add builtin ENV: __EXEC_FILENAME for getting executable filename. see…

… issue #40

_examples/027-global-option/main.go

@@ -16,7 +16,7 @@ func main() {
 // root command
 type rootT struct {
 	cli.Helper
-	Self *rootT `json:"-" cli:"c,config" usage:"config" parser:"jsonfile" dft:"1.json"`
+	Self *rootT `json:"-" cli:"c,config" usage:"config" parser:"jsonfile" dft:"$__EXEC_FILENAME.json"`
 	Host string `cli:"H,host" usage:"host addr" dft:"$HOST"`
 	Port int    `cli:"p,port" usage:"listening port"`
 }

_examples/027-global-option/run.sh

@@ -4,7 +4,7 @@ set -e
 
 go build -o app
 
-cat >1.json<<EOF
+cat >app.json<<EOF
 {
 	"host": "127.0.0.1",
 	"port": 8080
@@ -45,5 +45,5 @@ HOST=10.0.0.1 ./app sub -H 168.0.0.1 -w xxx
 echo
 
 echo
-rm 1.json
+rm app.json
 rm app

flag.go

@@ -6,7 +6,9 @@ import (
 	"fmt"
 	"math"
 	"os"
+	"path/filepath"
 	"reflect"
+	"runtime"
 	"strconv"
 	"strings"
 
@@ -115,6 +117,10 @@ func isWordByte(b byte) bool {
 		b == '_'
 }
 
+const (
+	builtinVar_EXEC_FILENAME = "__EXEC_FILENAME"
+)
+
 func parseExpression(s string, isNumber bool) (string, error) {
 	const escapeByte = '$'
 	var (
@@ -127,11 +133,25 @@ func parseExpression(s string, isNumber bool) (string, error) {
 		if envName == "" {
 			return fmt.Errorf("unexpected end after %v", escapeByte)
 		}
-		env := os.Getenv(envName)
-		if env == "" && isNumber {
-			env = "0"
+		var value string
+		switch envName {
+		case builtinVar_EXEC_FILENAME:
+			filename, err := filepath.Abs(os.Args[0])
+			if err != nil {
+				return err
+			}
+			if runtime.GOOS == "windows" {
+				value = strings.TrimSuffix(filename, ".exe")
+			} else {
+				value = filename
+			}
+		default:
+			value = os.Getenv(envName)
+			if value == "" && isNumber {
+				value = "0"
+			}
 		}
-		exprBuf.WriteString(env)
+		exprBuf.WriteString(value)
 		return nil
 	}
 	for i, b := range src {