shell变量状态赋值

${VAR:-string} 如果VAR变量为空则返回string
${VAR:+string} 如果VAR变量不为空则返回string
${VAR:=string} 如果VAR变量为空则重新赋值VAR变量值为string
${VAR:?string} 如果VAR变量为空则将string输出到stderr

[root@localhost ~]# var=
[root@localhost ~]# 
[root@localhost ~]# echo ${var:=12345}
12345
[root@localhost ~]# echo $var
12345