Source: exec.go in package os/exec

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. Unlike the "system" library call from C and other languages, the os/exec package intentionally does not invoke the system shell and does not expand any glob patterns or handle other expansions, pipelines, or redirections typically done by shells. The package behaves more like C's "exec" family of functions. To expand glob patterns, either call the shell directly, taking care to escape any dangerous input, or use the path/filepath package's Glob function. To expand environment variables, use package os's ExpandEnv. Note that the examples in this package assume a Unix system. They may not run on Windows, and they do not run in the Go Playground used by golang.org and godoc.org.

package exec

import (
	"bytes"
	"context"
	"errors"
	"internal/syscall/execenv"
	"io"
	"os"
	"path/filepath"
	"runtime"
	"strconv"
	"strings"
	"sync"
	"syscall"
)

Error is returned by LookPath when it fails to classify a file as an executable.

Name is the file name for which the error occurred.

Err is the underlying error.

	Err error
}

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

func (e *Error) Unwrap() error { return e.Err }

Cmd represents an external command being prepared or run. A Cmd cannot be reused after calling its Run, Output or CombinedOutput methods.

Path is the path of the command to run. This is the only field that must be set to a non-zero value. If Path is relative, it is evaluated relative to Dir.

	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. Each entry is of the form "key=value". If Env is nil, the new process uses the current process's environment. If Env contains duplicate environment keys, only the last value in the slice for each duplicate key is used. As a special case on Windows, SYSTEMROOT is always added if missing and not explicitly set to the empty string.

	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). If Stdin is an *os.File, the process's standard input is connected directly to that file. Otherwise, during the execution of the command a separate goroutine reads from Stdin and delivers that data to the command over a pipe. In this case, Wait does not complete until the goroutine stops copying, either because it has reached the end of Stdin (EOF or a read error) or because writing to the pipe returned an error.

	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 either is an *os.File, the corresponding output from the process is connected directly to that file. Otherwise, during the execution of the command a separate goroutine reads from the process over a pipe and delivers that data to the corresponding Writer. In this case, Wait does not complete until the goroutine reaches EOF or encounters an error. If Stdout and Stderr are the same writer, and have a type that can be compared with ==, 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. ExtraFiles is not supported on Windows.

	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

	ctx             context.Context // nil means none
	lookPathErr     error           // LookPath error, if any.
	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
	waitDone        chan struct{}
}

Command returns the Cmd struct to execute the named program with the given arguments. It sets only the Path and Args in the returned structure. If name contains no path separators, Command uses LookPath to resolve name to a complete path if possible. Otherwise it uses name directly as Path. 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"). Args[0] is always name, not the possibly resolved Path. On Windows, processes receive the whole command line as a single string and do their own parsing. Command combines and quotes Args into a command line string with an algorithm compatible with applications using CommandLineToArgvW (which is the most common way). Notable exceptions are msiexec.exe and cmd.exe (and thus, all batch files), which have a different unquoting algorithm. In these or other similar cases, you can do the quoting yourself and provide the full command line in SysProcAttr.CmdLine, leaving Args empty.

func Command(name string, arg ...string) *Cmd {
	cmd := &Cmd{
		Path: name,
		Args: append([]string{name}, arg...),
	}
	if filepath.Base(name) == name {
		if lp, err := LookPath(name); err != nil {
			cmd.lookPathErr = err
		} else {
			cmd.Path = lp
		}
	}
	return cmd
}

CommandContext is like Command but includes a context. The provided context is used to kill the process (by calling os.Process.Kill) if the context becomes done before the command completes on its own.

func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
	if ctx == nil {
		panic("nil Context")
	}
	cmd := Command(name, arg...)
	cmd.ctx = ctx
	return cmd
}

String returns a human-readable description of c. It is intended only for debugging. In particular, it is not suitable for use as input to a shell. The output of String may vary across Go releases.

func (c *Cmd) String() string {

failed to resolve path; report the original requested path (plus args)

		return strings.Join(c.Args, " ")

report the exact executable path (plus args)

	b := new(strings.Builder)
	b.WriteString(c.Path)
	for _, a := range c.Args[1:] {
		b.WriteByte(' ')
		b.WriteString(a)
	}
	return b.String()
}

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, error) {
	if c.Env != nil {
		return c.Env, nil
	}
	return execenv.Default(c.SysProcAttr)
}

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

skipStdinCopyError optionally specifies a function which reports whether the provided stdin copy error should be ignored.

