How can I split a text file using PowerShell?

I need to split a large (500 MB) text file (a log4net exception file) into manageable chunks like 100 5 MB files would be fine.

I would think this should be a walk in the park for PowerShell. How can I do it?

A word of warning about some of the existing answers - they will run very slow for very big files. For a 1.6 GB log file I gave up after a couple of hours, realising it would not finish before I returned to work the next day.

Two issues: the call to Add-Content opens, seeks and then closes the current destination file for every line in the source file. Reading a little of the source file each time and looking for the new lines will also slows things down, but my guess is that Add-Content is the main culprit.

The following variant produces slightly less pleasant output: it will split files in the middle of lines, but it splits my 1.6 GB log in less than a minute:

$from = "C:\temp\large_log.txt"
$rootName = "C:\temp\large_log_chunk"
$ext = "txt"
$upperBound = 100MB


$fromFile = [io.file]::OpenRead($from)
$buff = new-object byte[] $upperBound
$count = $idx = 0
try {
    do {
        "Reading $upperBound"
        $count = $fromFile.Read($buff, 0, $buff.Length)
        if ($count -gt 0) {
            $to = "{0}.{1}.{2}" -f ($rootName, $idx, $ext)
            $toFile = [io.file]::OpenWrite($to)
            try {
                "Writing $count to $to"
                $tofile.Write($buff, 0, $count)
            } finally {
                $tofile.Close()
            }
        }
        $idx ++
    } while ($count -gt 0)
}
finally {
    $fromFile.Close()
}

Simple one-liner to split based on number of lines (100 in this case):

$i=0; Get-Content .....log -ReadCount 100 | %{$i++; $_ | Out-File out_$i.txt}

This is a somewhat easy task for PowerShell, complicated by the fact that the standard Get-Content cmdlet doesn't handle very large files too well. What I would suggest to do is use the .NET StreamReader class to read the file line by line in your PowerShell script and use the Add-Content cmdlet to write each line to a file with an ever-increasing index in the filename. Something like this:

$upperBound = 50MB # calculated by Powershell
$ext = "log"
$rootName = "log_"

$reader = new-object System.IO.StreamReader("C:\Exceptions.log")
$count = 1
$fileName = "{0}{1}.{2}" -f ($rootName, $count, $ext)
while(($line = $reader.ReadLine()) -ne $null)
{
    Add-Content -path $fileName -value $line
    if((Get-ChildItem -path $fileName).Length -ge $upperBound)
    {
        ++$count
        $fileName = "{0}{1}.{2}" -f ($rootName, $count, $ext)
    }
}

$reader.Close()

Same as all the answers here, but using StreamReader/StreamWriter to split on new lines (line by line, instead of trying to read the whole file into memory at once). This approach can split big files in the fastest way I know of.

Note: I do very little error checking, so I can't guarantee it'll work smoothly for your case. It did for mine (1.7 GB TXT file of 4 million lines split in 100,000 lines per file in 95 seconds).

#split test
$sw = new-object System.Diagnostics.Stopwatch
$sw.Start()
$filename = "C:\Users\Vincent\Desktop\test.txt"
$rootName = "C:\Users\Vincent\Desktop\result"
$ext = ".txt"

$linesperFile = 100000#100k
$filecount = 1
$reader = $null
try{
    $reader = [io.file]::OpenText($filename)
    try{
        "Creating file number $filecount"
        $writer = [io.file]::CreateText("{0}{1}.{2}" -f ($rootName,$filecount.ToString("000"),$ext))
        $filecount++
        $linecount = 0

        while($reader.EndOfStream -ne $true) {
            "Reading $linesperFile"
            while( ($linecount -lt $linesperFile) -and ($reader.EndOfStream -ne $true)){
                $writer.WriteLine($reader.ReadLine());
                $linecount++
            }

            if($reader.EndOfStream -ne $true) {
                "Closing file"
                $writer.Dispose();

                "Creating file number $filecount"
                $writer = [io.file]::CreateText("{0}{1}.{2}" -f ($rootName,$filecount.ToString("000"),$ext))
                $filecount++
                $linecount = 0
            }
        }
    } finally {
        $writer.Dispose();
    }
} finally {
    $reader.Dispose();
}
$sw.Stop()

Write-Host "Split complete in " $sw.Elapsed.TotalSeconds "seconds"

Output splitting a 1.7 GB file:

...
Creating file number 45
Reading 100000
Closing file
Creating file number 46
Reading 100000
Closing file
Creating file number 47
Reading 100000
Closing file
Creating file number 48
Reading 100000
Split complete in  95.6308289 seconds

I often need to do the same thing. The trick is getting the header repeated into each of the split chunks. I wrote the following cmdlet (PowerShell v2 CTP 3) and it does the trick.

