Merging multiple CSV files into one using PowerShell

This is pretty trivial in PowerShell.

$CSVFolder = 'C:\Path\to\your\files';
$OutputFile = 'C:\Path\to\output\file.txt';

$CSV = Get-ChildItem -Path $CSVFolder -Filter *.csv | ForEach-Object { 
    Import-Csv -Path $_
}

$CSV | Export-Csv -Path $OutputFile -NoTypeInformation -Force;

Only drawback to this approach is that it does parse every file. It also loads all files into memory, so if we're talking about 4000 files that are 100 MB each you'll obviously run into problems.

You might get better performance with System.IO.File and System.IO.StreamWriter.

Try this, it worked for me

Get-Content *.csv| Add-Content output.csv

This will append all the files together reading them one at a time:

get-childItem "YOUR_DIRECTORY\*.txt" 
| foreach {[System.IO.File]::AppendAllText
 ("YOUR_DESTINATION_FILE", [System.IO.File]::ReadAllText($_.FullName))}

# Placed on seperate lines for readability

This one will place a new line at the end of each file entry if you need it:

get-childItem "YOUR_DIRECTORY\*.txt" | foreach
{[System.IO.File]::AppendAllText("YOUR_DESTINATION_FILE", 
[System.IO.File]::ReadAllText($_.FullName) + [System.Environment]::NewLine)}

Skipping the first line:

$getFirstLine = $true

get-childItem "YOUR_DIRECTORY\*.txt" | foreach {
    $filePath = $_

    $lines =  $lines = Get-Content $filePath  
    $linesToWrite = switch($getFirstLine) {
           $true  {$lines}
           $false {$lines | Select -Skip 1}

    }

    $getFirstLine = $false
    Add-Content "YOUR_DESTINATION_FILE" $linesToWrite
    }

If you're after a one-liner you can pipe each csv to an Import-Csv and then immediately pipe that to Export-Csv. This will retain the initial header row and exclude the remaining files header rows. It will also process each csv one at a time rather than loading all into memory and then dumping them into your merged csv.

Get-ChildItem -Filter *.csv | Select-Object -ExpandProperty FullName | Import-Csv | Export-Csv .\merged\merged.csv -NoTypeInformation -Append