$((运算式))或$[运算式]

[server@hadoop102 scripts]$ expr 1 + 2
3
[server@hadoop102 scripts]$ expr 1 - 2
-1
[server@hadoop102 scripts]$ expr 1 \* 2
2
[server@hadoop102 scripts]$ expr $[1*2]
2
[server@hadoop102 scripts]$ expr $((1*2))
2
[server@hadoop102 scripts]$ expr 6 / 2
3
[server@hadoop102 scripts]$ a=$[6*(5-3)]
[server@hadoop102 scripts]$ echo $a
12

[server@hadoop102 scripts]$ vim add.sh
[server@hadoop102 scripts]$ cat add.sh 
#!/bin/bash
sum=$[$1 + $2]
echo sum=$sum
[server@hadoop102 scripts]$ chmod +x add.sh 
[server@hadoop102 scripts]$ ./add.sh 7 8
sum=15