##############################################################################
#.SYNOPSIS
# Breaks a text file into multiple text files in a destination, where each
# file contains a maximum number of lines.
#
#.DESCRIPTION
# When working with files that have a header, it is often desirable to have
# the header information repeated in all of the split files. Split-File
# supports this functionality with the -rc (RepeatCount) parameter.
#
#.PARAMETER Path
# Specifies the path to an item. Wildcards are permitted.
#
#.PARAMETER LiteralPath
# Specifies the path to an item. Unlike Path, the value of LiteralPath is
# used exactly as it is typed. No characters are interpreted as wildcards.
# If the path includes escape characters, enclose it in single quotation marks.
# Single quotation marks tell Windows PowerShell not to interpret any
# characters as escape sequences.
#
#.PARAMETER Destination
# (Or -d) The location in which to place the chunked output files.
#
#.PARAMETER Count
# (Or -c) The maximum number of lines in each file.
#
#.PARAMETER RepeatCount
# (Or -rc) Specifies the number of "header" lines from the input file that will
# be repeated in each output file. Typically this is 0 or 1 but it can be any
# number of lines.
#
#.EXAMPLE
# Split-File bigfile.csv 3000 -rc 1
#
#.LINK 
# Out-TempFile
##############################################################################
function Split-File {

    [CmdletBinding(DefaultParameterSetName='Path')]
    param(
    
        [Parameter(ParameterSetName='Path', Position=1, Mandatory=$true, ValueFromPipeline=$true, ValueFromPipelineByPropertyName=$true)]
        [String[]]$Path,
        
        [Alias("PSPath")]
        [Parameter(ParameterSetName='LiteralPath', Mandatory=$true, ValueFromPipelineByPropertyName=$true)]
        [String[]]$LiteralPath,
        
        [Alias('c')]
        [Parameter(Position=2,Mandatory=$true)]
        [Int32]$Count,

        [Alias('d')]
        [Parameter(Position=3)]
        [String]$Destination='.',
                
        [Alias('rc')]
        [Parameter()]
        [Int32]$RepeatCount
                
    )

    process {

        # yeah! the cmdlet supports wildcards
        if ($LiteralPath) { $ResolveArgs = @{LiteralPath=$LiteralPath} }
        elseif ($Path) { $ResolveArgs = @{Path=$Path} }

        Resolve-Path @ResolveArgs | %{

            $InputName = [IO.Path]::GetFileNameWithoutExtension($_)
            $InputExt  = [IO.Path]::GetExtension($_)

            if ($RepeatCount) { $Header = Get-Content $_ -TotalCount:$RepeatCount }

            # get the input file in manageable chunks
            
            $Part = 1
            Get-Content $_ -ReadCount:$Count | %{
            
                # make an output filename with a suffix
                $OutputFile = Join-Path $Destination ('{0}-{1:0000}{2}' -f ($InputName,$Part,$InputExt))

                # In the first iteration the header will be
                # copied to the output file as usual
                # on subsequent iterations we have to do it
                if ($RepeatCount -and $Part -gt 1) {
                    Set-Content $OutputFile $Header
                }

                # write this chunk to the output file
                Write-Host "Writing $OutputFile"
                Add-Content $OutputFile $_

                $Part += 1
                
            }

        }

    }

}

I found this question while trying to split multiple contacts in a single vCard VCF file to separate files. Here's what I did based on Lee's code. I had to look up how to create a new StreamReader object and changed null to $null.

