Add ChangeToFlag to allow changing the current working directory.

util.go

@@ -2,6 +2,8 @@ package kong
 
 import (
 	"fmt"
+	"os"
+	"reflect"
 )
 
 // ConfigFlag uses the configured (via kong.Configuration(loader)) configuration loader to load configuration
@@ -33,3 +35,20 @@ func (v VersionFlag) BeforeApply(app *Kong, vars Vars) error {
 	app.Exit(0)
 	return nil
 }
+
+// ChangeDirFlag changes the current working directory to a path specified by a flag
+// early in the parsing process, changing how other flags resolve relative paths.
+//
+// Use this flag to provide a "git -C" like functionality.
+type ChangeDirFlag string
+
+// Decode is used to create a side effect of changing the current working directory.
+func (c ChangeDirFlag) Decode(ctx *DecodeContext) error {
+	var path string
+	err := ctx.Scan.PopValueInto("string", &path)
+	if err != nil {
+		return err
+	}
+	ctx.Value.Target.Set(reflect.ValueOf(ChangeDirFlag(path)))
+	return os.Chdir(path)
+}

util_test.go

@@ -3,6 +3,7 @@ package kong
 import (
 	"io/ioutil"
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -42,3 +43,24 @@ func TestVersionFlag(t *testing.T) {
 	require.Equal(t, "0.1.1", strings.TrimSpace(w.String()))
 	require.Equal(t, 0, called)
 }
+
+func TestChangeDirFlag(t *testing.T) {
+	cwd, err := os.Getwd()
+	require.NoError(t, err)
+	defer os.Chdir(cwd) // nolint: errcheck
+
+	dir := t.TempDir()
+	file := filepath.Join(dir, "out.txt")
+	err = os.WriteFile(file, []byte("foobar"), 0o600)
+	require.NoError(t, err)
+
+	var cli struct {
+		ChangeDir ChangeDirFlag `short:"C"`
+		Path      string        `arg:"" type:"existingfile"`
+	}
+
+	p := Must(&cli)
+	_, err = p.Parse([]string{"-C", dir, "out.txt"})
+	require.NoError(t, err)
+	require.Equal(t, file, cli.Path)
+}