Add EnvFlag util

go.mod

@@ -3,6 +3,7 @@ module github.com/alecthomas/kong
 require (
 	github.com/alecthomas/assert/v2 v2.1.0
 	github.com/alecthomas/repr v0.1.0
+	github.com/joho/godotenv v1.5.1
 )
 
 require github.com/hexops/gotextdiff v1.0.3 // indirect

go.sum

@@ -4,3 +4,5 @@ github.com/alecthomas/repr v0.1.0 h1:ENn2e1+J3k09gyj2shc0dHr/yjaWSHRlrJ4DPMevDqE
 github.com/alecthomas/repr v0.1.0/go.mod h1:2kn6fqh/zIyPLmm3ugklbEi5hg5wS435eygvNfaDQL8=
 github.com/hexops/gotextdiff v1.0.3 h1:gitA9+qJrrTCsiCl7+kh75nPqQt1cx4ZkudSTLoUqJM=
 github.com/hexops/gotextdiff v1.0.3/go.mod h1:pSWU5MAI3yDq+fZBTazCSJysOMbxWL1BSow5/V2vxeg=
+github.com/joho/godotenv v1.5.1 h1:7eLL/+HRGLY0ldzfGMeQkb7vMd0as4CfYvUVzLqw0N0=
+github.com/joho/godotenv v1.5.1/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=

util.go

@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"os"
 	"reflect"
+
+	"github.com/joho/godotenv"
 )
 
 // ConfigFlag uses the configured (via kong.Configuration(loader)) configuration loader to load configuration
@@ -26,6 +28,21 @@ func (c ConfigFlag) BeforeResolve(kong *Kong, ctx *Context, trace *Path) error {
 	return nil
 }
 
+// EnvFlag loads environment variables from a file specified by a flag.
+//
+// Use this as a flag value to support loading of environment variables from a file.
+type EnvFlag string
+
+// BeforeResolve loads env file.
+func (c EnvFlag) BeforeReset(ctx *Context, trace *Path) error {
+	path := string(ctx.FlagValue(trace.Flag).(EnvFlag)) // nolint
+	path = ExpandPath(path)
+	if err := godotenv.Load(path); err != nil {
+		return err
+	}
+	return nil
+}
+
 // VersionFlag is a flag type that can be used to display a version number, stored in the "version" variable.
 type VersionFlag bool
 

util_test.go

@@ -29,6 +29,24 @@ func TestConfigFlag(t *testing.T) {
 	assert.Equal(t, "hello world", cli.Flag)
 }
 
+func TestEnvFlag(t *testing.T) {
+	var cli struct {
+		EnvFile EnvFlag
+		Flag    string `env:"FLAG"`
+	}
+
+	w, err := ioutil.TempFile("", "")
+	assert.NoError(t, err)
+	defer os.Remove(w.Name())
+	w.WriteString(`FLAG=hello world`) // nolint: errcheck
+	w.Close()
+
+	p := Must(&cli)
+	_, err = p.Parse([]string{"--env-file", w.Name()})
+	assert.NoError(t, err)
+	assert.Equal(t, "hello world", cli.Flag)
+}
+
 func TestVersionFlag(t *testing.T) {
 	var cli struct {
 		Version VersionFlag