$reader = new-object System.IO.StreamReader("C:\Contacts.vcf")
$count = 1
$filename = "C:\Contacts\{0}.vcf" -f ($count) 

while(($line = $reader.ReadLine()) -ne $null)
{
    Add-Content -path $fileName -value $line

    if($line -eq "END:VCARD")
    {
        ++$count
        $filename = "C:\Contacts\{0}.vcf" -f ($count)
    }
}

$reader.Close()

Many of these answers were too slow for my source files. My source files were SQL files between 10 MB and 800 MB that needed to split into files of roughly equal line counts.

I found some of the previous answers which use Add-Content to be quite slow. Waiting many hours for a split to finish wasn't uncommon.

I didn't try Typhlosaurus's answer, but it looks to only do splits by file size, not line count.

The following has suited my purposes.

$sw = new-object System.Diagnostics.Stopwatch
$sw.Start()
Write-Host "Reading source file..."
$lines = [System.IO.File]::ReadAllLines("C:\Temp\SplitTest\source.sql")
$totalLines = $lines.Length

Write-Host "Total Lines :" $totalLines

$skip = 0
$count = 100000; # Number of lines per file

# File counter, with sort friendly name
$fileNumber = 1
$fileNumberString = $filenumber.ToString("000")

while ($skip -le $totalLines) {
    $upper = $skip + $count - 1
    if ($upper -gt ($lines.Length - 1)) {
        $upper = $lines.Length - 1
    }

    # Write the lines
    [System.IO.File]::WriteAllLines("C:\Temp\SplitTest\result$fileNumberString.txt",$lines[($skip..$upper)])

    # Increment counters
    $skip += $count
    $fileNumber++
    $fileNumberString = $filenumber.ToString("000")
}

$sw.Stop()

Write-Host "Split complete in " $sw.Elapsed.TotalSeconds "seconds"

For a 54 MB file, I get the output...

Reading source file...
Total Lines : 910030
Split complete in  1.7056578 seconds

I hope others looking for a simple, line-based splitting script that matches my requirements will find this useful.

There's also this quick (and somewhat dirty) one-liner:

$linecount=0; $i=0; Get-Content .\BIG_LOG_FILE.txt | %{ Add-Content OUT$i.log "$_"; $linecount++; if ($linecount -eq 3000) {$I++; $linecount=0 } }

You can tweak the number of first lines per batch by changing the hard-coded 3000 value.

Do this:

FILE 1

There's also this quick (and somewhat dirty) one-liner:

    $linecount=0; $i=0; 
    Get-Content .\BIG_LOG_FILE.txt | %
    { 
      Add-Content OUT$i.log "$_"; 
      $linecount++; 
      if ($linecount -eq 3000) {$I++; $linecount=0 } 
    }

You can tweak the number of first lines per batch by changing the hard-coded 3000 value.

Get-Content C:\TEMP\DATA\split\splitme.txt | Select -First 5000 | out-File C:\temp\file1.txt -Encoding ASCII

FILE 2

Get-Content C:\TEMP\DATA\split\splitme.txt | Select -Skip 5000 | Select -First 5000 | out-File C:\temp\file2.txt -Encoding ASCII

FILE 3

Get-Content C:\TEMP\DATA\split\splitme.txt | Select -Skip 10000 | Select -First 5000 | out-File C:\temp\file3.txt -Encoding ASCII

etc…

I've made a little modification to split files based on size of each part.

##############################################################################
#.SYNOPSIS
# Breaks a text file into multiple text files in a destination, where each
# file contains a maximum number of lines.
#
#.DESCRIPTION
# When working with files that have a header, it is often desirable to have
# the header information repeated in all of the split files. Split-File
# supports this functionality with the -rc (RepeatCount) parameter.
#
#.PARAMETER Path
# Specifies the path to an item. Wildcards are permitted.
#
#.PARAMETER LiteralPath
# Specifies the path to an item. Unlike Path, the value of LiteralPath is
# used exactly as it is typed. No characters are interpreted as wildcards.
# If the path includes escape characters, enclose it in single quotation marks.
# Single quotation marks tell Windows PowerShell not to interpret any
# characters as escape sequences.
#
#.PARAMETER Destination
# (Or -d) The location in which to place the chunked output files.
#
#.PARAMETER Size
# (Or -s) The maximum size of each file. Size must be expressed in MB.
#
#.PARAMETER RepeatCount
# (Or -rc) Specifies the number of "header" lines from the input file that will
# be repeated in each output file. Typically this is 0 or 1 but it can be any
# number of lines.
#
#.EXAMPLE
# Split-File bigfile.csv -s 20 -rc 1
#
#.LINK 
# Out-TempFile
##############################################################################
function Split-File {

