do not pass null value variables

cli/args.go

@@ -104,6 +104,8 @@ func parseTerragruntOptionsFromArgs(terragruntVersion string, args []string, wri
 
 	includeExternalDependencies := parseBooleanArg(args, OPT_TERRAGRUNT_INCLUDE_EXTERNAL_DEPENDENCIES, false)
 
+	passNullVars := parseBooleanArg(args, OPT_TERRAGRUNT_PASS_NULL_VARS, false)
+
 	iamRole, err := parseStringArg(args, OPT_TERRAGRUNT_IAM_ROLE, os.Getenv("TERRAGRUNT_IAM_ROLE"))
 	if err != nil {
 		return nil, err
@@ -170,6 +172,7 @@ func parseTerragruntOptionsFromArgs(terragruntVersion string, args []string, wri
 	opts.HclFile = filepath.ToSlash(terragruntHclFilePath)
 	opts.Debug = debug
 	opts.AwsProviderPatchOverrides = awsProviderPatchOverrides
+	opts.PassNullVars = passNullVars
 
 	return opts, nil
 }
@@ -386,18 +389,21 @@ func parseMutliStringKeyValueArg(args []string, argName string, defaultValue map
 // Convert the given variables to a map of environment variables that will expose those variables to Terraform. The
 // keys will be of the format TF_VAR_xxx and the values will be converted to JSON, which Terraform knows how to read
 // natively.
-func toTerraformEnvVars(vars map[string]interface{}) (map[string]string, error) {
+func toTerraformEnvVars(vars map[string]interface{}, passNullVars bool) (map[string]string, error) {
 	out := map[string]string{}
 
 	for varName, varValue := range vars {
-		envVarName := fmt.Sprintf("TF_VAR_%s", varName)
+		// Do not pass null value variables if option is not set
+		// null assigned variables in terraform are "unset" variables (do not exist)
+		if varValue != nil || passNullVars {
+			envVarName := fmt.Sprintf("TF_VAR_%s", varName)
+			envVarValue, err := asTerraformEnvVarJsonValue(varValue)
+			if err != nil {
+				return nil, err
+			}
 
-		envVarValue, err := asTerraformEnvVarJsonValue(varValue)
-		if err != nil {
-			return nil, err
+			out[envVarName] = string(envVarValue)
 		}
-
-		out[envVarName] = string(envVarValue)
 	}
 
 	return out, nil

cli/cli_app.go

@@ -43,6 +43,7 @@ const OPT_TERRAGRUNT_CHECK = "terragrunt-check"
 const OPT_TERRAGRUNT_HCLFMT_FILE = "terragrunt-hclfmt-file"
 const OPT_TERRAGRUNT_DEBUG = "terragrunt-debug"
 const OPT_TERRAGRUNT_OVERRIDE_ATTR = "terragrunt-override-attr"
+const OPT_TERRAGRUNT_PASS_NULL_VARS = "terragrunt-pass-null-vars"
 
 var ALL_TERRAGRUNT_BOOLEAN_OPTS = []string{
 	OPT_NON_INTERACTIVE,
@@ -56,6 +57,7 @@ var ALL_TERRAGRUNT_BOOLEAN_OPTS = []string{
 	OPT_TERRAGRUNT_CHECK,
 	OPT_TERRAGRUNT_STRICT_INCLUDE,
 	OPT_TERRAGRUNT_DEBUG,
+	OPT_TERRAGRUNT_PASS_NULL_VARS,
 }
 var ALL_TERRAGRUNT_STRING_OPTS = []string{
 	OPT_TERRAGRUNT_CONFIG,
@@ -181,6 +183,7 @@ GLOBAL OPTIONS:
    terragrunt-hclfmt-file                       The path to a single terragrunt.hcl file that the hclfmt command should run on.
    terragrunt-override-attr                     A key=value attribute to override in a provider block as part of the aws-provider-patch command. May be specified multiple times.
    terragrunt-debug                             Write terragrunt-debug.tfvars to working folder to help root-cause issues.
+   terragrunt-pass-null-vars                    Pass null value input variables to Terraform (as "null" string)
 
 VERSION:
    {{.Version}}{{if len .Authors}}
@@ -567,7 +570,7 @@ func runActionWithHooks(description string, terragruntOptions *options.Terragrun
 // The Terragrunt configuration can contain a set of inputs to pass to Terraform as environment variables. This method
 // sets these environment variables in the given terragruntOptions.
 func setTerragruntInputsAsEnvVars(terragruntOptions *options.TerragruntOptions, terragruntConfig *config.TerragruntConfig) error {
-	asEnvVars, err := toTerraformEnvVars(terragruntConfig.Inputs)
+	asEnvVars, err := toTerraformEnvVars(terragruntConfig.Inputs, terragruntOptions.PassNullVars)
 	if err != nil {
 		return err
 	}

docs/_docs/04_reference/cli-options.md

@@ -284,6 +284,7 @@ prefix `--terragrunt-` (e.g., `--terragrunt-config`). The currently available op
 - [terragrunt-check](#terragrunt-check)
 - [terragrunt-hclfmt-file](#terragrunt-hclfmt-file)
 - [terragrunt-override-attr](#terragrunt-override-attr)
+- [terragrunt-pass-null-vars](#terragrunt-pass-null-vars)
 
 
 ### terragrunt-config
@@ -502,3 +503,9 @@ When passed in, run `hclfmt` only on specified `*/terragrunt.hcl` file.
 
 A `KEY=VALUE` attribute to override in a `provider` block as part of the [aws-provider-patch 
 command](#aws-provider-patch). May be specified multiple times.
+
+### terragrunt-pass-null-vars
+
+**CLI Arg**: `--terragrunt-pass-null-vars`
+
+When passed in, Terragrunt will pass null value input variables to Terraform as string (i.e: "null")

options/options.go

@@ -146,6 +146,9 @@ type TerragruntOptions struct {
 	// Attributes to override in AWS provider nested within modules as part of the aws-provider-patch command. See that
 	// command for more info.
 	AwsProviderPatchOverrides map[string]string
+
+	// Pass to Terraform null value variables as string (terraform does not export null variables)
+	PassNullVars bool
 }
 
 // Create a new TerragruntOptions object with reasonable defaults for real usage
@@ -260,6 +263,7 @@ func (terragruntOptions *TerragruntOptions) Clone(terragruntConfigPath string) *
 		StrictInclude:               terragruntOptions.StrictInclude,
 		RunTerragrunt:               terragruntOptions.RunTerragrunt,
 		AwsProviderPatchOverrides:   terragruntOptions.AwsProviderPatchOverrides,
+		PassNullVars:                terragruntOptions.PassNullVars,
 	}
 }