src/pkg/os/exec/exec.go - go - Git at Google

 // Copyright 2009 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 runs external commands. It wraps os.StartProcess to make it
 // easier to remap stdin and stdout, connect I/O with pipes, and do other
 // adjustments.
 package exec

 import (
 	"bytes"
 	"errors"
 	"io"
 	"os"
 	"strconv"
 	"sync"
 	"syscall"
 )

 // Error records the name of a binary that failed to be executed
 // and the reason it failed.
 type Error struct {
 	Name string
 	Err  error
 }

 func (e *Error) Error() string {
 	return "exec: " + strconv.Quote(e.Name) + ": " + e.Err.Error()
 }

 // Cmd represents an external command being prepared or run.
 type Cmd struct {
 	// Path is the path of the command to run.
 	//
 	// This is the only field that must be set to a non-zero
 	// value.
 	Path string

 	// Args holds command line arguments, including the command as Args[0].
 	// If the Args field is empty or nil, Run uses {Path}.
 	//
 	// In typical use, both Path and Args are set by calling Command.
 	Args []string

 	// Env specifies the environment of the process.
 	// If Env is nil, Run uses the current process's environment.
 	Env []string

 	// Dir specifies the working directory of the command.
 	// If Dir is the empty string, Run runs the command in the
 	// calling process's current directory.
 	Dir string

 	// Stdin specifies the process's standard input. If Stdin is
 	// nil, the process reads from the null device (os.DevNull).
 	Stdin io.Reader

 	// Stdout and Stderr specify the process's standard output and error.
 	//
 	// If either is nil, Run connects the corresponding file descriptor
 	// to the null device (os.DevNull).
 	//
 	// If Stdout and Stderr are the same writer, at most one
 	// goroutine at a time will call Write.
 	Stdout io.Writer
 	Stderr io.Writer

 	// ExtraFiles specifies additional open files to be inherited by the
 	// new process. It does not include standard input, standard output, or
 	// standard error. If non-nil, entry i becomes file descriptor 3+i.
 	//
 	// BUG: on OS X 10.6, child processes may sometimes inherit unwanted fds.
 	// http://golang.org/issue/2603
 	ExtraFiles []*os.File

 	// SysProcAttr holds optional, operating system-specific attributes.
 	// Run passes it to os.StartProcess as the os.ProcAttr's Sys field.
 	SysProcAttr *syscall.SysProcAttr

 	// Process is the underlying process, once started.
 	Process *os.Process

 	// ProcessState contains information about an exited process,
 	// available after a call to Wait or Run.
 	ProcessState *os.ProcessState

 	err             error // last error (from LookPath, stdin, stdout, stderr)
 	finished        bool  // when Wait was called
 	childFiles      []*os.File
 	closeAfterStart []io.Closer
 	closeAfterWait  []io.Closer
 	goroutine       []func() error
 	errch           chan error // one send per goroutine
 }

 // Command returns the Cmd struct to execute the named program with
 // the given arguments.
 //
 // It sets Path and Args in the returned structure and zeroes the
 // other fields.
 //
 // If name contains no path separators, Command uses LookPath to
 // resolve the path to a complete name if possible. Otherwise it uses
 // name directly.
 //
 // The returned Cmd's Args field is constructed from the command name
 // followed by the elements of arg, so arg should not include the
 // command name itself. For example, Command("echo", "hello")
 func Command(name string, arg ...string) *Cmd {
 	aname, err := LookPath(name)
 	if err != nil {
 		aname = name
 	}
 	return &Cmd{
 		Path: aname,
 		Args: append([]string{name}, arg...),
 		err:  err,
 	}
 }

 // interfaceEqual protects against panics from doing equality tests on
 // two interfaces with non-comparable underlying types
 func interfaceEqual(a, b interface{}) bool {
 	defer func() {
 		recover()
 	}()
 	return a == b
 }

 func (c *Cmd) envv() []string {
 	if c.Env != nil {
 		return c.Env
 	}
 	return os.Environ()
 }

 func (c *Cmd) argv() []string {
 	if len(c.Args) > 0 {
 		return c.Args
 	}
 	return []string{c.Path}
 }

 func (c *Cmd) stdin() (f *os.File, err error) {
 	if c.Stdin == nil {
 		f, err = os.Open(os.DevNull)
 		if err != nil {
 			return
 		}
 		c.closeAfterStart = append(c.closeAfterStart, f)
 		return
 	}