    [CmdletBinding(DefaultParameterSetName='Path')]
    param(

        [Parameter(ParameterSetName='Path', Position=1, Mandatory=$true, ValueFromPipeline=$true, ValueFromPipelineByPropertyName=$true)]
        [String[]]$Path,

        [Alias("PSPath")]
        [Parameter(ParameterSetName='LiteralPath', Mandatory=$true, ValueFromPipelineByPropertyName=$true)]
        [String[]]$LiteralPath,

        [Alias('s')]
        [Parameter(Position=2,Mandatory=$true)]
        [Int32]$Size,

        [Alias('d')]
        [Parameter(Position=3)]
        [String]$Destination='.',

        [Alias('rc')]
        [Parameter()]
        [Int32]$RepeatCount

    )

    process {

  # yeah! the cmdlet supports wildcards
        if ($LiteralPath) { $ResolveArgs = @{LiteralPath=$LiteralPath} }
        elseif ($Path) { $ResolveArgs = @{Path=$Path} }

        Resolve-Path @ResolveArgs | %{

            $InputName = [IO.Path]::GetFileNameWithoutExtension($_)
            $InputExt  = [IO.Path]::GetExtension($_)
   
            if ($RepeatCount) { $Header = Get-Content $_ -TotalCount:$RepeatCount }
   
   Resolve-Path @ResolveArgs | %{

    $InputName = [IO.Path]::GetFileNameWithoutExtension($_)
    $InputExt  = [IO.Path]::GetExtension($_)
 
    if ($RepeatCount) { $Header = Get-Content $_ -TotalCount:$RepeatCount }
 
    # get the input file in manageable chunks
 
    $Part = 1
    $buffer = ""
    Get-Content $_ -ReadCount:1 | %{
 
     # make an output filename with a suffix
     $OutputFile = Join-Path $Destination ('{0}-{1:0000}{2}' -f ($InputName,$Part,$InputExt))
 
     # In the first iteration the header will be
     # copied to the output file as usual
     # on subsequent iterations we have to do it
     if ($RepeatCount -and $Part -gt 1) {
      Set-Content $OutputFile $Header
     }
 
     # test buffer size and dump data only if buffer is greater than size
     if ($buffer.length -gt ($Size * 1MB)) {
      # write this chunk to the output file
      Write-Host "Writing $OutputFile"
      Add-Content $OutputFile $buffer
      $Part += 1
      $buffer = ""
     } else {
      $buffer += $_ + "`r"
     }
    }
   }
        }
    }
}

Sounds like a job for the UNIX command split:

split MyBigFile.csv

Just split my 55 GB csv file in 21k chunks in less than 10 minutes.

It's not native to PowerShell though, but comes with, for instance, the git for windows package https://git-scm.com/download/win

As the lines can be variable in logs I thought it best to take a number of lines per file approach. The following code snippet processed a 4 million line log file in under 19 seconds (18.83.. seconds)splitting it into 500,000 line chunks:

$sourceFile = "c:\myfolder\mylargeTextyFile.csv"
$partNumber = 1
$batchSize = 500000
$pathAndFilename = "c:\myfolder\mylargeTextyFile part $partNumber file.csv"

[System.Text.Encoding]$enc = [System.Text.Encoding]::GetEncoding(65001)  # utf8 this one

$fs=New-Object System.IO.FileStream ($sourceFile,"OpenOrCreate", "Read", "ReadWrite",8,"None") 
$streamIn=New-Object System.IO.StreamReader($fs, $enc)
$streamout = new-object System.IO.StreamWriter $pathAndFilename

