PowerShell：Start-Job 中的 $input 到底是什么？

Question

考虑以下代码。我只是通过使用-InputObject 参数将一个32 位有符号整数数组[Int32[]] 传递到Start-Job cmdlet。

$Job = Start-Job -ScriptBlock { $input.GetType().FullName; } -InputObject @(1,2,3);
Wait-Job -Job $Job;
Receive-Job -Keep $Job;

这段代码的结果是：

System.Management.Automation.Runspaces.PipelineReader`1+<GetReadEnumerator>d__0[[System.Object, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]

查看PipelineReader .NET class 的文档，它有一个ReadToEnd() 方法。因此，下面的代码应该可以工作：

$Job = Start-Job -ScriptBlock { $input.ReadToEnd(); } -InputObject @(1,2,3);
Wait-Job -Job $Job;
Receive-Job -Keep $Job;

但是，我收到一条错误消息：

方法调用失败，因为 [System.Int32] 不包含名为“ReadToEnd”的方法。 + CategoryInfo : InvalidOperation: (:) [], RuntimeException + FullyQualifiedErrorId : MethodNotFound + PSComputerName : 本地主机

那么我想，我将只使用PSBase 属性来获取“真实”对象。

$Job = Start-Job -ScriptBlock { $input.psbase.ReadToEnd(); } -InputObject @(1,2,3);
Wait-Job -Job $Job;
Receive-Job -Keep $Job;

然后我收到类似的错误消息：

方法调用失败，因为 [System.Management.Automation.PSInternalMemberSet] 不包含名为“ReadToEnd”的方法。 + CategoryInfo : InvalidOperation: (ReadToEnd:String) [], RuntimeException + FullyQualifiedErrorId : MethodNotFound + PSComputerName : 本地主机

我注意到围绕这种混乱有一个Microsoft Connect bug filed，但它让我更加困惑。显然PipelineReader 类有一个名称混淆的属性<>4__this，它有一个Read() 方法，使用Get-Member 是看不到的。

底线：当通过Start-Job cmdlet 上的-InputObject 参数提交输入时，有谁知道如何简单地“解包”$input 自动变量的内容，所以我可以单独处理这些对象吗？

这个脚本应该只返回1，而不是1, 2, 3。

$Job = Start-Job -ScriptBlock { $input[0]; } -InputObject @(1,2,3);
Wait-Job -Job $Job;
Receive-Job -Keep $Job;

Answer 1

大概$input 是一个枚举器，就像在标准管道中一样。 为了处理项目，我们应该使用 process 块和 自动变量$_ 或在end 的另一个管道中传递$input 块（如果未指定，则为隐式）。

# process each item separately
$Job = Start-Job -ScriptBlock {process{$_}} -InputObject @(1,2,3);
Wait-Job -Job $Job;
Receive-Job -Keep $Job;

# process the whole $input
$Job = Start-Job -ScriptBlock {$input | %{$_}} -InputObject @(1,2,3);
Wait-Job -Job $Job;
Receive-Job -Keep $Job;

# compare with script blocks in standard pipelines

# each item
@(1,2,3) | . {process{$_}}

# whole input
@(1,2,3) | . {$input | %{$_}}

也许还有其他方法可以枚举$input 的项目，但它们是 据我所知，在实践中并不经常使用。

Answer 2

这似乎有效：

$Job = Start-Job -ScriptBlock { $input.movenext();$input.current[0] } -InputObject @(1,2,3);
Wait-Job -Job $Job;
Receive-Job -Keep $Job;

Answer 3

$Input
   Contains an enumerator that enumerates all input that is passed to a
   function. The $input variable is available only to functions and script
   blocks (which are unnamed functions).  In the Process block of a
   function, the $input variable enumerates the object that is currently
   in the pipeline. When the Process block  completes, there are no objects
   left in the pipeline, so the $input variable enumerates an empty
   collection. If the function does not have a Process block, then in the
   End block, the $input variable enumerates the collection of all input to
   the function.

来源：about_Automatic_Variables

总结一下：$input 是一个自动变量，它以枚举器的形式包含整个管道，而 $_ 是管道中的“当前对象”。

这是一个如何使用它的示例。

function test {
    #$input is an enumerator that you should use $input | foreach-object { } to access the objects.

    #To to get all items you could e.g. convert the enumerator to an array.
    $arr = @($input)

    #If you need to use the $input enumerator for something else, you need to call .Reset() first as the enumerator has reached the end.
    $input.Reset()

    #Print some values from the data
    $arr.count
    $arr[0]
}

PS> "hello", "world" | test


2
hello

更新：这是证明这也适用于您Start-Job 场景的证据。示例注释说明。

$Job = Start-Job -ScriptBlock { 
    #Read the complete pipeline to an array
    $data= @($input)
    "`$data is an $($data.GetType().Name) with $($data.count) objects"

    #Unlike in a pipeline, where the `Start-Job` command would be called once per object like `Start-Job ...... -InputObject $_`, 
    #you're inputing a single `object[]` object the the pipeline. So you only have one item in your $input pipeline.

    #Get our inputobject (our array)
    $arr= $data[0]
    "`$arr is an $($arr.GetType().Name)"

    #Use array
    "$($arr[0]) is less than $($arr[1]) which is less than $($arr[2])"
} -InputObject @(1,2,3);

Wait-Job -Job $Job;
Receive-Job -Keep $Job;

Id     Name            PSJobTypeName   State         HasMoreData     Location             Command                  
--     ----            -------------   -----         -----------     --------             -------                  
54     Job54           BackgroundJob   Completed     True            localhost             ...                     
$data is an Object[] with 1 objects
$arr is an ArrayList
1 is less then 2 which is less then 3

并证明我关于管道的“理论”。这是流水线版本：

$Job = 1, 2, 3 | Start-Job -ScriptBlock { 
    #Read the complete pipeline to an array
    $data= @($input)
    "`$data is an $($data.GetType().Name) with $($data.count) objects"

    #Now the command is run per object, so the $input enumerator contained our 3 seperate Int32 values.

    #Get a single object in the pipeline
    $OneOfTheValues= $data[0]
    "`$OneOfTheValues is an $($OneOfTheValues.GetType().Name)"

    #Use data
    "$($data[0]) is less than $($data[1]) which is less than $($data[2])"
}

Wait-Job -Job $Job;
Receive-Job -Keep $Job;

Id     Name            PSJobTypeName   State         HasMoreData     Location             Command                  
--     ----            -------------   -----         -----------     --------             -------                  
58     Job58           BackgroundJob   Completed     True            localhost             ...                     
$data is an Object[] with 3 objects
$OneOfTheValues is an Int32
1 is less then 2 which is less then 3

所以我坚持我原来的答案。行为是相同的，您只是以不同的方式使用管道/cmdlet。 :)