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I maintain a set of virtual machine templates that use Packer. For VMware VMs, the workflow is to build them locally using the vmware-iso builder, then push to vSphere using the vsphere post-processor. The idea behind this approach is to be able to use the same templates for local VMs (which are exported as an ova archive) as those that end up on vSphere. Unfortunately, there’s some differences between VMware Workstation and vSphere that create some tricky problems like mismatched operating system versions.

Fortunately, PowerCLI exists which makes it fairly pleasant to work with vSphere from PowerShell. Packer can run local scripts, so I hatched the plan to run a PowerShell script (using the shell-local provisioner) that would adjust settings after the build had run. Alas, this is one of those problems that took many months of evenings to figure out how to do.

I’m using Packer v1.11.2 and PowerShell 7.4.4, and running everything on Linux. Here’s an example that works, and an explanation of how it got here below:

# hello-world.ps1
Write-Output "Hello world!"

Write-Output $env:MY_VAR
Write-Output $env:WITH_SPACES
Write-Output $env:PACKER_BUILDER_TYPE
Write-Output $env:PACKER_BUILD_NAME

Write-Output "and done!"

# example.pkr.hcl
source "null" "example" {
  communicator = "none"
}

build {
  source "source.null.example" {}

  provisioner "shell-local" {
    env                = {
      "MY_VAR": "hi",
      "WITH_SPACES": "and again"
    }

    execute_command    = ["pwsh", "-Command", "& {   }"]
    env_var_format     = "$env:%s=\"%s\"; "
    script             = "hello-world.ps1"
  }
}

$ packer build example.pkr.hcl
null.example: output will be in this color.

==> null.example: Running local shell script: hello-world.ps1
    null.example: Hello world!
    null.example: hi
    null.example: hello world
    null.example: null
    null.example: example
    null.example: and done!
Build 'null.example' finished after 450 milliseconds 889 microseconds.

==> Wait completed after 450 milliseconds 927 microseconds

==> Builds finished. The artifacts of successful builds are:
--> null.example: Did not export anything. This is the null builder

• This example uses the “null” builder, as we just want to run a local file, it’s really helpful for testing,
• Using “shell-local” we can execute a file on the local filesystem, which would usually default to a shell script (on a Unix),
• But, this can be any command, and there’s a few options we can use to adjust how the command is put together,
• Packer exposes the builder type and build name as environment variables or anything set in environment_vars or env, which will be key to providing things like secrets to the script later on,
• Unfortunately, PowerShell doesn’t accept environment variables as arguments, and seems to fail when assigned ahead of the command (e.g.: ` bash ‘’` would usually work in an existing shell session),
• Instead we can use a PowerShell block ({ }), and use & in front to execute immediately,
• There’s two key things this is doing to make this work reliably, first, we provide a different template to env_var_format so that the output is how PowerShell expects it’s environment variables (and how it ends up in the block), secondly, we use env rather than environment_vars and provide a map of values as this means that the output is escaped correctly