() 和 $() 的区别 [英] Difference between () and $()

问题描述

有什么区别

Write-Host (Get-Date) # just paren

和

Write-Host $(Get-Date) # dollar-paren

括号内的内容可以是任何内容，只是举一个简单的例子.两者有区别吗?

Content within the parens could be anything, just going with a simple example. Is there any difference between the two?

我认为自己在 PS 方面的经验相当丰富，但正是这些小事情让我感到困扰，尤其是在代码审查等过程中.有没有人找到这就是语言的工作原理"的良好来源，其中包含足够详细的信息来推导出此类问题的答案?

I consider myself reasonably experienced with PS, but it's these little things that bug me, especially during code review and the like. Has anyone come across a good source for "here is how the language works" with enough detail to derive answers to these sorts of questions?

推荐答案

子表达式 ($(...)) 包含一个 StatementBlockAst.它可以采用任意数量的语句，意思是关键字(if、foreach等)、管道、命令等.解析类似于在像 begin/process/end 这样的命名块内.

The sub expression ($(...)) contains a StatementBlockAst. It can take any number of statements, meaning keywords (if, foreach, etc), pipelines, commands, etc. Parsing is similar to the inside of a named block like begin/process/end.

paren 表达式 ((...)) 可以包含一个 single ExpressionAst，它是 AST 的一个有限子集.语句和表达式之间最显着的区别是不解析关键字.

The paren expression ((...)) can contain a single ExpressionAst which is a limited subset of the AST. The most notable difference between a statement and an expression is that keywords are not parsed.

$(if ($true) { 'It worked!' })
# It worked!

(if ($true) { 'It worked!' })
# if : The term 'if' is not recognized as the name of a cmdlet, function, 
# script file, or operable program. Check the spelling of the name, or
# if a path was included, verify that the path is correct and try again.
# At line:1 char:2
# + (if ($true) { 'It worked' })
# +  ~~
#     + CategoryInfo          : ObjectNotFound: (if:String) [], CommandNotFoundException
#     + FullyQualifiedErrorId : CommandNotFoundException

正如其他人所指出的，子表达式将在双引号字符串中展开.

Also as others have noted, the sub expression will expand in double quoted strings.

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