Add post on running PowerShell scripts with Packer

_posts/2024-08-15-running-powershell-scripts-locally-with-packer.md

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+---
+title: "Running PowerShell scripts locally with Packer"
+tags: packer powershell
+---
+
+I maintain a set of virtual machine templates that use [Packer][4]. For
+[VMware][5] 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][6] 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:
+
+```pwsh
+# 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 $env:PACKER_BUILD_NAME
+
+Write-Output "and done!"
+```
+
+```hcl
+# example.pkr.hcl
+source "null" "example" {
+  communicator = "none"
+}
+
+build {
+  source "source.null.example" {}
+
+  provisioner "shell-local" {
+    env                = {
+      "MY_VAR": "hi",
+      "WITH_SPACES": "and again"
+    }
+
+    execute_command    = ["pwsh", "-Command", "& { {{.Vars}} {{.Script}} }"]
+    env_var_format     = "$env:%s=\"%s\"; "
+    script             = "hello-world.ps1"
+  }
+}
+```
+
+```sh
+$ 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"][2] 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][1]
+  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][3], and seems to fail when assigned ahead of the command (e.g.:
+  `{{.Vars}} bash '{{.Path}}'` 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
+
+[1]: https://developer.hashicorp.com/packer/docs/post-processors/shell-local#default-environmental-variables
+[2]: https://developer.hashicorp.com/packer/docs/post-processors/shell-local
+[3]: https://github.com/PowerShell/PowerShell/issues/3316
+[4]: https://www.packer.io
+[5]: https://www.vmware.com
+[6]: https://docs.vmware.com/en/VMware-PowerCLI/index.html