 	if f, ok := c.Stdin.(*os.File); ok {
 		return f, nil
 	}

 	pr, pw, err := os.Pipe()
 	if err != nil {
 		return
 	}

 	c.closeAfterStart = append(c.closeAfterStart, pr)
 	c.closeAfterWait = append(c.closeAfterWait, pw)
 	c.goroutine = append(c.goroutine, func() error {
 		_, err := io.Copy(pw, c.Stdin)
 		if err1 := pw.Close(); err == nil {
 			err = err1
 		}
 		return err
 	})
 	return pr, nil
 }

 func (c *Cmd) stdout() (f *os.File, err error) {
 	return c.writerDescriptor(c.Stdout)
 }

 func (c *Cmd) stderr() (f *os.File, err error) {
 	if c.Stderr != nil && interfaceEqual(c.Stderr, c.Stdout) {
 		return c.childFiles[1], nil
 	}
 	return c.writerDescriptor(c.Stderr)
 }

 func (c *Cmd) writerDescriptor(w io.Writer) (f *os.File, err error) {
 	if w == nil {
 		f, err = os.OpenFile(os.DevNull, os.O_WRONLY, 0)
 		if err != nil {
 			return
 		}
 		c.closeAfterStart = append(c.closeAfterStart, f)
 		return
 	}

 	if f, ok := w.(*os.File); ok {
 		return f, nil
 	}

 	pr, pw, err := os.Pipe()
 	if err != nil {
 		return
 	}

 	c.closeAfterStart = append(c.closeAfterStart, pw)
 	c.closeAfterWait = append(c.closeAfterWait, pr)
 	c.goroutine = append(c.goroutine, func() error {
 		_, err := io.Copy(w, pr)
 		return err
 	})
 	return pw, nil
 }

 func (c *Cmd) closeDescriptors(closers []io.Closer) {
 	for _, fd := range closers {
 		fd.Close()
 	}
 }

 // Run starts the specified command and waits for it to complete.
 //
 // The returned error is nil if the command runs, has no problems
 // copying stdin, stdout, and stderr, and exits with a zero exit
 // status.
 //
 // If the command fails to run or doesn't complete successfully, the
 // error is of type *ExitError. Other error types may be
 // returned for I/O problems.
 func (c *Cmd) Run() error {
 	if err := c.Start(); err != nil {
 		return err
 	}
 	return c.Wait()
 }

 // Start starts the specified command but does not wait for it to complete.
 func (c *Cmd) Start() error {
 	if c.err != nil {
 		c.closeDescriptors(c.closeAfterStart)
 		c.closeDescriptors(c.closeAfterWait)
 		return c.err
 	}
 	if c.Process != nil {
 		return errors.New("exec: already started")
 	}

 	type F func(*Cmd) (*os.File, error)
 	for _, setupFd := range []F{(*Cmd).stdin, (*Cmd).stdout, (*Cmd).stderr} {
 		fd, err := setupFd(c)
 		if err != nil {
 			c.closeDescriptors(c.closeAfterStart)
 			c.closeDescriptors(c.closeAfterWait)
 			return err
 		}
 		c.childFiles = append(c.childFiles, fd)
 	}
 	c.childFiles = append(c.childFiles, c.ExtraFiles...)

 	var err error
 	c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
 		Dir:   c.Dir,
 		Files: c.childFiles,
 		Env:   c.envv(),
 		Sys:   c.SysProcAttr,
 	})
 	if err != nil {
 		c.closeDescriptors(c.closeAfterStart)
 		c.closeDescriptors(c.closeAfterWait)
 		return err
 	}

 	c.closeDescriptors(c.closeAfterStart)

 	c.errch = make(chan error, len(c.goroutine))
 	for _, fn := range c.goroutine {
 		go func(fn func() error) {
 			c.errch <- fn()
 		}(fn)
 	}

 	return nil
 }

 // An ExitError reports an unsuccessful exit by a command.
 type ExitError struct {
 	*os.ProcessState
 }

