1.${variable:-word}
${variable:-word} 如果variable已经被设置了，且不为空，则代入它的值，否则代入word;
$ fruit=peach
$ echo ${fruit:-plum}
peach
$ echo ${newfruit:-apple}
apple
2.${variable:=word}
${variable:=word}如果variable已经被设置且不为空，则代入它的值，否则代入word，并且在后面variable始终为word的值。位置参量不能用这种方式赋值。
$ name=
$ echo ${name:=peter}
peter
$ echo $name
peter
3.${variable:+word}
${variable:+word} 如果变量variable已被设置且值为非空，代入word值，否则什么也不代入，
$ foo=grapes
$ echo ${foo:+pears}
pears
$ echo $foo
grapes
4. ${variable:?word}
${variable:?word} 如果变量variable已被设置值且为非空，就代入它的值，否则输出word并从shell中退出，如果有省略了word,就会显示信息：parameter null or not set.
$ echo ${namex:?"namex is undefined"}
namex: namex is undefined
5.$ echo ${y?]
$ echo ${y?]
y: parameter null or not set
6.${variable:offset}
${variable:offset} 获得变量从variable值中位置从offset开始的子串，偏移为从0到串尾。
${variable:offset:length} 获得variable值位置从offset开始长度为length的子串。
$var=notebook
$ echo ${var:0:4}
note
$ echo ${var:4:4]
book