*\.pub" ;then

   echo "is .pub file"

else

   echo "is not *.pub file"

fi

[[email protected]~_~ ~]# sh exprfile.sh test.pub

8

is .pub file

[[email protected]~_~ ~]# sh exprfile.sh test.pu

0

Is not *.pub file

(3).判断一个数是否为整数

[[email protected]~_~ day4]# cat isInteger.sh

#!/bin/bash

expr $1 + 1 &>/dev/null

if [ $? -eq 0 ]

   then

       echo "Is Integer"

else

         echo "Is not a Integer"

fi

[[email protected]~_~ day4]# sh isInteger.sh 3.9

Is not a Integer

[[email protected]~_~ day4]# sh isInteger.sh 3

Is Integer

(4).计算字符串长度

[[email protected]~_~ day4]# echo `expr length "Hello"`

5

还有其他运用，查看man expr

5.bc命令

一个计算器，用yum安装;直接bc进入计算器; 支持浮点数计算.

(1).交互环境

[[email protected]~_~ day4]# bc

bc 1.06.95

Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.

This is free software with ABSOLUTELY NO WARRANTY.

For details type `warranty‘.

2*3

6

3+7+5

15

3.4+5.6

9.0

(2).加管道符

[[email protected]~_~ day4]# echo "3+5"|bc

8

(3).通过scale指定计算精度

[[email protected]~_~ day4]# var=3.14

[[email protected]~_~ day4]# var=`echo "scale=2;$var*3"|bc`

[[email protected]~_~ day4]# echo $var

9.42

6.awk命令进行计算

支持浮点运算，内置有log、sqr、cos、sin等等函数

[[email protected]~_~ day4]# var=`echo "$var"|awk ‘{printf("%g",sin($1))}‘`

[[email protected]~_~ day4]# echo $var

0.841471

[[email protected]~_~ day4]# var=`echo "$var 2"|awk ‘{printf("%g",cos($1/$2))}‘`

[[email protected]~_~ day4]# echo $var

0.97922