diff --git a/common/command/build_flags.go b/common/command/build_flags.go
index 477c83ef8..167e13310 100644
--- a/common/command/build_flags.go
+++ b/common/command/build_flags.go
@@ -2,6 +2,8 @@ package command
 
 import (
 	"flag"
+	"fmt"
+	"strings"
 )
 
 // BuildOptionFlags sets the proper command line flags needed for
@@ -9,4 +11,28 @@ import (
 func BuildOptionFlags(fs *flag.FlagSet, f *BuildOptions) {
 	fs.Var((*SliceValue)(&f.Except), "except", "build all builds except these")
 	fs.Var((*SliceValue)(&f.Only), "only", "only build the given builds by name")
+	fs.Var((*userVarValue)(&f.UserVars), "var", "specify a user variable")
+}
+
+// userVarValue is a flag.Value that parses out user variables in
+// the form of 'key=value' and sets it on this map.
+type userVarValue map[string]string
+
+func (v *userVarValue) String() string {
+	return ""
+}
+
+func (v *userVarValue) Set(raw string) error {
+	idx := strings.Index(raw, "=")
+	if idx == -1 {
+		return fmt.Errorf("No '=' value in arg: %s", raw)
+	}
+
+	if *v == nil {
+		*v = make(map[string]string)
+	}
+
+	key, value := raw[0:idx], raw[idx+1:]
+	(*v)[key] = value
+	return nil
 }
diff --git a/common/command/build_flags_test.go b/common/command/build_flags_test.go
new file mode 100644
index 000000000..b4f9753fe
--- /dev/null
+++ b/common/command/build_flags_test.go
@@ -0,0 +1,97 @@
+package command
+
+import (
+	"flag"
+	"reflect"
+	"testing"
+)
+
+func TestBuildOptionFlags(t *testing.T) {
+	opts := new(BuildOptions)
+	fs := flag.NewFlagSet("test", flag.ContinueOnError)
+	BuildOptionFlags(fs, opts)
+
+	args := []string{
+		"-except=foo,bar,baz",
+		"-only=a,b",
+		"-var=foo=bar",
+		"-var", "bar=baz",
+		"-var=foo=bang",
+	}
+
+	err := fs.Parse(args)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	expected := []string{"foo", "bar", "baz"}
+	if !reflect.DeepEqual(opts.Except, expected) {
+		t.Fatalf("bad: %#v", opts.Except)
+	}
+
+	expected = []string{"a", "b"}
+	if !reflect.DeepEqual(opts.Only, expected) {
+		t.Fatalf("bad: %#v", opts.Only)
+	}
+
+	if len(opts.UserVars) != 2 {
+		t.Fatalf("bad: %#v", opts.UserVars)
+	}
+
+	if opts.UserVars["foo"] != "bang" {
+		t.Fatalf("bad: %#v", opts.UserVars)
+	}
+
+	if opts.UserVars["bar"] != "baz" {
+		t.Fatalf("bad: %#v", opts.UserVars)
+	}
+}
+
+func TestUserVarValue_implements(t *testing.T) {
+	var raw interface{}
+	raw = new(userVarValue)
+	if _, ok := raw.(flag.Value); !ok {
+		t.Fatalf("userVarValue should be a Value")
+	}
+}
+
+func TestUserVarValueSet(t *testing.T) {
+	sv := new(userVarValue)
+	err := sv.Set("key=value")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	vars := map[string]string(*sv)
+	if vars["key"] != "value" {
+		t.Fatalf("Bad: %#v", vars)
+	}
+
+	// Empty value
+	err = sv.Set("key=")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	vars = map[string]string(*sv)
+	if vars["key"] != "" {
+		t.Fatalf("Bad: %#v", vars)
+	}
+
+	// Equal in value
+	err = sv.Set("key=foo=bar")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	vars = map[string]string(*sv)
+	if vars["key"] != "foo=bar" {
+		t.Fatalf("Bad: %#v", vars)
+	}
+
+	// No equal
+	err = sv.Set("key")
+	if err == nil {
+		t.Fatal("should have error")
+	}
+}
diff --git a/common/command/template.go b/common/command/template.go
index 90280de9f..a323523b0 100644
--- a/common/command/template.go
+++ b/common/command/template.go
@@ -10,8 +10,9 @@ import (
 // BuildOptions is a set of options related to builds that can be set
 // from the command line.
 type BuildOptions struct {
-	Except []string
-	Only   []string
+	UserVars map[string]string
+	Except   []string
+	Only     []string
 }
 
 // Validate validates the options