declare:声明变量的类型

declare [-aixr] var

参数：
-a：array
-i：int
-x：同export，声明为环境变量
+x：从环境变量声明为变量
-r：只读，无法重设

#变量的默认类型是 string
zmc@SUSE11:~> sum=1+2+3 
zmc@SUSE11:~> echo $sum
1+2+3
#声明为i
zmc@SUSE11:~> declare -i sum
zmc@SUSE11:~> sum=1+2+3
zmc@SUSE11:~> echo $sum
6
#设定为环境变量
zmc@SUSE11:~> declare -x sum 
zmc@SUSE11:~> export | grep sum
declare -ix sum="6"
#设定为只读
zmc@SUSE11:~> declare -r sum 
zmc@SUSE11:~> sum=$sum+4
bash: sum: readonly variable
#不可取消
zmc@SUSE11:~> declare +r sum 
bash: declare: sum: readonly variable