With PowerShell, how do I insert a header row to the first line of a text file?

I was able to prepend the file with the content of $newstring.

$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."
}

Though, the new "modified" file has only two rows. Somewhere this code stripped out all of the carriage returns and new lines that were in original file ($filename).

Not sure where I went wrong here.

Change the following:

if ($containsKeyword -notmatch $keyword) {
    # Read the contents of the latest file
    $fileContents = Get-Content $latestFile.FullName

To (notice the use of -Raw):

if ($containsKeyword -notmatch $keyword) {
    # Read the contents of the latest file
    $fileContents = Get-Content $latestFile.FullName -Raw

This is so $fileContents becomes a single multi-line string instead of an array of strings.

When you try to concatenate a string in the left-hand side of the operation with an array, said array gets coerced into string thus joined by $OFS (a space by default).

Example:

$myArr = 'foo', 'bar', 'baz'
$newString = 'hello world'
$newString + "`r`n" + $myArr

Outputs:

hello world
foo bar baz

Instead of what you expected:

hello world
foo
bar
baz

<!-- language-all: sh -->

Get-Content by default returns the individual lines of a text file, one by one, which, when captured, becomes an array.

Therefore, the following does not work as intended:

> $fileContents = $newString + "`r`n" + $fileContents

It appends an array to $newString + "`r`n" and when PowerShell stringifies an array, it joins its element with spaces (by default; the rarely used $OFS preference variable can change that); try "foo: " + 1, 2, 3, which yields verbatim foo: 1 2 3.

The simplest - and better-performing - solution in your case is to use Get-Content's -Raw switch to read your file as a single, multiline string, in which case your string concatenation would work as intended.

$fileContents = Get-Content -Raw $latestFile.FullName

However, given how Set-Content works, you don't even need the string concatenation and can pass the two strings - the new header line and the multiline string with the existing file content -
separately:

$newString, $fileContents | Set-Content $newFilePath