shell 算数运算

expr:

[root@xiaoyuan ~]# expr 2+4  # 需要空格

2+4
[root@xiaoyuan ~]# expr 2 + 4
6
[root@xiaoyuan ~]# expr 2 * 4  #  *需要转义，不然报错
expr: syntax error
[root@xiaoyuan ~]# expr 2 \* 4  
8

[root@xiaoyuan ~]# sum=`expr 3 + 4`
[root@xiaoyuan ~]# echo $sum
7
[root@xiaoyuan ~]#

bc:

[root@xiaoyuan ~]# echo 3*5 | bc
15
[root@xiaoyuan ~]# echo 3/5 | bc
0
[root@xiaoyuan ~]# echo 3/5 | bc -l
.60000000000000000000

$(())

[root@xiaoyuan ~]# sum=$((3+4))
[root@xiaoyuan ~]# echo $sum
7
[root@xiaoyuan ~]# sum=$((3*4))
[root@xiaoyuan ~]# echo $sum
12

[root@xiaoyuan ~]# sum=$((3/4))
[root@xiaoyuan ~]# echo $sum
0
[root@xiaoyuan ~]# sum=$((4/3))
[root@xiaoyuan ~]# echo $sum
1
$[ ]

[root@xiaoyuan ~]# echo $sum
7
[root@xiaoyuan ~]# sum=$[3*4]
[root@xiaoyuan ~]# echo $sum
12
[root@xiaoyuan ~]# sum=$[3+4]
[root@xiaoyuan ~]# echo $sum
7
[root@xiaoyuan ~]# sum=$[3/4]
[root@xiaoyuan ~]# echo $sum
0
[root@xiaoyuan ~]# sum=$[3-4]
[root@xiaoyuan ~]# echo $sum
-1