是否可以在不分配给变量的情况下通过管道传递 while 循环的结果 [英] Is it possible to pipe the result of a while loop without assigning to a variable

问题描述

以下代码无效

$result = (而 ($true) {<#生成psobject的长列表#>}) |Tee-Object -FilePath 'fname.csv' |组-对象-属性 xxx |选择名称，计数

<块引用>

术语while"未被识别为 cmdlet、函数、脚本文件或可运行程序的名称.检查名称的拼写，如果包含路径，请验证路径是否正确，然后重试.

解决方案

虽然:

• 您可以使用表达式作为管道的第一段(例如，
  1..2 | Tee-Object - 变量结果),

• 你不能使用复合语句，例如if、while>、foreach 和 do 很遗憾.

也就是说，这样的复合语句不是真正的表达式，尽管在​​赋值的上下文中它们可以起到这样的作用.也就是说，您可以执行 $result = while ($true) ... - 无需在 (...) 中包含 while -但是你不能直接通过管道发送while循环.

参见 这个 GitHub 问题，其中讨论了这个有问题的半表达式状态";复合语句，并询问是否可以使它们成为完整的表达式；事实证明，PowerShell 语法的基础阻止了这种情况.

解决方法:

• 如果你想让你的循环复合语句流，即把它的对象一个一个输出到管道，如它们变得可用 - 即，如果您想要标准的流式管道行为:

  • 用 & 包裹你的复合语句{ ... } (或 . { ... }，如果您想直接在当前范围内运行而不是在子范围内运行).
  • 例如，&{ foreach ($i in 1..2) { $i } } |Tee-Object - 变量结果

• 如果您想收集循环复合语句的所有输出预先，在通过管道发送它们之前:

  • 将复合语句包装在 $(...) 中，子表达式运算符.
  • 例如，$(foreach ($i in 1..2) { $i }) |Tee-Object - 变量结果
  • 注意:
    • 同时使用 $(...) 而不是 &{ ... } 可以加速您的管道，它只是略微加速，并且它可能以内存消耗为代价，因为所有输出总是em> 首先收集到内存中.
    • 这同样适用于 @(...)，array-subexpression operator 和 (...)，分组运算符，但请注意(...) 仅适用于单个 表达式 或 命令.这个答案对比了(...)、$(...) 和 @(...) 详细说明.

---

适用于您的案例:

$result = &{而 ($true) {<#生成psobject的长列表#>}} |Tee-Object -FilePath 'fname.csv' |...

The following code doesn't work

$result = (
    while ($true) { 
        <# Generating long list of psobject #> 
    }) | Tee-Object -FilePath 'fname.csv' | Group-Object -Property xxx | Select Name,Count

The term 'while' 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.

解决方案

While:

• you can use expressions as the first segment of a pipeline (e.g.,
  1..2 | Tee-Object -Variable result),

• you cannot use compound statements such as if, while, foreach, and do as-is, unfortunately.

That is, such compound statements aren't true expressions, even though in the context of assignments they can act a such. That is, you could do $result = while ($true) ... - without enclosing the while in (...) - yet you cannot send the while loop directly through a pipeline.

See this GitHub issue, which discusses this problematic "half-expression status" of compound statements, and asks if it's feasible to make them full expressions; as it turns out, the fundamentals of PowerShell's grammar prevent that.

Workarounds:

• If you want your looping compound statement to stream, i.e. to output its objects one by one to the pipeline, as they become available - i.e. if you want the standard, streaming pipeline behavior:

  • Wrap your compound statement in & { ... } (or . { ... }, if you want to run directly in the current rather than in a child scope).
  • E.g., & { foreach ($i in 1..2) { $i } } | Tee-Object -Variable result

• If you want to collect all outputs from your looping compound statement up front, before sending them through the pipeline:

  • Wrap your compound statement in $(...), the subexpression operator.
  • E.g., $(foreach ($i in 1..2) { $i }) | Tee-Object -Variable result
  • Note:
    • While using $(...) over & { ... } can speed up your pipeline, it does so only slightly, and it potentially comes at the expense of memory consumption, given that all outputs are invariably collected in memory first.
    • The same applies to @(...), the array-subexpression operator and (...), the grouping operator, but note that (...) only works with a single expression or command. This answer contrasts (...), $(...) and @(...) in detail.

---

Applied to your case:

$result = & {
    while ($true) { 
        <# Generating long list of psobject #> 
    } 
  } | Tee-Object -FilePath 'fname.csv' | ...

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