默认值表达式1
${a-defaultvalue}
a如果没有定义，则表达式返回默认值，否则返回a的值；
demo1
a=""
ret1=${a-"/usr/local"}
echo "ret1:" $ret1
output:
ret1:
demo2
ret1=${a-"/usr/local"}
echo "ret1:" $ret1
output:
ret1:/usr/local
默认值表达式2
${a:-defaultvalue}
a没有定义或者为空字符串，则表达式返回默认值，否则返回a的值；
demo1
a=""
ret1=${a:-"/usr/local"}
echo "ret1:" $ret1
output:
ret1:/usr/local
demo2
ret1=${a:-"/usr/local"}
echo "ret1:" $ret1
output:
ret1:/usr/local