 func (e *ExitError) Error() string {
 	return e.ProcessState.String()
 }

 // Wait waits for the command to exit.
 // It must have been started by Start.
 //
 // The returned error is nil if the command runs, has no problems
 // copying stdin, stdout, and stderr, and exits with a zero exit
 // status.
 //
 // If the command fails to run or doesn't complete successfully, the
 // error is of type *ExitError. Other error types may be
 // returned for I/O problems.
 func (c *Cmd) Wait() error {
 	if c.Process == nil {
 		return errors.New("exec: not started")
 	}
 	if c.finished {
 		return errors.New("exec: Wait was already called")
 	}
 	c.finished = true
 	state, err := c.Process.Wait()
 	c.ProcessState = state

 	var copyError error
 	for _ = range c.goroutine {
 		if err := <-c.errch; err != nil && copyError == nil {
 			copyError = err
 		}
 	}

 	c.closeDescriptors(c.closeAfterWait)

 	if err != nil {
 		return err
 	} else if !state.Success() {
 		return &ExitError{state}
 	}

 	return copyError
 }

 // Output runs the command and returns its standard output.
 func (c *Cmd) Output() ([]byte, error) {
 	if c.Stdout != nil {
 		return nil, errors.New("exec: Stdout already set")
 	}
 	var b bytes.Buffer
 	c.Stdout = &b
 	err := c.Run()
 	return b.Bytes(), err
 }

 // CombinedOutput runs the command and returns its combined standard
 // output and standard error.
 func (c *Cmd) CombinedOutput() ([]byte, error) {
 	if c.Stdout != nil {
 		return nil, errors.New("exec: Stdout already set")
 	}
 	if c.Stderr != nil {
 		return nil, errors.New("exec: Stderr already set")
 	}
 	var b bytes.Buffer
 	c.Stdout = &b
 	c.Stderr = &b
 	err := c.Run()
 	return b.Bytes(), err
 }

 // StdinPipe returns a pipe that will be connected to the command's
 // standard input when the command starts.
 // The pipe will be closed automatically after Wait sees the command exit.
 // A caller need only call Close to force the pipe to close sooner.
 // For example, if the command being run will not exit until standard input
 // is closed, the caller must close the pipe.
 func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
 	if c.Stdin != nil {
 		return nil, errors.New("exec: Stdin already set")
 	}
 	if c.Process != nil {
 		return nil, errors.New("exec: StdinPipe after process started")
 	}
 	pr, pw, err := os.Pipe()
 	if err != nil {
 		return nil, err
 	}
 	c.Stdin = pr
 	c.closeAfterStart = append(c.closeAfterStart, pr)
 	wc := &closeOnce{File: pw}
 	c.closeAfterWait = append(c.closeAfterWait, wc)
 	return wc, nil
 }

 type closeOnce struct {
 	*os.File

 	close    sync.Once
 	closeErr error
 }

 func (c *closeOnce) Close() error {
 	c.close.Do(func() {
 		c.closeErr = c.File.Close()
 	})
 	return c.closeErr
 }

 // StdoutPipe returns a pipe that will be connected to the command's
 // standard output when the command starts.
 //
 // Wait will close the pipe after seeing the command exit, so most callers
 // need not close the pipe themselves; however, an implication is that
 // it is incorrect to call Wait before all reads from the pipe have completed.
 // For the same reason, it is incorrect to call Run when using StdoutPipe.
 // See the example for idiomatic usage.
 func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
 	if c.Stdout != nil {
 		return nil, errors.New("exec: Stdout already set")
 	}
 	if c.Process != nil {
 		return nil, errors.New("exec: StdoutPipe after process started")
 	}
 	pr, pw, err := os.Pipe()
 	if err != nil {
 		return nil, err
 	}
 	c.Stdout = pw
 	c.closeAfterStart = append(c.closeAfterStart, pw)
 	c.closeAfterWait = append(c.closeAfterWait, pr)
 	return pr, nil
 }