var skipStdinCopyError func(error) bool

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 skip := skipStdinCopyError; skip != nil && skip(err) {
			err = nil
		}
		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)
		pr.Close() // in case io.Copy stopped due to write error
		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 starts but does not complete successfully, the error is of type *ExitError. Other error types may be returned for other situations. If the calling goroutine has locked the operating system thread with runtime.LockOSThread and modified any inheritable OS-level thread state (for example, Linux or Plan 9 name spaces), the new process will inherit the caller's thread state.

func (c *Cmd) Run() error {
	if err := c.Start(); err != nil {
		return err
	}
	return c.Wait()
}

lookExtensions finds windows executable by its dir and path. It uses LookPath to try appropriate extensions. lookExtensions does not search PATH, instead it converts `prog` into `.\prog`.

func lookExtensions(path, dir string) (string, error) {
	if filepath.Base(path) == path {
		path = filepath.Join(".", path)
	}
	if dir == "" {
		return LookPath(path)
	}
	if filepath.VolumeName(path) != "" {
		return LookPath(path)
	}
	if len(path) > 1 && os.IsPathSeparator(path[0]) {
		return LookPath(path)
	}

We assume that LookPath will only add file extension.

	lp, err := LookPath(dirandpath)
	if err != nil {
		return "", err
	}
	ext := strings.TrimPrefix(lp, dirandpath)
	return path + ext, nil
}

Start starts the specified command but does not wait for it to complete. If Start returns successfully, the c.Process field will be set. The Wait method will return the exit code and release associated resources once the command exits.

func (c *Cmd) Start() error {
	if c.lookPathErr != nil {
		c.closeDescriptors(c.closeAfterStart)
		c.closeDescriptors(c.closeAfterWait)
		return c.lookPathErr
	}
	if runtime.GOOS == "windows" {
		lp, err := lookExtensions(c.Path, c.Dir)
		if err != nil {
			c.closeDescriptors(c.closeAfterStart)
			c.closeDescriptors(c.closeAfterWait)
			return err
		}
		c.Path = lp
	}
	if c.Process != nil {
		return errors.New("exec: already started")
	}
	if c.ctx != nil {
		select {
		case <-c.ctx.Done():
			c.closeDescriptors(c.closeAfterStart)
			c.closeDescriptors(c.closeAfterWait)
			return c.ctx.Err()
		default:
		}
	}

	c.childFiles = make([]*os.File, 0, 3+len(c.ExtraFiles))
	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...)

	envv, err := c.envv()
	if err != nil {
		return err
	}

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

	c.closeDescriptors(c.closeAfterStart)

Don't allocate the channel unless there are goroutines to fire.

	if len(c.goroutine) > 0 {
		c.errch = make(chan error, len(c.goroutine))
		for _, fn := range c.goroutine {
			go func(fn func() error) {
				c.errch <- fn()
			}(fn)
		}
	}

	if c.ctx != nil {
		c.waitDone = make(chan struct{})
		go func() {
			select {
			case <-c.ctx.Done():
				c.Process.Kill()
			case <-c.waitDone:
			}
		}()
	}

	return nil
}

An ExitError reports an unsuccessful exit by a command.

type ExitError struct {
	*os.ProcessState

Stderr holds a subset of the standard error output from the Cmd.Output method if standard error was not otherwise being collected. If the error output is long, Stderr may contain only a prefix and suffix of the output, with the middle replaced with text about the number of omitted bytes. Stderr is provided for debugging, for inclusion in error messages. Users with other needs should redirect Cmd.Stderr as needed.

	Stderr []byte
}

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

Wait waits for the command to exit and waits for any copying to stdin or copying from stdout or stderr to complete. The command 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. If any of c.Stdin, c.Stdout or c.Stderr are not an *os.File, Wait also waits for the respective I/O loop copying to or from the process to complete. Wait releases any resources associated with the Cmd.

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()
	if c.waitDone != nil {
		close(c.waitDone)
	}
	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{ProcessState: state}
	}

	return copyError
}

Output runs the command and returns its standard output. Any returned error will usually be of type *ExitError. If c.Stderr was nil, Output populates ExitError.Stderr.

