变量声明declare，简单运算符运算，变量测试与内容替换

declare -/+ 选项 变量名

- 设类型

+ 取消类型

-i 设为整型

-x 设为环境变量

-p 显示类型属性（property）

[root@localhost ~]# a=
[root@localhost ~]# declare -p a
declare -- a=""
[root@localhost ~]# export a
[root@localhost ~]# declare -p a
declare -x a=""
[root@localhost ~]# declare -i a
[root@localhost ~]# declare -p a
declare -ix a=""
[root@localhost ~]# b=
[root@localhost ~]# c=$a+$b
[root@localhost ~]# echo $c
+
[root@localhost ~]# declare -i d=$a+$b
[root@localhost ~]# echo $d

[root@localhost ~]# e=$(expr $a + $b)
[root@localhost ~]# echo $e

[root@localhost ~]# f=$(($a+$b))
[root@localhost ~]# echo $f

[root@localhost ~]# g=$(expr $a+$b)
[root@localhost ~]# echo $g
+
[root@localhost ~]# h=$[$a+$b]
[root@localhost ~]# echo $h

---

变量测试与内容替换

[root@localhost ~]# x=${y-new}
[root@localhost ~]# echo $x
new
[root@localhost ~]# y=""
[root@localhost ~]# x=${y-new}
[root@localhost ~]# echo $x   

[root@localhost ~]# y=
[root@localhost ~]# x=${y-new}
[root@localhost ~]# echo $x