expr判断整数是相加的值,返回命令的返回值$? 是0,但是少数情况是1,例如1 + -1 ,$? 的结果是1 ,判断要大于1最准确

[[email protected] ~]# expr 1 + 1
2
[[email protected] ~]# echo $?
0
[[email protected] ~]# echo 1 - 5
1 - 5
[[email protected] ~]# expr 1 - 5
-4
[[email protected] ~]# echo $?
0
[[email protected] ~]# expr 1 + -1
0
[[email protected] ~]# echo $?
1
[[email protected] ~]# expr 1 + f
expr: non-numeric argument
[[email protected] ~]# echo $?
2