如果你想使用[System.IO.File]::WriteAllLines()，你应该将第二个参数转换为String[](如果$MyFile的类型是Object[])，并指定绝对路径$ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath($MyPath)，如:

$Utf8NoBomEncoding = New-Object System.Text.UTF8Encoding $False
Get-ChildItem | ConvertTo-Csv | Set-Variable MyFile
[System.IO.File]::WriteAllLines($ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath($MyPath), [String[]]$MyFile, $Utf8NoBomEncoding)

如果你想使用[System.IO.File]::WriteAllText()，有时你应该将第二个参数管道到| Out-String |中，以显式地将crlf添加到每行的末尾(特别是当你使用ConvertTo-Csv时):

$Utf8NoBomEncoding = New-Object System.Text.UTF8Encoding $False
Get-ChildItem | ConvertTo-Csv | Out-String | Set-Variable tmp
[System.IO.File]::WriteAllText("/absolute/path/to/foobar.csv", $tmp, $Utf8NoBomEncoding)

或者你可以使用[Text.Encoding]::UTF8.GetBytes()与Set-Content -Encoding Byte:

$Utf8NoBomEncoding = New-Object System.Text.UTF8Encoding $False
Get-ChildItem | ConvertTo-Csv | Out-String | % { [Text.Encoding]::UTF8.GetBytes($_) } | Set-Content -Encoding Byte -Path "/absolute/path/to/foobar.csv"

参见:如何将ConvertTo-Csv的结果写入没有BOM的UTF-8文件

可以使用下面得到UTF8没有BOM

$MyFile | Out-File -Encoding ASCII

重要!:这只适用于当一个额外的空格或换行符在开始是没有问题的文件用例 (例如，如果是SQL文件、Java文件或人类可读的文本文件)

可以结合使用创建一个空(非utf8或ASCII (utf8兼容))文件并追加它(如果源文件是一个文件，则将$str替换为gc $src):

" "    |  out-file  -encoding ASCII  -noNewline  $dest
$str  |  out-file  -encoding UTF8   -append     $dest

当一行程序

根据你的用例替换$dest和$str:

$_ofdst = $dest ; " " | out-file -encoding ASCII -noNewline $_ofdst ; $src | out-file -encoding UTF8 -append $_ofdst

作为简单函数

function Out-File-UTF8-noBOM { param( $str, $dest )
  " "    |  out-file  -encoding ASCII  -noNewline  $dest
  $str  |  out-file  -encoding UTF8   -append     $dest
}

与源文件一起使用:

Out-File-UTF8-noBOM  (gc $src),  $dest

与字符串一起使用:

Out-File-UTF8-noBOM  $str,  $dest

可选:继续追加Out-File: "more foo bar" | Out-File -encoding UTF8 -append $dest

这是为我工作(使用“默认”而不是“UTF8”):

$MyFile = Get-Content $MyPath
$MyFile | Out-File -Encoding "Default" $MyPath

结果是没有BOM的ASCII。

我认为这不会是UTF，但我只是发现了一个相当简单的解决方案，似乎工作…

Get-Content path/to/file.ext | out-file -encoding ASCII targetFile.ext

对我来说，这导致了一个没有bom文件的utf-8，不管源格式如何。

注意:这个答案适用于Windows PowerShell;相比之下，在跨平台的PowerShell核心版(v6+)中，在所有cmdlet中，不含BOM的UTF-8是默认编码。

换句话说:如果您使用PowerShell [Core]版本6或更高版本，默认情况下您将获得无bom的UTF-8文件(您也可以显式地使用-Encoding utf8 / -Encoding utf8NoBOM请求，而使用-utf8BOM获得- bom编码)。 如果你正在运行Windows 10，并且你愿意在系统范围内切换到无bom的UTF-8编码——这可能会有副作用——甚至Windows PowerShell也可以一直使用无bom的UTF-8编码——请看这个答案。

为了补充M. Dudley自己简单而务实的回答(以及ForNeVeR更简洁的重新表述):

A simple, (non-streaming) PowerShell-native alternative is to use New-Item, which (curiously) creates BOM-less UTF-8 files by default even in Windows PowerShell: # Note the use of -Raw to read the file as a whole. # Unlike with Set-Content / Out-File *no* trailing newline is appended. $null = New-Item -Force $MyPath -Value (Get-Content -Raw $MyPath) Note: To save the output from arbitrary commands in the same format as Out-File would, pipe to Out-String first; e.g.: $null = New-Item -Force Out.txt -Value (Get-ChildItem | Out-String) For convenience, below is advanced function Out-FileUtf8NoBom, a pipeline-based alternative that mimics Out-File, which means: you can use it just like Out-File in a pipeline. input objects that aren't strings are formatted as they would be if you sent them to the console, just like with Out-File. an additional -UseLF switch allows you use Unix-format LF-only newlines ("`n") instead of the Windows-format CRLF newlines ("`r`n") you normally get.

