Tip: Get a Windows VM to run a batch file at boot

With the virt-win-reg tool built on top of libguestfs and hivex it’s now relatively straightforward to modify a Windows virtual machine so that it runs a batch file, script or program at next boot.

Note: The Windows VM must be shut down before you attempt this.

The plan is that we upload the batch script to some place in the VM, and then add a “RunOnce” key in the Windows Registry (explained in this MSDN article and this article). First let’s just take a look at what’s in the key. In most cases it will be empty:

# virt-win-reg Windows7x64 \
    'HKLM\Software\Microsoft\Windows\CurrentVersion\RunOnce'
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce]

Now we’ll prepare our batch file and upload it:

# cat test.bat
ECHO HELLO > C:\TEST.LOG
TIME /T >> C:\TEST.LOG
# guestfish -i Windows7x64

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for a list of commands
      'man' to read the manual
      'quit' to quit the shell

><fs> upload test.bat /test.bat
><fs> ^D

And finally we modify the RunOnce registry key:

# virt-win-reg --merge Windows7x64
[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunOnce]
"Test"="c:\\test.bat"
^D

One potential gotcha: You must be running hivex ≥ 1.2.2.

Now you can boot your Windows guest and check that the script runs after the user logs in. Look for the file C:\TEST.LOG:

><fs> cat /TEST.LOG
HELLO 
09:21

Because we’re using the RunOnce key, the script will run just one time. If you want it to run every time, use the Run key.

Now, how do we make the script work without the user needing to log in? (Clue: The answer is not RunServicesOnce — this does not work in Windows 7). What’s surprising (coming from a Linux background) is the huge amount of incomplete, contradictory and simply false information contained in MSDN about this topic.