Shell parameter Extensions

Recently looking at the script inside the OpenWrt, with too handsome, the following 2 parameters extended expression of large use, all recorded.

${parameter:+expression} 
If parameter has a value and is not empty, use the value of expression

#!/bin/sh

path1= "/usr/bin"
Path2= "/usr/sbin"
path=${path1:+ $path 2}

echo $path The

result is:/usr/sbin
if path1 undefined or empty result is empty


${ Parameter:-expression} 

#!/bin/sh

path1= "/usr/bin"
path2= "/usr/sbin" path=${path1:-
$path 2}


Echo $path

Run as:/usr/bin