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Hashicorp vagrant and the Vagrantfiles.

This post is about a fully documented but easy to miss feature of vagrant, which is the evaluation of the Vagrantfiles when a vagrant ‘box’ is started. As you can see, I wrote ‘Vagrantfiles’, which is plural. I (very naively) thought that the Vagrantfile in the directory where you want to start the ‘box’ determines the specifics of the vagrant ‘box’.

This begins with me trying to create a vagrant virtualbox VM using packer, where I specify a Vagrantfile during the packer build. I normally don’t spend a lot of thought on that, and just put in a very simple Vagrantfile, essentially just defining the box and setting memory and cpu amounts.

In this case, I decided to put in the Vagrantfile that I wanted to use for this special purpose ‘box’ as the Vagrantfile for the packer build, which has some ruby in it which adds a disk:

data1_disk = "data1.vdi"
if !File.exist?(data1_disk)
  vb.customize [ 'createhd', '--filename', data1_disk, '--size', 20480 ]
end
vb.customize [ 'storageattach', :id, '--storagectl', 'SATA Controller', '--port', 2, '--device', 0, '--type', 'hdd', '--medium', data1_disk ]

My (wrong) assumption was that this Vagrantfile was kept as a validation or template.

After the box was created, I tested running it, and therefore I copied the Vagrantfile including the above addition of checking for a file, if it doesn’t exist creating it, and then attaching the disk to the VM. However, this did fail with an error in the provisioning stage that a disk was already added, and therefore could not be added again.

I didn’t understand the error when I first encountered it, so I investigated the issue by removing the box (vagrant destroy), and commenting out the disk addition code (the above 5 lines) from the Vagrantfile, and then run the box addition again (vagrant up). Much to my surprise, it did add the disk for which I explicitly removed the code that provided that functionality.

This severely puzzled me.

After going over the vagrant documentation and searching for the error messages, I found out that the Vagrantfile embedded with the ‘box’ is actually used, and parsed, along with the user specified Vagrantfile, and the contents of both are merged before a vagrant ‘box’ is started. This Vagrantfile can be seen in the directory which holds the zipped boxes after downloading from app.vagrantup.com. On my Mac this is ‘~/.vagrant.d/boxes/{box name}/{version}/{provider}/Vagrantfile’.

This perfectly explained what I witnessed: because the disk creation steps are in the ’embedded’ Vagrantfile, it will simply be executed, even if it’s not in the normal Vagrantfile. And of course it threw an error when the disk creation steps were added to the normal Vagrantfile, because then these steps were executed twice, which would show at the vb.customize ‘storageattach’ execution, because that is not protected by a check, so the second occurrence would be tried and fails.

This was really my ignorance, and maybe a bit the documentation not being overly verbose about the existence of more than one Vagrantfile. It also gives great opportunities: lots of steps or logic that would normally be in the Vagrantfile could be put in the embedded Vagrantfile so that regular Vagrantfile can be kept really simple.

Conclusion is that if you create vagrant boxes yourself, when you want to perform provisioning steps against a vagrant box that simply need to be done, you might as well put them in the embedded Vagrantfile.