config: Add getenv interpolation function

config/interpolate_funcs.go

@@ -11,6 +11,7 @@ import (
 	"io/ioutil"
 	"math"
 	"net"
+	"os"
 	"path/filepath"
 	"regexp"
 	"sort"
@@ -70,6 +71,7 @@ func Funcs() map[string]ast.Function {
 		"distinct":     interpolationFuncDistinct(),
 		"element":      interpolationFuncElement(),
 		"file":         interpolationFuncFile(),
+		"getenv":       interpolationFuncGetenv(),
 		"matchkeys":    interpolationFuncMatchKeys(),
 		"floor":        interpolationFuncFloor(),
 		"format":       interpolationFuncFormat(),
@@ -1344,3 +1346,17 @@ func interpolationFuncSubstr() ast.Function {
 		},
 	}
 }
+
+// interpolationFuncGetenv implements the "getenv" function, which literally
+// passes off to os.Getenv in the Go standard library. This function returns
+// the value of the requested environment variable, or an empty string if it
+// does not exist.
+func interpolationFuncGetenv() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeString},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			return os.Getenv(args[0].(string)), nil
+		},
+	}
+}

config/interpolate_funcs_test.go

@@ -2317,3 +2317,35 @@ func TestInterpolateFuncSubstr(t *testing.T) {
 		},
 	})
 }
+
+func TestInterpolateFuncGetenv(t *testing.T) {
+	os.Setenv("TF_INTERP_FUNC_GETENV_TEST", "foobar")
+	testFunction(t, testFunctionConfig{
+		Vars: map[string]ast.Variable{
+			"var.getenv_test":         interfaceToVariableSwallowError("TF_INTERP_FUNC_GETENV_TEST"),
+			"var.getenv_test_missing": interfaceToVariableSwallowError("TF_INTERP_FUNC_GETENV_TEST_MISSING"),
+		},
+		Cases: []testFunctionCase{
+			{
+				`${getenv("TF_INTERP_FUNC_GETENV_TEST")}`,
+				"foobar",
+				false,
+			},
+			{
+				`${getenv("TF_INTERP_FUNC_GETENV_TEST_MISSING")}`,
+				"",
+				false,
+			},
+			{
+				`${getenv(var.getenv_test)}`,
+				"foobar",
+				false,
+			},
+			{
+				`${getenv(var.getenv_test_missing)}`,
+				"",
+				false,
+			},
+		},
+	})
+}

website/source/docs/configuration/environment-variables.html.md

@@ -99,3 +99,11 @@ tests, but by setting this variable you can force these tests to be skipped.
 export TF_SKIP_REMOTE_TESTS=1
 make test
 ```
+
+## The `getenv` Interpolation Function
+
+The [`getenv`](/docs/configuration/interpolation.html#getenv-key-) interpolation
+function can be used to get the value of any environment variable available to
+the Terraform process. Note that this function is subject to the same rules as
+any other interpolation function and as such will not work within `variable`
+blocks - use `TF_VAR_` variables to assign values instead.

website/source/docs/configuration/interpolation.html.md

@@ -246,6 +246,14 @@ The supported built-in functions are:
       `formatlist("instance %v has private ip %v", aws_instance.foo.*.id, aws_instance.foo.*.private_ip)`.
       Passing lists with different lengths to formatlist results in an error.
 
+  * `getenv(key)` - Gets the value for the environment variable defined by
+     `key`. Returns an empty string if the environment variable does not exist.
+     Note that this functionality differs from the
+     [`TF_VAR_`](/docs/configuration/environment-variables.html#tf_var_name) pattern
+     in that it does not assign values to
+     [variables](/docs/configuration/variables.html), and allows you to access any
+     environment variable available to the Terraform process.
+
   * `index(list, elem)` - Finds the index of a given element in a list.
       This function only works on flat lists.
       Example: `index(aws_instance.foo.*.tags.Name, "foo-test")`