Shell参数展开需要解释

我正在学习bash手册

在第27/166页，我真的无法理解最后一个参数展开：

${parameter^pattern} 
${parameter^^pattern} 
${parameter,pattern}
${parameter,,pattern}

有人能告诉我这些扩张意味着什么吗

也可以举一些例子。

到小写

$ string="A FEW WORDS"
$ echo ${string,}
a FEW WORDS
$ echo ${string,,}
a few words

大写

$ string="a few words"
$ echo ${string^}
A few words
$ echo ${string^^}
A FEW WORDS

转换为小写

$ string="A FEW WORDS"
$ echo ${string,}
a FEW WORDS
$ echo ${string,,}
a few words

大写

$ string="a few words"
$ echo ${string^}
A few words
$ echo ${string^^}
A FEW WORDS