func (c *Cmd) Output() ([]byte, error) {
	if c.Stdout != nil {
		return nil, errors.New("exec: Stdout already set")
	}
	var stdout bytes.Buffer
	c.Stdout = &stdout

	captureErr := c.Stderr == nil
	if captureErr {
		c.Stderr = &prefixSuffixSaver{N: 32 << 10}
	}

	err := c.Run()
	if err != nil && captureErr {
		if ee, ok := err.(*ExitError); ok {
			ee.Stderr = c.Stderr.(*prefixSuffixSaver).Bytes()
		}
	}
	return stdout.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

	once sync.Once
	err  error
}

func (c *closeOnce) Close() error {
	c.once.Do(c.close)
	return c.err
}

func (c *closeOnce) close() {
	c.err = c.File.Close()
}

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. It is thus 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. It is thus 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
}

prefixSuffixSaver is an io.Writer which retains the first N bytes and the last N bytes written to it. The Bytes() methods reconstructs it with a pretty error message.

type prefixSuffixSaver struct {
	N         int // max size of prefix or suffix
	prefix    []byte
	suffix    []byte // ring buffer once len(suffix) == N
	suffixOff int    // offset to write into suffix
	skipped   int64

TODO(bradfitz): we could keep one large []byte and use part of it for the prefix, reserve space for the '... Omitting N bytes ...' message, then the ring buffer suffix, and just rearrange the ring buffer suffix when Bytes() is called, but it doesn't seem worth it for now just for error messages. It's only ~64KB anyway.

}

func (w *prefixSuffixSaver) Write(p []byte) (n int, err error) {
	lenp := len(p)
	p = w.fill(&w.prefix, p)

Only keep the last w.N bytes of suffix data.

	if overage := len(p) - w.N; overage > 0 {
		p = p[overage:]
		w.skipped += int64(overage)
	}
	p = w.fill(&w.suffix, p)

w.suffix is full now if p is non-empty. Overwrite it in a circle.

	for len(p) > 0 { // 0, 1, or 2 iterations.
		n := copy(w.suffix[w.suffixOff:], p)
		p = p[n:]
		w.skipped += int64(n)
		w.suffixOff += n
		if w.suffixOff == w.N {
			w.suffixOff = 0
		}
	}
	return lenp, nil
}

fill appends up to len(p) bytes of p to *dst, such that *dst does not grow larger than w.N. It returns the un-appended suffix of p.

func (w *prefixSuffixSaver) fill(dst *[]byte, p []byte) (pRemain []byte) {
	if remain := w.N - len(*dst); remain > 0 {
		add := minInt(len(p), remain)
		*dst = append(*dst, p[:add]...)
		p = p[add:]
	}
	return p
}

func (w *prefixSuffixSaver) Bytes() []byte {
	if w.suffix == nil {
		return w.prefix
	}
	if w.skipped == 0 {
		return append(w.prefix, w.suffix...)
	}
	var buf bytes.Buffer
	buf.Grow(len(w.prefix) + len(w.suffix) + 50)
	buf.Write(w.prefix)
	buf.WriteString("\n... omitting ")
	buf.WriteString(strconv.FormatInt(w.skipped, 10))
	buf.WriteString(" bytes ...\n")
	buf.Write(w.suffix[w.suffixOff:])
	buf.Write(w.suffix[:w.suffixOff])
	return buf.Bytes()
}

func minInt(a, b int) int {
	if a < b {
		return a
	}
	return b
}

dedupEnv returns a copy of env with any duplicates removed, in favor of later values. Items not of the normal environment "key=value" form are preserved unchanged.

func dedupEnv(env []string) []string {
	return dedupEnvCase(runtime.GOOS == "windows", env)
}

dedupEnvCase is dedupEnv with a case option for testing. If caseInsensitive is true, the case of keys is ignored.

func dedupEnvCase(caseInsensitive bool, env []string) []string {
	out := make([]string, 0, len(env))
	saw := make(map[string]int, len(env)) // key => index into out
	for _, kv := range env {
		eq := strings.Index(kv, "=")
		if eq < 0 {
			out = append(out, kv)
			continue
		}
		k := kv[:eq]
		if caseInsensitive {
			k = strings.ToLower(k)
		}
		if dupIdx, isDup := saw[k]; isDup {
			out[dupIdx] = kv
			continue
		}
		saw[k] = len(out)
		out = append(out, kv)
	}
	return out
}

addCriticalEnv adds any critical environment variables that are required (or at least almost always required) on the operating system. Currently this is only used for Windows.

func addCriticalEnv(env []string) []string {
	if runtime.GOOS != "windows" {
		return env
	}
	for _, kv := range env {
		eq := strings.Index(kv, "=")
		if eq < 0 {
			continue
		}
		k := kv[:eq]

We already have it.

			return env
		}
	}
	return append(env, "SYSTEMROOT="+os.Getenv("SYSTEMROOT"))