This is the first post in a series I’m calling “Scripting Notebook,” in which I will explore ways of accomplishing various scripting tasks across several languages: Bash, CMD, PowerShell, and Python.

Creating a File of Arbitrary Size

Sometimes when you’re testing, for whatever reason, you need one or more files that are a particular size. You don’t care what they are, you just need some data to take up disk space, transfer over the network, etc. So here, in our four target languages,¹ I’m going to show how to do exactly that.

CMD

In CMD, otherwise known as the Windows Command Prompt, we rely on an old tool called Fsutil:

REM Create 1 GB test file:
fsutil file createnew one-gigabyte-file-cmd.dat 1073741824

PowerShell

We’re cheating a little bit here, using .NET Framework’s System.IO.FileStream Class to do most of the work…

# Create 1 GB test file
$f = New-Object System.IO.FileStream .\one-gigabyte-file-powershell.dat, Create, ReadWrite
$f.SetLength(1GB)
$f.Close()

You may try a “pure” PowerShell implementation like this, it will take a very, very long time:

# DO NOT do this. Poor memory utilization, very long execution time
1..1024 | %{ Add-Content -Value (New-Object byte[] 1mb) -Encoding Byte -Path .\one-gigabyte-file-ps1.dat }

And while you may be tempted to “optimize” that last implementation by using larger objects, it consumes a ton of memory and nasty things will happen. You have been warned!

Python

Here’s a clean and fast implementation in Python. The with operator handles closing out the file for us, as a bonus:

with open("one-gigabyte-file-python.dat", "wb") as out:
    out.truncate(1024**3)

Bash

Then there’s the tried-and-true dd command in bash:

dd if=/dev/zero of=one-gigabyte-file-bash.dat bs=1073741824 count=1

This works well on Windows too, if you’ve got WSL, MinGW or Cygwin installed.

Additional Reading

• Windows Server Documentation: Fsutil
• .NET Framework Documentation: FileStream Class (System.IO)
• GNU Coreutils Documentation: dd invocation
• Python 3 Documentation: Reading and Writing Files

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