// Copyright 2010 The Go Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package exec

import (
	"os"
	"strings"
)

// ErrNotFound is the error resulting if a path search failed to find an executable file.
var ErrNotFound = os.NewError("executable file not found in $PATH")

func findExecutable(file string) os.Error {
	d, err := os.Stat(file)
	if err != nil {
		return err
	}
	if d.IsRegular() && d.Permission()&0111 != 0 {
		return nil
	}
	return os.EPERM
}

// LookPath searches for an executable binary named file
// in the directories named by the PATH environment variable.
// If file contains a slash, it is tried directly and the PATH is not consulted.
func LookPath(file string) (string, os.Error) {
	// NOTE(rsc): I wish we could use the Plan 9 behavior here
	// (only bypass the path if file begins with / or ./ or ../)
	// but that would not match all the Unix shells.

	if strings.Contains(file, "/") {
		err := findExecutable(file)
		if err == nil {
			return file, nil
		}
		return "", &Error{file, err}
	}
	pathenv := os.Getenv("PATH")
	for _, dir := range strings.Split(pathenv, ":") {
		if dir == "" {
			// Unix shell semantics: path element "" means "."
			dir = "."
		}
		if err := findExecutable(dir + "/" + file); err == nil {
			return dir + "/" + file, nil
		}
	}
	return "", &Error{file, ErrNotFound}
}