我可以在文件前面加上$newstring的内容。

$sourceDirectory = "C:\SFTP_Root\kcmosdftp\"
$keyword = "User_ID|User_Alt_ID"
$newString = "User_ID|User_Alt_ID|Home_Room_Teacher|User_Group_ID|User_First_Name|User_Middle_Name|User_Last_Name|User_Library|User_Profile|User_Category1|User_Category2|User_Category3|User_Category4|User_Category5|User_Category10|User_Priv_Granted|User_Priv_Expires|Password|Street|City_State|zip|Email|PHONE|User_Birth_Date"
# Set the file name to search for
$fileName = "INBOUND-Student_New_File_Format.txt"
# Get the latest file in the source directory
$latestFile = Get-ChildItem -Path $sourceDirectory | Sort-Object LastWriteTime -Descending | Select-Object -First 1
# Check if the latest file contains the specified keyword
$containsKeyword = (Get-Content $latestFile.FullName) | Select-Object -first 1
if ($containsKeyword -notmatch $keyword) {
    # Read the contents of the latest file
    $fileContents = Get-Content $latestFile.FullName
    # Insert the new string at the beginning of the contents
    $fileContents = $newString + "`r`n" + $fileContents
    # Determine the new file path
    $newFilePath = Join-Path -Path $sourceDirectory -ChildPath "Modified_$($latestFile.Name)"
    # Write the modified contents to the new file
    $fileContents | Set-Content $newFilePath
    Write-Host "Added the string to the first row of $($latestFile.Name). New file saved as $($newFilePath)."
}
else {
    Write-Host "The latest file already contains the keyword."
}

不过，新的“修改”文件只有两行。在某些地方，这段代码删除了原始文件（$filename）中的所有回车和新行。
不知道我在这里哪里错了。