例子:

(Get-Content $MyPath) | Out-FileUtf8NoBom $MyPath # Add -UseLF for Unix newlines

注意(Get-Content $MyPath)是如何包含在(…)中，这确保在通过管道发送结果之前打开、完整读取和关闭整个文件。为了能够写回相同的文件(在适当的位置更新它)，这是必要的。 一般来说，这种技术是不可取的，原因有二:(a)整个文件必须适合内存;(b)如果命令被中断，数据将丢失。

内存使用注意事项:

达德利先生自己的回答 和上面的New-Item替代方案要求首先在内存中构建整个文件内容，这对于大的输入集可能是个问题。 下面的函数不需要这样做，因为它是作为一个代理(包装器)函数实现的(关于如何定义这样的函数的简明摘要，请参阅这个答案)。

Out-FileUtf8NoBom函数源代码:

注意:该功能也可以作为麻省理工学院授权的Gist使用，并且今后只会维护它。

你可以直接使用以下命令安装它(虽然我个人可以向你保证这样做是安全的，但在直接执行脚本之前，你应该总是检查脚本的内容):

# Download and define the function.
irm https://gist.github.com/mklement0/8689b9b5123a9ba11df7214f82a673be/raw/Out-FileUtf8NoBom.ps1 | iex

function Out-FileUtf8NoBom {

  <#
  .SYNOPSIS
    Outputs to a UTF-8-encoded file *without a BOM* (byte-order mark).

  .DESCRIPTION

    Mimics the most important aspects of Out-File:
      * Input objects are sent to Out-String first.
      * -Append allows you to append to an existing file, -NoClobber prevents
        overwriting of an existing file.
      * -Width allows you to specify the line width for the text representations
        of input objects that aren't strings.
    However, it is not a complete implementation of all Out-File parameters:
      * Only a literal output path is supported, and only as a parameter.
      * -Force is not supported.
      * Conversely, an extra -UseLF switch is supported for using LF-only newlines.

  .NOTES
    The raison d'être for this advanced function is that Windows PowerShell
    lacks the ability to write UTF-8 files without a BOM: using -Encoding UTF8 
    invariably prepends a BOM.

    Copyright (c) 2017, 2022 Michael Klement <mklement0@gmail.com> (http://same2u.net), 
    released under the [MIT license](https://spdx.org/licenses/MIT#licenseText).

  #>

  [CmdletBinding(PositionalBinding=$false)]
  param(
    [Parameter(Mandatory, Position = 0)] [string] $LiteralPath,
    [switch] $Append,
    [switch] $NoClobber,
    [AllowNull()] [int] $Width,
    [switch] $UseLF,
    [Parameter(ValueFromPipeline)] $InputObject
  )

  begin {

    # Convert the input path to a full one, since .NET's working dir. usually
    # differs from PowerShell's.
    $dir = Split-Path -LiteralPath $LiteralPath
    if ($dir) { $dir = Convert-Path -ErrorAction Stop -LiteralPath $dir } else { $dir = $pwd.ProviderPath }
    $LiteralPath = [IO.Path]::Combine($dir, [IO.Path]::GetFileName($LiteralPath))
    
    # If -NoClobber was specified, throw an exception if the target file already
    # exists.
    if ($NoClobber -and (Test-Path $LiteralPath)) {
      Throw [IO.IOException] "The file '$LiteralPath' already exists."
    }
    
    # Create a StreamWriter object.
    # Note that we take advantage of the fact that the StreamWriter class by default:
    # - uses UTF-8 encoding
    # - without a BOM.
    $sw = New-Object System.IO.StreamWriter $LiteralPath, $Append
    
    $htOutStringArgs = @{}
    if ($Width) { $htOutStringArgs += @{ Width = $Width } }

    try { 
      # Create the script block with the command to use in the steppable pipeline.
      $scriptCmd = { 
        & Microsoft.PowerShell.Utility\Out-String -Stream @htOutStringArgs | 
          . { process { if ($UseLF) { $sw.Write(($_ + "`n")) } else { $sw.WriteLine($_) } } }
      }  
      
      $steppablePipeline = $scriptCmd.GetSteppablePipeline($myInvocation.CommandOrigin)
      $steppablePipeline.Begin($PSCmdlet)
    }
    catch { throw }

  }

  process
  {
    $steppablePipeline.Process($_)
  }

  end {
    $steppablePipeline.End()
    $sw.Dispose()
  }

}