冒号：用来检验变量是否设置过，如果没有冒号，则认为设置过，不替换
$fruit=peach
$echo ${fruit:-plum}
peach

$fruit=
$echo ${fruit:-plum}
plum

$echo $fruit
$

$name=
$echo ${name:=Peter}
Peter

$echo $name
Peter

$foo=grapes
$echo ${foo:+pears}
pears
$echo $foo
grapes

$echo ${namex:?"namex is undefined"}
namex: namex is undefined

$echo name=${1:?"requires an argument"} //适用于脚本中检测位置参量
1: requires an argument

echo ${y?}
y: parameter null or not set //打印默认消息

$var=notebook

$echo ${var:0}
notebook

$echo ${var:0:4}
note

$echo ${var:4:4}
book?