为什么 Powershell 的 Set-Content -Value 参数不能正确读取管道变量

Question

So I'm trying to get the contents of a bunch of files to replace the header string:

$replaces = dir | select Name, @{n='content';e={(get-content $_) -replace "header-foo","header-bar"}}

That then gives me a list of:

Name       Content
----       ----------
a.txt      header-foo blah blah
b.txt      header-foo blah blah blah
c.txt      header-foo blah

Then I want to pass the value of this to set-content -value like so:

$replaces | set-content -Path {"alt/" + $_.Name} -Value {$_.content}

Only all of my files have the content $_.content now. I also tried -Value ($_.content) but that doesn't do the right thing either.

It's only when I use a foreach does it work:

$replaces | foreach { set-content -Path ("alt/" + $_.Name) -Value $_.content }

Why is this the case? Why does it not work correctly without the foreach ?

Answer 1

You're trying to use a delay-bind script block ( {... } ) in order to dynamically determine the argument for Set-Content 's -Value parameter, based on each pipeline input object.

However, delay-bind script blocks cannot be used with -Value , because that parameter's type is [object[]] ( System.Object[] ) (see Get-Help -Parameter Value Set-Content ); the same limitation applies to [scriptblock] -typed parameters.

To work around this limitation, you do need a loop-like construct in order to invoke Set-Content once for every intended -Value argument , as you have done with the ForEach-Object (whose built-in alias is foreach ) cmdlet.

---

The reason that [object] and [scriptblock] parameters (and their array variants) do not work as delay-bind script blocks is that passing as script block to such parameters binds it instantly, as-is , because the argument type matches the parameter type.

With any other parameter type - assuming that the parameter is designated as accepting pipeline input - PowerShell infers that a script-block argument is a delay-bind script block and evaluates the script block for each pipeline input object , as intended, and the script block must then ensure that its output is of a type that matches the target parameter.

Answer 2

You problem is in "get-content $ " you must use get-content $ .Name

But you should modify you script like this:

1. Use Get-childItem (norme powershell)
2. Use -file for take only file and not directory
3. Use FullName, you can then use recurse if necessary
4. Use.Replace and not -replace operator (doesnt work in this case)

-replace use regular expression: detail here

$replaces = Get-ChildItem -file | select FullName, @{n='content';e={(get-content $_.FullName).Replace('header-foo', 'header-bar')}}
$replaces | %{set-content -Path $_.FullName -Value $_.content}