用法

Parameter	What does it do?
`${VAR:-STRING}`	If `VAR` is empty or unset, use `STRING` as its value.
`${VAR-STRING}`	If `VAR` is unset, use `STRING` as its value.
`${VAR:=STRING}`	If `VAR` is empty or unset, set the value of `VAR` to `STRING`.
`${VAR=STRING}`	If `VAR` is unset, set the value of `VAR` to `STRING`.
`${VAR:+STRING}`	If `VAR` is not empty, use `STRING` as its value.
`${VAR+STRING}`	If `VAR` is set, use `STRING` as its value.
`${VAR:?STRING}`	Display an error if empty or unset.
`${VAR?STRING}`	Display an error if unset.

例子

执行下面的例子，如果环境变量中 CONF 的值存在，则取 CONF 的值，否则用默认值 7

#/bin/bash

a=${CONF:-"7"}
echo $a;