add colum to merged csv file - excel

Question

Ok heres what I have code wise:
$a = "C:\Users\some.deranged.character\Desktop\SomeAwfulPlace\Checklists\C_F\*.csv"
$b = "C:\Users\some.deranged.character\Desktop\SomeAwfulPlace\Checklists\C_F\merge.csv"
(get-content $a) | set-content $b
This pulls all the data of all the files into one merged file, but I need one additional item, I need to pull the name of the individual files and append it to the first column of the file for multiple files, several hundred at a time.

Answer 1

Not tested but something like this should do it:
$a = "C:\Users\some.deranged.character\Desktop\SomeAwfulPlace\Checklists\C_F\*.csv"
$b = "C:\Users\some.deranged.character\Desktop\SomeAwfulPlace\Checklists\C_F\merge.csv"
Get-ChildItem $a | % {
Import-Csv $_.Fullname | Add-Member -MemberType NoteProperty -Name 'File Name' -Value $_.Name
} | Export-Csv $b

Answer 2

Assuming the CSV files each have the same column headings, I would lean toward using Import-CSV instead of Get-Content so that you can work with the CSV contents as arrays of objects with named properties.
Then all you need to do is iterate through each item of the array and append a property containing the file path, which you can do using the Add-Member cmdlet. Once that's done, export the array of objects using the Export-CSV cmdlet.
$directory = "C:\Users\some.deranged.character\Desktop\SomeAwfulPlace\Checklists\C_F\"
$search = $directory + "*.csv"
$exportpath = "C:\Users\some.deranged.character\Desktop\SomeAwfulPlace\Checklists\C_F\merge.csv"
$paths = get-childitem $search
$objectArrays = #()
$paths | %{
$filepath = $_.fullname;
$objectArray = Import-CSV $filepath;
$objectArray | %{
Add-Member -inputobject $_ -Name "SourceFile" -Value $filepath -MemberType NoteProperty};
$objectArrays += $objectArray}
$objectArrays | export-csv -path $exportpath -notype
This puts the SourceFile property as the last column in the outputted CSV file

Answer 3

Ok, simplification... Search target folder, pipe to a ForEach-Object loop (shorthand % used), capture the file name as variable, import the CSV, add the sourcefile using the Select-Object cmdlet, convert it back to a CSV, end loop, pipe to destination file.
$a = "C:\Users\some.deranged.character\Desktop\SomeAwfulPlace\Checklists\C_F\*.csv"
$b = "C:\Users\some.deranged.character\Desktop\SomeAwfulPlace\Checklists\C_F\merge.csv"
GCI $a | %{$FileName=$_.Name;Import-CSV $_|Select #{l='SourceFile';e={$FileName}},*|ConvertTo-CSV -NoType} | set-content $b