 // StderrPipe returns a pipe that will be connected to the command's
 // standard error when the command starts.
 //
 // Wait will close the pipe after seeing the command exit, so most callers
 // need not close the pipe themselves; however, an implication is that
 // it is incorrect to call Wait before all reads from the pipe have completed.
 // For the same reason, it is incorrect to use Run when using StderrPipe.
 // See the StdoutPipe example for idiomatic usage.
 func (c *Cmd) StderrPipe() (io.ReadCloser, error) {
 	if c.Stderr != nil {
 		return nil, errors.New("exec: Stderr already set")
 	}
 	if c.Process != nil {
 		return nil, errors.New("exec: StderrPipe after process started")
 	}
 	pr, pw, err := os.Pipe()
 	if err != nil {
 		return nil, err
 	}
 	c.Stderr = pw
 	c.closeAfterStart = append(c.closeAfterStart, pw)
 	c.closeAfterWait = append(c.closeAfterWait, pr)
 	return pr, nil
 }
	// Copyright 2009 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 runs external commands. It wraps os.StartProcess to make it
	// easier to remap stdin and stdout, connect I/O with pipes, and do other
	// adjustments.
	package exec

	import (
	"bytes"
	"errors"
	"io"
	"os"
	"strconv"
	"sync"
	"syscall"
	)

	// Error records the name of a binary that failed to be executed
	// and the reason it failed.
	type Error struct {
	Name string
	Err error
	}

	func (e *Error) Error() string {
	return "exec: " + strconv.Quote(e.Name) + ": " + e.Err.Error()
	}

	// Cmd represents an external command being prepared or run.
	type Cmd struct {
	// Path is the path of the command to run.
	//
	// This is the only field that must be set to a non-zero
	// value.
	Path string

	// Args holds command line arguments, including the command as Args[0].
	// If the Args field is empty or nil, Run uses {Path}.
	//
	// In typical use, both Path and Args are set by calling Command.
	Args []string

	// Env specifies the environment of the process.
	// If Env is nil, Run uses the current process's environment.
	Env []string

	// Dir specifies the working directory of the command.
	// If Dir is the empty string, Run runs the command in the
	// calling process's current directory.
	Dir string

	// Stdin specifies the process's standard input. If Stdin is
	// nil, the process reads from the null device (os.DevNull).
	Stdin io.Reader

	// Stdout and Stderr specify the process's standard output and error.
	//
	// If either is nil, Run connects the corresponding file descriptor
	// to the null device (os.DevNull).
	//
	// If Stdout and Stderr are the same writer, at most one
	// goroutine at a time will call Write.
	Stdout io.Writer
	Stderr io.Writer

	// ExtraFiles specifies additional open files to be inherited by the
	// new process. It does not include standard input, standard output, or
	// standard error. If non-nil, entry i becomes file descriptor 3+i.
	//
	// BUG: on OS X 10.6, child processes may sometimes inherit unwanted fds.
	// http://golang.org/issue/2603
	ExtraFiles []*os.File

	// SysProcAttr holds optional, operating system-specific attributes.
	// Run passes it to os.StartProcess as the os.ProcAttr's Sys field.
	SysProcAttr *syscall.SysProcAttr

	// Process is the underlying process, once started.
	Process *os.Process

	// ProcessState contains information about an exited process,
	// available after a call to Wait or Run.
	ProcessState *os.ProcessState

	err error // last error (from LookPath, stdin, stdout, stderr)
	finished bool // when Wait was called
	childFiles []*os.File
	closeAfterStart []io.Closer
	closeAfterWait []io.Closer
	goroutine []func() error
	errch chan error // one send per goroutine
	}

	// Command returns the Cmd struct to execute the named program with
	// the given arguments.
	//
	// It sets Path and Args in the returned structure and zeroes the
	// other fields.
	//
	// If name contains no path separators, Command uses LookPath to
	// resolve the path to a complete name if possible. Otherwise it uses
	// name directly.
	//
	// The returned Cmd's Args field is constructed from the command name
	// followed by the elements of arg, so arg should not include the
	// command name itself. For example, Command("echo", "hello")
	func Command(name string, arg ...string) *Cmd {
	aname, err := LookPath(name)
	if err != nil {
	aname = name
	}
	return &Cmd{
	Path: aname,
	Args: append([]string{name}, arg...),
	err: err,
	}
	}

