Current File : //usr/local/go119/src/os/exec/exec.go
// 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.
//
// 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.
//
// # Executables in the current directory
//
// The functions Command and LookPath look for a program
// in the directories listed in the current path, following the
// conventions of the host operating system.
// Operating systems have for decades included the current
// directory in this search, sometimes implicitly and sometimes
// configured explicitly that way by default.
// Modern practice is that including the current directory
// is usually unexpected and often leads to security problems.
//
// To avoid those security problems, as of Go 1.19, this package will not resolve a program
// using an implicit or explicit path entry relative to the current directory.
// That is, if you run exec.LookPath("go"), it will not successfully return
// ./go on Unix nor .\go.exe on Windows, no matter how the path is configured.
// Instead, if the usual path algorithms would result in that answer,
// these functions return an error err satisfying errors.Is(err, ErrDot).
//
// For example, consider these two program snippets:
//
// path, err := exec.LookPath("prog")
// if err != nil {
// log.Fatal(err)
// }
// use(path)
//
// and
//
// cmd := exec.Command("prog")
// if err := cmd.Run(); err != nil {
// log.Fatal(err)
// }
//
// These will not find and run ./prog or .\prog.exe,
// no matter how the current path is configured.
//
// Code that always wants to run a program from the current directory
// can be rewritten to say "./prog" instead of "prog".
//
// Code that insists on including results from relative path entries
// can instead override the error using an errors.Is check:
//
// path, err := exec.LookPath("prog")
// if errors.Is(err, exec.ErrDot) {
// err = nil
// }
// if err != nil {
// log.Fatal(err)
// }
// use(path)
//
// and
//
// cmd := exec.Command("prog")
// if errors.Is(cmd.Err, exec.ErrDot) {
// cmd.Err = nil
// }
// if err := cmd.Run(); err != nil {
// log.Fatal(err)
// }
//
// Setting the environment variable GODEBUG=execerrdot=0
// disables generation of ErrDot entirely, temporarily restoring the pre-Go 1.19
// behavior for programs that are unable to apply more targeted fixes.
// A future version of Go may remove support for this variable.
//
// Before adding such overrides, make sure you understand the
// security implications of doing so.
// See https://go.dev/blog/path-security for more information.
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.
type Error struct {
// Name is the file name for which the error occurred.
Name string
// 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 }
// wrappedError wraps an error without relying on fmt.Errorf.
type wrappedError struct {
prefix string
err error
}
func (w wrappedError) Error() string {
return w.prefix + ": " + w.err.Error()
}
func (w wrappedError) Unwrap() error {
return w.err
}
// Cmd represents an external command being prepared or run.
//
// A Cmd cannot be reused after calling its Run, Output or CombinedOutput
// methods.
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. 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
Err error // LookPath error, if any.
childFiles []*os.File
closeAfterStart []io.Closer
closeAfterWait []io.Closer
goroutine []func() error
goroutineErrs <-chan error // one receive per goroutine
ctxErr <-chan error // if non nil, receives the error from watchCtx exactly once
// For a security release long ago, we created x/sys/execabs,
// which manipulated the unexported lookPathErr error field
// in this struct. For Go 1.19 we exported the field as Err error,
// above, but we have to keep lookPathErr around for use by
// old programs building against new toolchains.
// The String and Start methods look for an error in lookPathErr
// in preference to Err, to preserve the errors that execabs sets.
//
// In general we don't guarantee misuse of reflect like this,
// but the misuse of reflect was by us, the best of various bad
// options to fix the security problem, and people depend on
// those old copies of execabs continuing to work.
// The result is that we have to leave this variable around for the
// rest of time, a compatibility scar.
//
// See https://go.dev/blog/path-security
// and https://go.dev/issue/43724 for more context.
lookPathErr error
}
// 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 {
lp, err := LookPath(name)
if lp != "" {
// Update cmd.Path even if err is non-nil.
