1.(())双圆结构扩展介绍
使用linux下(())双圆结构扩展并计算一个算术表达式的值时，如果表达式的结果为0，那么返回的退出状态码为1，或者是“假/false”；如果表达式的结果为一个非零值，那么返回的退出状态码为0，或者是“真/true”。这种情况和先前介绍的test命令和[]结构的行为正好相反。
2.(())双圆结构扩展样例
样例：
[root@elasticsearch ~]# vim double_circle.sh
[root@elasticsearch ~]#
[root@elasticsearch ~]#
[root@elasticsearch ~]# cat double_circle.sh
#!/bin/bash
#(())结构可以用来计算并测试算术表达式的结果，退出状态将会与[]结构完全相反!
(( 0 ))
echo "Exit status of \"(( 0 ))\" is $?."
(( 1 ))
echo "Exit status of \"(( 1 ))\" is $?."
(( 5 > 4 ))
echo "Exit status of \"(( 5 > 4 ))\" is $?."
(( 5 > 9 ))
echo "Exit status of \"(( 5 > 9 ))\" is $?."
(( 5 - 5 ))
echo "Exit status of \"(( 5 - 5 ))\" is $?."
(( 5 / 4 ))
echo "Exit status of \"(( 5 / 4 ))\" is $?."
(( 1 / 2 )) #除法的计算结果 < 1，截取之后的结果为0，$?的输出就为1.
echo "Exit status of \"(( 1 / 2 ))\" is $?."
(( 1 / 0 )) 2>/dev/null #除数为0，非法计算.
echo "Exit status of \"(( 1 / 0 ))\" is $?."
exit 0
[root@elasticsearch ~]# sh double_circle.sh
Exit status of "(( 0 ))" is 1.
Exit status of "(( 1 ))" is 0.
Exit status of "(( 5 > 4 ))" is 0.
Exit status of "(( 5 > 9 ))" is 1.
Exit status of "(( 5 - 5 ))" is 1.
Exit status of "(( 5 / 4 ))" is 0.
Exit status of "(( 1 / 2 ))" is 1.
Exit status of "(( 1 / 0 ))" is 1.
[root@elasticsearch ~]#