$line = $streamIn.readline()
$counter = 0
while ($line -ne $null)
{
	$streamout.writeline($line)
	$counter +=1
	if ($counter -eq $batchsize)
	{
		$partNumber+=1
		$counter =0
		$streamOut.close()
		$pathAndFilename = "c:\myfolder\mylargeTextyFile part $partNumber file.csv"
		$streamout = new-object System.IO.StreamWriter $pathAndFilename
	
	}
	$line = $streamIn.readline()
}
$streamin.close()
$streamout.close()

This can easily be turned into a function or script file with parameters to make it more versatile. It uses a StreamReader and StreamWriter to achieve its speed and tiny memory footprint

My requirement was a bit different. I often work with Comma Delimited and Tab Delimited ASCII files where a single line is a single record of data. And they're really big, so I need to split them into manageable parts (whilst preserving the header row).

So, I reverted back to my classic VBScript method and bashed together a small .vbs script that can be run on any Windows computer (it gets automatically executed by the WScript.exe script host engine on Window).

The benefit of this method is that it uses Text Streams, so the underlying data isn't loaded into memory (or, at least, not all at once). The result is that it's exceptionally fast and it doesn't really need much memory to run. The test file I just split using this script on my i7 was about 1 GB in file size, had about 12 million lines of text and was split into 25 part files (each with about 500k lines each) – the processing took about 2 minutes and it didn’t go over 3 MB memory used at any point.

The caveat here is that it relies on the text file having "lines" (meaning each record is delimited with a CRLF) as the Text Stream object uses the "ReadLine" function to process a single line at a time. But hey, if you're working with TSV or CSV files, it's perfect.

Option Explicit

Private Const INPUT_TEXT_FILE = "c:\bigtextfile.txt"  
Private Const REPEAT_HEADER_ROW = True                
Private Const LINES_PER_PART = 500000                 

Dim oFileSystem, oInputFile, oOutputFile, iOutputFile, iLineCounter, sHeaderLine, sLine, sFileExt, sStart

sStart = Now()

sFileExt = Right(INPUT_TEXT_FILE,Len(INPUT_TEXT_FILE)-InstrRev(INPUT_TEXT_FILE,".")+1)
iLineCounter = 0
iOutputFile = 1

Set oFileSystem = CreateObject("Scripting.FileSystemObject")
Set oInputFile = oFileSystem.OpenTextFile(INPUT_TEXT_FILE, 1, False)
Set oOutputFile = oFileSystem.OpenTextFile(Replace(INPUT_TEXT_FILE, sFileExt, "_" & iOutputFile & sFileExt), 2, True)

If REPEAT_HEADER_ROW Then
	iLineCounter = 1
	sHeaderLine = oInputFile.ReadLine()
	Call oOutputFile.WriteLine(sHeaderLine)
End If

Do While Not oInputFile.AtEndOfStream
	sLine = oInputFile.ReadLine()
	Call oOutputFile.WriteLine(sLine)
	iLineCounter = iLineCounter + 1
	If iLineCounter Mod LINES_PER_PART = 0 Then
		iOutputFile = iOutputFile + 1
		Call oOutputFile.Close()
		Set oOutputFile = oFileSystem.OpenTextFile(Replace(INPUT_TEXT_FILE, sFileExt, "_" & iOutputFile & sFileExt), 2, True)
		If REPEAT_HEADER_ROW Then
			Call oOutputFile.WriteLine(sHeaderLine)
		End If
	End If
Loop

Call oInputFile.Close()
Call oOutputFile.Close()
Set oFileSystem = Nothing

Call MsgBox("Done" & vbCrLf & "Lines Processed:" & iLineCounter & vbCrLf & "Part Files: " & iOutputFile & vbCrLf & "Start Time: " & sStart & vbCrLf & "Finish Time: " & Now())

Here is my solution to split a file called patch6.txt (about 32,000 lines) into separate files of 1000 lines each. Its not quick, but it does the job.

$infile = "D:\Malcolm\Test\patch6.txt"
$path = "D:\Malcolm\Test\"
$lineCount = 1
$fileCount = 1

foreach ($computername in get-content $infile)
{
    write $computername | out-file -Append $path_$fileCount".txt"
    $lineCount++

    if ($lineCount -eq 1000)
    {
        $fileCount++
        $lineCount = 1
    }
}