What causes "Error processing command: %s" in hashicorp/packer?

Fix the template syntax in execute_command — check for balanced {{ }} braces

Error processing command: %s

Error message

Error processing command: %s

What it means

The shell provisioner renders the user-supplied `execute_command` HCL/Go template (with generated data like .Path, .Vars, .EnvVarFile) before running it on the remote machine. If template interpolation fails — e.g. a syntax error or a reference to an unknown variable/sprig function — Provision aborts with this message wrapping the underlying render error. It is thrown before anything is uploaded, so no remote state was changed.

Source

Thrown at provisioner/shell/provisioner.go:315

		ui.Say(fmt.Sprintf("Provisioning with shell script: %s", path))

		log.Printf("Opening %s for reading", path)
		f, err := os.Open(path)
		if err != nil {
			return fmt.Errorf("Error opening shell script: %s", err)
		}
		defer f.Close()

		// Compile the command
		// These are extra variables that will be made available for interpolation.
		generatedData["Vars"] = flattenedEnvVars
		generatedData["EnvVarFile"] = p.config.envVarFile
		generatedData["Path"] = p.config.RemotePath
		p.config.ctx.Data = generatedData

		command, err := interpolate.Render(p.config.ExecuteCommand, &p.config.ctx)
		if err != nil {
			return fmt.Errorf("Error processing command: %s", err)
		}

		// Upload the file and run the command. Do this in the context of
		// a single retryable function so that we don't end up with
		// the case that the upload succeeded, a restart is initiated,
		// and then the command is executed but the file doesn't exist
		// any longer.
		var cmd *packersdk.RemoteCmd
		err = retry.Config{StartTimeout: p.config.StartRetryTimeout}.Run(ctx, func(ctx context.Context) error {
			if _, err := f.Seek(0, 0); err != nil {
				return err
			}

			var r io.Reader = f
			if !p.config.Binary {
				r = &UnixReader{Reader: r}
			}

View on GitHub (pinned to eb36e3c3e4)

Solutions

Fix the template syntax in execute_command — check for balanced {{ }} braces
Use only the provided variables: .Path, .Vars, .EnvVarFile (and configured template variables)
Run `packer validate` / `packer inspect` to catch template errors before a build
If using custom env vars, ensure they are defined so {{ .Vars }} renders correctly

Example fix

// before
execute_command = "chmod +x {{ .Pathy }}; {{ .Pathy }}"
// after
execute_command = "chmod +x {{ .Path }}; {{ .Vars }} {{ .Path }}"

Defensive patterns

Strategy: validation

Validate before calling

# Validate the template renders before a build; catch bad vars/braces early.
execute_command = "chmod +x {{ .Path }}; {{ .Vars }} {{ .Path }}"
# then:
# packer validate template.pkr.hcl

Prevention

Only reference generated variables Packer provides (.Path, .Vars, .EnvVarFile) unless you added custom ones
Keep execute_command templates simple; test complex logic inside the script file itself
Run packer validate/inspect before builds to catch template syntax errors
Avoid hand-copying {{ }} snippets from other templates without checking variable names

When it happens

Trigger: Calling provision (via `packer build`) with an execute_command containing invalid template syntax such as `{{` without closing braces, an unknown template function like `{{ .Foo }}` when Foo is not a provided variable, or invalid sprig pipeline usage.

Common situations: Typo'd template variables (e.g. `{{ .Pathy }}` instead of `{{ .Path }}`); copying a command with `{{.Vars}}` into a context where env vars were not defined; hand-edited templates with unbalanced `{{` `}}`; mixing legacy JSON template syntax with HCL2 variables.

Related errors

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/320cb7e35217a546. Report an issue: GitHub.