	// interfaceEqual protects against panics from doing equality tests on
	// two interfaces with non-comparable underlying types
	func interfaceEqual(a, b interface{}) bool {
	defer func() {
	recover()
	}()
	return a == b
	}

	func (c *Cmd) envv() []string {
	if c.Env != nil {
	return c.Env
	}
	return os.Environ()
	}

	func (c *Cmd) argv() []string {
	if len(c.Args) > 0 {
	return c.Args
	}
	return []string{c.Path}
	}

	func (c Cmd) stdin() (f os.File, err error) {
	if c.Stdin == nil {
	f, err = os.Open(os.DevNull)
	if err != nil {
	return
	}
	c.closeAfterStart = append(c.closeAfterStart, f)
	return
	}

	if f, ok := c.Stdin.(*os.File); ok {
	return f, nil
	}

	pr, pw, err := os.Pipe()
	if err != nil {
	return
	}

	c.closeAfterStart = append(c.closeAfterStart, pr)
	c.closeAfterWait = append(c.closeAfterWait, pw)
	c.goroutine = append(c.goroutine, func() error {
	_, err := io.Copy(pw, c.Stdin)
	if err1 := pw.Close(); err == nil {
	err = err1
	}
	return err
	})
	return pr, nil
	}

	func (c Cmd) stdout() (f os.File, err error) {
	return c.writerDescriptor(c.Stdout)
	}

	func (c Cmd) stderr() (f os.File, err error) {
	if c.Stderr != nil && interfaceEqual(c.Stderr, c.Stdout) {
	return c.childFiles[1], nil
	}
	return c.writerDescriptor(c.Stderr)
	}

	func (c Cmd) writerDescriptor(w io.Writer) (f os.File, err error) {
	if w == nil {
	f, err = os.OpenFile(os.DevNull, os.O_WRONLY, 0)
	if err != nil {
	return
	}
	c.closeAfterStart = append(c.closeAfterStart, f)
	return
	}

	if f, ok := w.(*os.File); ok {
	return f, nil
	}

	pr, pw, err := os.Pipe()
	if err != nil {
	return
	}

	c.closeAfterStart = append(c.closeAfterStart, pw)
	c.closeAfterWait = append(c.closeAfterWait, pr)
	c.goroutine = append(c.goroutine, func() error {
	_, err := io.Copy(w, pr)
	return err
	})
	return pw, nil
	}

	func (c *Cmd) closeDescriptors(closers []io.Closer) {
	for _, fd := range closers {
	fd.Close()
	}
	}

	// Run starts the specified command and waits for it to complete.
	//
	// The returned error is nil if the command runs, has no problems
	// copying stdin, stdout, and stderr, and exits with a zero exit
	// status.
	//
	// If the command fails to run or doesn't complete successfully, the
	// error is of type *ExitError. Other error types may be
	// returned for I/O problems.
	func (c *Cmd) Run() error {
	if err := c.Start(); err != nil {
	return err
	}
	return c.Wait()
	}

	// Start starts the specified command but does not wait for it to complete.
	func (c *Cmd) Start() error {
	if c.err != nil {
	c.closeDescriptors(c.closeAfterStart)
	c.closeDescriptors(c.closeAfterWait)
	return c.err
	}
	if c.Process != nil {
	return errors.New("exec: already started")
	}

	type F func(Cmd) (os.File, error)
	for _, setupFd := range []F{(Cmd).stdin, (Cmd).stdout, (*Cmd).stderr} {
	fd, err := setupFd(c)
	if err != nil {
	c.closeDescriptors(c.closeAfterStart)
	c.closeDescriptors(c.closeAfterWait)
	return err
	}
	c.childFiles = append(c.childFiles, fd)
	}
	c.childFiles = append(c.childFiles, c.ExtraFiles...)

	var err error
	c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
	Dir: c.Dir,
	Files: c.childFiles,
	Env: c.envv(),
	Sys: c.SysProcAttr,
	})
	if err != nil {
	c.closeDescriptors(c.closeAfterStart)
	c.closeDescriptors(c.closeAfterWait)
	return err
	}

	c.closeDescriptors(c.closeAfterStart)

	c.errch = make(chan error, len(c.goroutine))
	for _, fn := range c.goroutine {
	go func(fn func() error) {
	c.errch <- fn()
	}(fn)
	}

	return nil
	}

	// An ExitError reports an unsuccessful exit by a command.
	type ExitError struct {
	*os.ProcessState
	}

	func (e *ExitError) Error() string {
	return e.ProcessState.String()
	}

	// Wait waits for the command to exit.
	// It must have been started by Start.
	//
	// The returned error is nil if the command runs, has no problems
	// copying stdin, stdout, and stderr, and exits with a zero exit
	// status.
	//
	// If the command fails to run or doesn't complete successfully, the
	// error is of type *ExitError. Other error types may be
	// returned for I/O problems.
	func (c *Cmd) Wait() error {
	if c.Process == nil {
	return errors.New("exec: not started")
	}
	if c.finished {
	return errors.New("exec: Wait was already called")
	}
	c.finished = true
	state, err := c.Process.Wait()
	c.ProcessState = state

