Allow extvars to be populated from envvars

cmd/root.go

@@ -90,10 +90,17 @@ func JsonnetVM(cmd *cobra.Command) (*jsonnet.VM, error) {
 	}
 	for _, extvar := range extvars {
 		kv := strings.SplitN(extvar, "=", 2)
-		if len(kv) != 2 {
-			return nil, fmt.Errorf("Failed to parse extvar: missing '=' in %s", extvar)
+		switch len(kv) {
+		case 1:
+			v, present := os.LookupEnv(kv[0])
+			if present {
+				vm.ExtVar(kv[0], v)
+			} else {
+				return nil, fmt.Errorf("Missing environment variable: %s", kv[0])
+			}
+		case 2:
+			vm.ExtVar(kv[0], kv[1])
 		}
-		vm.ExtVar(kv[0], kv[1])
 	}
 
 	resolver, err := buildResolver(cmd)

cmd/show_test.go

@@ -3,6 +3,7 @@ package cmd
 import (
 	"bytes"
 	"encoding/json"
+	"os"
 	"path/filepath"
 	"reflect"
 	"testing"
@@ -46,6 +47,7 @@ func TestShow(t *testing.T) {
   "number": 42,
   "string": "bar",
   "notAVal": "aVal",
+  "notAnotherVal": "aVal2",
   "array": ["one", 2, [3]],
   "object": {"foo": "bar"}
 }
@@ -57,11 +59,15 @@ func TestShow(t *testing.T) {
 			t.Errorf("error parsing *expected* value: %s", err)
 		}
 
+		os.Setenv("anVar", "aVal2")
+		defer os.Unsetenv("anVar")
+
 		output := cmdOutput(t, []string{"show",
 			"-J", filepath.FromSlash("../testdata/lib"),
 			"-o", format,
 			filepath.FromSlash("../testdata/test.jsonnet"),
 			"-V", "aVar=aVal",
+			"-V", "anVar",
 		})
 
 		t.Log("output is", output)

testdata/test.jsonnet

 local test = import "test.libsonnet";
 local aVar = std.extVar("aVar");
+local anVar = std.extVar("anVar");
 
 {
   apiVersion: "v1",
@@ -8,6 +9,7 @@ local aVar = std.extVar("aVar");
     test {
       string: "bar",
       notAVal : aVar,
+      notAnotherVal : anVar,
     }
   ],
 }