Make GetOSEnv private and use OS env when nil passed to Evaluate

cmd/bkl/main.go

@@ -89,7 +89,7 @@ Related tools:
 	}
 
 	fsys := os.DirFS(opts.RootPath)
-	output, err := bkl.Evaluate(fsys, files, format, opts.RootPath, wd, bkl.GetOSEnv())
+	output, err := bkl.Evaluate(fsys, files, format, opts.RootPath, wd, nil)
 	if err != nil {
 		fatal(err)
 	}

parser.go

@@ -338,8 +338,8 @@ func PreparePathsFromCwd(paths []string, rootPath string) ([]string, error) {
 	return preparePathsForParser(paths, rootPath, wd)
 }
 
-// GetOSEnv returns the current OS environment as a map.
-func GetOSEnv() map[string]string {
+// getOSEnv returns the current OS environment as a map.
+func getOSEnv() map[string]string {
 	env := make(map[string]string)
 	for _, e := range os.Environ() {
 		parts := strings.SplitN(e, "=", 2)
@@ -364,9 +364,14 @@ func FormatOutput(data any, format string) ([]byte, error) {
 
 // Evaluate processes the specified files and returns the formatted output.
 // It creates a new bkl instance internally to process the files.
+// If env is nil, it uses the current OS environment.
 func Evaluate(fsys fs.FS, files []string, format string, rootPath string, workingDir string, env map[string]string) ([]byte, error) {
 	b := &bkl{}
 
+	if env == nil {
+		env = getOSEnv()
+	}
+
 	evalFiles, err := preparePathsForParser(files, rootPath, workingDir)
 	if err != nil {
 		return nil, err

wrapper/wrapper.go

@@ -50,7 +50,7 @@ func WrapOrDie(cmd string) {
 		}
 
 		// Use Evaluate to process the file
-		output, err := bkl.Evaluate(fsys, []string{realPath}, f, "/", wd, bkl.GetOSEnv())
+		output, err := bkl.Evaluate(fsys, []string{realPath}, f, "/", wd, nil)
 		if err != nil {
 			fatal(err)
 		}