Split a text file by lines - windows

Question

With Powershell i'm trying to split a text file into multiple files using the the beginning of each line as a delimiter
Input file (transfer.txt):
3M|9935551876|11.99|2235641|001|1|100|N|780
3M|1135741031|13.99|8735559|003|1|100|N|145
3M|5835551001|20.50|4556481|002|1|100|N|222
3M|4578420001|33.00|1125785|001|1|100|N|652
8L|00811444243|134148|4064080040|1|02/05/2017 21:15:13|8|170502707|19.85
8L|00811444243|130925|4189133003|1|02/05/2017 21:15:13|8|170502707|4.69
8L|00811444243|136513|4186144003|2|02/05/2017 21:15:13|8|170502707|10.83
Output file (Article.txt):
3M|9935551876|11.99|2235641|001|1|100|N|780
3M|1135741031|13.99|8735559|003|1|100|N|145
3M|5835551001|20.50|4556481|002|1|100|N|222
3M|4578420001|33.00|1125785|001|1|100|N|652
Here's a snippet of my code:
$Path = "D:\BATCH\"
$InputFile = (Join-Path $Path "transfer.txt")
$Reader = New-Object System.IO.StreamReader($InputFile)
while (($Line = $Reader.ReadLine()) -ne $null) {
if ($Line.StartsWith("3M")) {
$OutputFile = "Article.txt"
}
Add-Content (Join-Path $Path $OutputFile) $Line
}
This as a result, creates the same file as the input file. What's wrong with the code?

Answer 1

The below line is the problem. It is outside the If loop and adding the content of each line to the output file. But as I understand, that is not what you want. You want only the content that pass the If condition to be added to the output file. Hence, it needs to be inside the If loop.
Add-Content (Join-Path $Path $OutputFile) $Line
Although I am not too found of this approach because you would be making as many Disk I/O operations as there are lines that pass the if condition. Not very good for scalability.
You can change your code to something like this to reduce number of Disk I/O to just 1.
$out = While (($Line = $Reader.ReadLine()) -ne $null) {
If ($Line.StartsWith("3M")) {
$Line
}
}
$OutputFile = "Article.txt"
Add-Content (Join-Path $Path $OutputFile) $Out

Answer 2

As others have already pointed out, you never change the output file to anything different from "Article.txt", and you write all input lines to the defined output file.
If you want to write the lines of the input file to different files depending on the value of the first field I'd recommend naming the output files after that value. And since you're writing the output with Add-Content I'd also suggest reading the input file via Get-Content for simplicity reasons. Use a StreamReader when performance is an issue (in which case you'll want to use a StreamWriter too), but not just because.
Get-Content $InputFile | ForEach-Object {
$basename, $null = $_.Split('|', 2)
Add-Content (Join-Path $Path "${basename}.txt") $_
}