// If err is ErrDot (especially on Windows), lp may include a resolved
// extension (like .exe or .bat) that should be preserved.
cmd.Path = lp
}
if err != nil {
cmd.Err = err
}
}
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 {
if c.Err != nil || c.lookPathErr != nil {
// 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 any) bool {
defer func() {
recover()
}()
return a == b
}
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 skipStdinCopyError(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 = "." + string(filepath.Separator) + path
}
if dir == "" {
return LookPath(path)
}
if filepath.VolumeName(path) != "" {
return LookPath(path)
}
if len(path) > 1 && os.IsPathSeparator(path[0]) {
return LookPath(path)
}
dirandpath := filepath.Join(dir, 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.
//
// After a successful call to Start the Wait method must be called in
// order to release associated system resources.
func (c *Cmd) Start() error {
if c.Path == "" && c.Err == nil && c.lookPathErr == nil {
c.Err = errors.New("exec: no command")
}
if c.Err != nil || c.lookPathErr != nil {
c.closeDescriptors(c.closeAfterStart)
c.closeDescriptors(c.closeAfterWait)
if c.lookPathErr != nil {
return c.lookPathErr
}
return c.Err
}
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...)
env, err := c.environ()
if err != nil {
return err
}
c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
Dir: c.Dir,
Files: c.childFiles,
Env: env,
Sys: c.SysProcAttr,
})
if err != nil {
c.closeDescriptors(c.closeAfterStart)
c.closeDescriptors(c.closeAfterWait)
return err
}
c.closeDescriptors(c.closeAfterStart)
// Don't allocate the goroutineErrs channel unless there are goroutines to fire.
if len(c.goroutine) > 0 {
errc := make(chan error, len(c.goroutine))
c.goroutineErrs = errc
for _, fn := range c.goroutine {
go func(fn func() error) {
errc <- fn()
}(fn)
}
}
c.ctxErr = c.watchCtx()
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.ProcessState != nil {
return errors.New("exec: Wait was already called")
}
state, err := c.Process.Wait()
if err == nil && !state.Success() {
err = &ExitError{ProcessState: state}
}
c.ProcessState = state
// Wait for the pipe-copying goroutines to complete.
var copyError error
for range c.goroutine {
if err := <-c.goroutineErrs; err != nil && copyError == nil {
copyError = err
}
}
c.goroutine = nil // Allow the goroutines' closures to be GC'd.
if c.ctxErr != nil {
interruptErr := <-c.ctxErr
// If c.Process.Wait returned an error, prefer that.
// Otherwise, report any error from the interrupt goroutine.
if interruptErr != nil && err == nil {
err = interruptErr
}
}
// Report errors from the copying goroutines only if the program otherwise
// exited normally on its own. Otherwise, the copying error may be due to the
// abnormal termination.
if err == nil {
err = copyError
}
c.closeDescriptors(c.closeAfterWait)
c.closeAfterWait = nil
return err
}
// watchCtx conditionally starts a goroutine that waits until either c.ctx is
// done or c.Process.Wait has completed (called from Wait).
// If c.ctx is done first, the goroutine terminates c.Process.
//
// If a goroutine was started, watchCtx returns a channel on which its result
// must be received.
func (c *Cmd) watchCtx() <-chan error {
if c.ctx == nil {
return nil
}
errc := make(chan error)
go func() {
select {
case errc <- nil:
return
case <-c.ctx.Done():
}
var err error
if killErr := c.Process.Kill(); killErr == nil {
// We appear to have successfully delivered a kill signal, so any
// program behavior from this point may be due to ctx.
err = c.ctx.Err()
} else if !errors.Is(killErr, os.ErrProcessDone) {
err = wrappedError{
prefix: "exec: error sending signal to Cmd",
err: killErr,
}
}
errc <- err
}()
return errc
}
// 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
}
// environ returns a best-effort copy of the environment in which the command
// would be run as it is currently configured. If an error occurs in computing
// the environment, it is returned alongside the best-effort copy.
func (c *Cmd) environ() ([]string, error) {
var err error
env := c.Env
if env == nil {
env, err = execenv.Default(c.SysProcAttr)
if err != nil {
env = os.Environ()
// Note that the non-nil err is preserved despite env being overridden.
