I use -NoTypeInformation so why do I get header back when using Out-File? - powershell

Question

I filtered by date this file data1.csv
2017.11.1,09:55,1.1,1.2,1.3,1.4,1
2017.11.2,09:55,1.5,1.6,1.7,1.8,2
I don't get a header with -NoTypeInformation:
$CutOff = (Get-Date).AddDays(-2)
$filePath = "data1.csv"
$Data = Import-Csv $filePath -Header Date,Time,A,B,C,D,E
$Data2 = $Data | Where-Object {$_.Date -as [datetime] -gt $Cutoff} | convertto-csv -NoTypeInformation -Delimiter "," | % {$_ -replace '"',''}
But when rewriting with Out-File
$Data2 | Out-File "data2.csv" -Encoding utf8 -Force
I get header back as data2.csv contains:
Date,Time,A,B,C,D,E
2017.11.2,09:55,1.5,1.6,1.7,1.8,2
Why do I have Date,Time,A,B,C,D,E ?

Answer 1

-NoTypeInformation is not about the header but the data type of the rows in the file. Remove it to see what shows up. From Microsoft
Omits the type information header from the output. By default, the string in the output contains #TYPE followed by the fully-qualified name of the object type.
Emphasis mine.
CSVs need headers. That is why it is making one. If you don't want to see the header in the output use Select-Object -Skip 1 to remove it.
$Data |
Where-Object {$_.Date -as [datetime] -gt $Cutoff} |
ConvertTo-CSV -NoTypeInformation -Delimiter "," |
Select-Object -Skip 1 |
% {$_ -replace '"'}
I would not pipe Out-File to itself. You could pipe to Set-Content here just as well.
I am guessing this whole process is to keep the source file in the same state just with some lines filtered out based on date. You could skip most of this just by parsing the date out in each line.
$threshold = (Get-Date).AddDays(-2)
$filePath = "c:\temp\bagel.txt"
(Get-Content $filePath) | Where-Object{
$date,$null=$_.Split(",",2)
[datetime]$date -gt $threshold
} | Set-Content $filePath
Now you don't have to worry about PowerShell CSV object structure or output since we act on the raw data of the file itself.
That will take each line of the input file and filter it out if the parsed date does not match the threshold. Change encoding on the input output cmdlets as you see necessary. What $date,$null=$_.Split(",",2) is doing is splitting the line
on the comma into 2 parts. First of which becomes $date and since this is just a filtering condition we dump the rest of the line into $null.

Answer 2

Properly-formed CSV files must have column headers. Your use of -NoTypeInformation in generating the CSV does not affect column headers; instead, it affects whether the PowerShell object type information is included. If you Export-CSV without -NoTypeInformation, the first line of your CSV file will have a line that looks like #TYPE System.PSCustomObject, which you don't want if you're going to open the CSV in a spreadsheet program.
If you subsequently Import-CSV, the headers (Date, Time, A, B, C) are used to create the fields of a PSObject, so that you can refer to them using the standard dot notation (e.g., $CSV[$line].Date).
The ability to specify -Header on Import-CSV is essentially a "hack" to allow the cmdlet to handle files that are comma-separated, but which did not include column headers.