zsh 参数扩展语法：结合默认值和转换为大写

Question

在 zsh 脚本中，

echo ${X:-4711}

输出变量X的值，如果有none则输出4711。

echo ${X:u}

输出变量 X 的值，转换为大写。

请问有没有办法将两者结合起来，即达到

的效果

tmp=${X:-4711}
echo $X:u

不引入辅助变量

Answer 1

$ echo ${${X:-4711}:u}
4711
$ X=hello
$ echo ${${X:-4711}:u}
HELLO

来自man zshexpn：

If  a  `${...}`  type  parameter expression or a `$(...)` type command
substitution is used in place of name above, it is expanded first and
the result is used as if it were the value of name.  Thus it is possible
to perform nested operations:  `${${foo#head}%tail}` substitutes the value
of `$foo` with both 'head' and 'tail' deleted.

zsh 参数扩展语法：结合默认值和转换为大写

zsh parameter expansion syntax: combining default value and conversion to upper case

unix

linux

shell

zsh