	var copyError error
	for _ = range c.goroutine {
	if err := <-c.errch; err != nil && copyError == nil {
	copyError = err
	}
	}

	c.closeDescriptors(c.closeAfterWait)

	if err != nil {
	return err
	} else if !state.Success() {
	return &ExitError{state}
	}

	return copyError
	}

	// Output runs the command and returns its standard output.
	func (c *Cmd) Output() ([]byte, error) {
	if c.Stdout != nil {
	return nil, errors.New("exec: Stdout already set")
	}
	var b bytes.Buffer
	c.Stdout = &b
	err := c.Run()
	return b.Bytes(), err
	}

	// CombinedOutput runs the command and returns its combined standard
	// output and standard error.
	func (c *Cmd) CombinedOutput() ([]byte, error) {
	if c.Stdout != nil {
	return nil, errors.New("exec: Stdout already set")
	}
	if c.Stderr != nil {
	return nil, errors.New("exec: Stderr already set")
	}
	var b bytes.Buffer
	c.Stdout = &b
	c.Stderr = &b
	err := c.Run()
	return b.Bytes(), err
	}

	// StdinPipe returns a pipe that will be connected to the command's
	// standard input when the command starts.
	// The pipe will be closed automatically after Wait sees the command exit.
	// A caller need only call Close to force the pipe to close sooner.
	// For example, if the command being run will not exit until standard input
	// is closed, the caller must close the pipe.
	func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
	if c.Stdin != nil {
	return nil, errors.New("exec: Stdin already set")
	}
	if c.Process != nil {
	return nil, errors.New("exec: StdinPipe after process started")
	}
	pr, pw, err := os.Pipe()
	if err != nil {
	return nil, err
	}
	c.Stdin = pr
	c.closeAfterStart = append(c.closeAfterStart, pr)
	wc := &closeOnce{File: pw}
	c.closeAfterWait = append(c.closeAfterWait, wc)
	return wc, nil
	}

	type closeOnce struct {
	*os.File

	close sync.Once
	closeErr error
	}

	func (c *closeOnce) Close() error {
	c.close.Do(func() {
	c.closeErr = c.File.Close()
	})
	return c.closeErr
	}

	// StdoutPipe returns a pipe that will be connected to the command's
	// standard output when the command starts.
	//
	// Wait will close the pipe after seeing the command exit, so most callers
	// need not close the pipe themselves; however, an implication is that
	// it is incorrect to call Wait before all reads from the pipe have completed.
	// For the same reason, it is incorrect to call Run when using StdoutPipe.
	// See the example for idiomatic usage.
	func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
	if c.Stdout != nil {
	return nil, errors.New("exec: Stdout already set")
	}
	if c.Process != nil {
	return nil, errors.New("exec: StdoutPipe after process started")
	}
	pr, pw, err := os.Pipe()
	if err != nil {
	return nil, err
	}
	c.Stdout = pw
	c.closeAfterStart = append(c.closeAfterStart, pw)
	c.closeAfterWait = append(c.closeAfterWait, pr)
	return pr, nil
	}

	// StderrPipe returns a pipe that will be connected to the command's
	// standard error when the command starts.
	//
	// Wait will close the pipe after seeing the command exit, so most callers
	// need not close the pipe themselves; however, an implication is that
	// it is incorrect to call Wait before all reads from the pipe have completed.
	// For the same reason, it is incorrect to use Run when using StderrPipe.
	// See the StdoutPipe example for idiomatic usage.
	func (c *Cmd) StderrPipe() (io.ReadCloser, error) {
	if c.Stderr != nil {
	return nil, errors.New("exec: Stderr already set")
	}
	if c.Process != nil {
	return nil, errors.New("exec: StderrPipe after process started")
	}
	pr, pw, err := os.Pipe()
	if err != nil {
	return nil, err
	}
	c.Stderr = pw
	c.closeAfterStart = append(c.closeAfterStart, pw)
	c.closeAfterWait = append(c.closeAfterWait, pr)
	return pr, nil
	}