}
if c.Dir != "" {
switch runtime.GOOS {
case "windows", "plan9":
// Windows and Plan 9 do not use the PWD variable, so we don't need to
// keep it accurate.
default:
// On POSIX platforms, PWD represents βan absolute pathname of the
// current working directory.β Since we are changing the working
// directory for the command, we should also update PWD to reflect that.
//
// Unfortunately, we didn't always do that, so (as proposed in
// https://go.dev/issue/50599) to avoid unintended collateral damage we
// only implicitly update PWD when Env is nil. That way, we're much
// less likely to override an intentional change to the variable.
if pwd, absErr := filepath.Abs(c.Dir); absErr == nil {
env = append(env, "PWD="+pwd)
} else if err == nil {
err = absErr
}
}
}
}
env, dedupErr := dedupEnv(env)
if err == nil {
err = dedupErr
}
return addCriticalEnv(env), err
}
// Environ returns a copy of the environment in which the command would be run
// as it is currently configured.
func (c *Cmd) Environ() []string {
// Intentionally ignore errors: environ returns a best-effort environment no matter what.
env, _ := c.environ()
return env
}
// 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.
// Except on Plan 9, items containing NUL characters are removed, and
// an error is returned along with the remaining values.
func dedupEnv(env []string) ([]string, error) {
return dedupEnvCase(runtime.GOOS == "windows", runtime.GOOS == "plan9", env)
}
// dedupEnvCase is dedupEnv with a case option for testing.
// If caseInsensitive is true, the case of keys is ignored.
// If nulOK is false, items containing NUL characters are allowed.
func dedupEnvCase(caseInsensitive, nulOK bool, env []string) ([]string, error) {
// Construct the output in reverse order, to preserve the
// last occurrence of each key.
var err error
out := make([]string, 0, len(env))
saw := make(map[string]bool, len(env))
for n := len(env); n > 0; n-- {
kv := env[n-1]
// Reject NUL in environment variables to prevent security issues (#56284);
// except on Plan 9, which uses NUL as os.PathListSeparator (#56544).
if !nulOK && strings.IndexByte(kv, 0) != -1 {
err = errors.New("exec: environment variable contains NUL")
continue
}
i := strings.Index(kv, "=")
if i == 0 {
// We observe in practice keys with a single leading "=" on Windows.
// TODO(#49886): Should we consume only the first leading "=" as part
// of the key, or parse through arbitrarily many of them until a non-"="?
i = strings.Index(kv[1:], "=") + 1
}
if i < 0 {
if kv != "" {
// The entry is not of the form "key=value" (as it is required to be).
// Leave it as-is for now.
// TODO(#52436): should we strip or reject these bogus entries?
out = append(out, kv)
}
continue
}
k := kv[:i]
if caseInsensitive {
k = strings.ToLower(k)
}
if saw[k] {
continue
}
saw[k] = true
out = append(out, kv)
}
// Now reverse the slice to restore the original order.
for i := 0; i < len(out)/2; i++ {
j := len(out) - i - 1
out[i], out[j] = out[j], out[i]
}
return out, err
}
// 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 {
k, _, ok := strings.Cut(kv, "=")
if !ok {
continue
}
if strings.EqualFold(k, "SYSTEMROOT") {
// We already have it.
return env
}
}
return append(env, "SYSTEMROOT="+os.Getenv("SYSTEMROOT"))
}
// ErrDot indicates that a path lookup resolved to an executable
// in the current directory due to β.β being in the path, either
// implicitly or explicitly. See the package documentation for details.
//
// Note that functions in this package do not return ErrDot directly.
// Code should use errors.Is(err, ErrDot), not err == ErrDot,
// to test whether a returned error err is due to this condition.
var ErrDot = errors.New("cannot run executable found relative to current directory")
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