Involved Source Files 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 [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.exec_unix.golp_unix.go
Code Examples
package main
import (
"fmt"
"log"
"os/exec"
)
func main() {
cmd := exec.Command("sh", "-c", "echo stdout; echo 1>&2 stderr")
stdoutStderr, err := cmd.CombinedOutput()
if err != nil {
log.Fatal(err)
}
fmt.Printf("%s\n", stdoutStderr)
}
package main
import (
"fmt"
"log"
"os/exec"
)
func main() {
cmd := exec.Command("pwd")
// Set Dir before calling cmd.Environ so that it will include an
// updated PWD variable (on platforms where that is used).
cmd.Dir = ".."
cmd.Env = append(cmd.Environ(), "POSIXLY_CORRECT=1")
out, err := cmd.Output()
if err != nil {
log.Fatal(err)
}
fmt.Printf("%s\n", out)
}
package main
import (
"fmt"
"log"
"os/exec"
)
func main() {
out, err := exec.Command("date").Output()
if err != nil {
log.Fatal(err)
}
fmt.Printf("The date is %s\n", out)
}
package main
import (
"log"
"os/exec"
)
func main() {
cmd := exec.Command("sleep", "1")
log.Printf("Running command and waiting for it to finish...")
err := cmd.Run()
log.Printf("Command finished with error: %v", err)
}
package main
import (
"log"
"os/exec"
)
func main() {
cmd := exec.Command("sleep", "5")
err := cmd.Start()
if err != nil {
log.Fatal(err)
}
log.Printf("Waiting for command to finish...")
err = cmd.Wait()
log.Printf("Command finished with error: %v", err)
}
package main
import (
"fmt"
"io"
"log"
"os/exec"
)
func main() {
cmd := exec.Command("sh", "-c", "echo stdout; echo 1>&2 stderr")
stderr, err := cmd.StderrPipe()
if err != nil {
log.Fatal(err)
}
if err := cmd.Start(); err != nil {
log.Fatal(err)
}
slurp, _ := io.ReadAll(stderr)
fmt.Printf("%s\n", slurp)
if err := cmd.Wait(); err != nil {
log.Fatal(err)
}
}
package main
import (
"fmt"
"io"
"log"
"os/exec"
)
func main() {
cmd := exec.Command("cat")
stdin, err := cmd.StdinPipe()
if err != nil {
log.Fatal(err)
}
go func() {
defer stdin.Close()
io.WriteString(stdin, "values written to stdin are passed to cmd's standard input")
}()
out, err := cmd.CombinedOutput()
if err != nil {
log.Fatal(err)
}
fmt.Printf("%s\n", out)
}
package main
import (
"encoding/json"
"fmt"
"log"
"os/exec"
)
func main() {
cmd := exec.Command("echo", "-n", `{"Name": "Bob", "Age": 32}`)
stdout, err := cmd.StdoutPipe()
if err != nil {
log.Fatal(err)
}
if err := cmd.Start(); err != nil {
log.Fatal(err)
}
var person struct {
Name string
Age int
}
if err := json.NewDecoder(stdout).Decode(&person); err != nil {
log.Fatal(err)
}
if err := cmd.Wait(); err != nil {
log.Fatal(err)
}
fmt.Printf("%s is %d years old\n", person.Name, person.Age)
}
package main
import (
"fmt"
"log"
"os/exec"
"strings"
)
func main() {
cmd := exec.Command("tr", "a-z", "A-Z")
cmd.Stdin = strings.NewReader("some input")
var out strings.Builder
cmd.Stdout = &out
err := cmd.Run()
if err != nil {
log.Fatal(err)
}
fmt.Printf("in all caps: %q\n", out.String())
}
package main
import (
"context"
"os/exec"
"time"
)
func main() {
ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
defer cancel()
if err := exec.CommandContext(ctx, "sleep", "5").Run(); err != nil {
// This will fail after 100 milliseconds. The 5 second sleep
// will be interrupted.
}
}
package main
import (
"log"
"os"
"os/exec"
)
func main() {
cmd := exec.Command("prog")
cmd.Env = append(os.Environ(),
"FOO=duplicate_value", // ignored
"FOO=actual_value", // this value is used
)
if err := cmd.Run(); err != nil {
log.Fatal(err)
}
}
package main
import (
"fmt"
"log"
"os/exec"
)
func main() {
path, err := exec.LookPath("fortune")
if err != nil {
log.Fatal("installing fortune is in your future")
}
fmt.Printf("fortune is available at %s\n", path)
}
Package-Level Type Names (total 3)
/* sort by: | */
Cmd represents an external command being prepared or run.
A Cmd cannot be reused after calling its [Cmd.Run], [Cmd.Output] or [Cmd.CombinedOutput]
methods. 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. If Cancel is non-nil, the command must have been created with
CommandContext and Cancel will be called when the command's
Context is done. By default, CommandContext sets Cancel to
call the Kill method on the command's Process.
Typically a custom Cancel will send a signal to the command's
Process, but it may instead take other actions to initiate cancellation,
such as closing a stdin or stdout pipe or sending a shutdown request on a
network socket.
If the command exits with a success status after Cancel is
called, and Cancel does not return an error equivalent to
os.ErrProcessDone, then Wait and similar methods will return a non-nil
error: either an error wrapping the one returned by Cancel,
or the error from the Context.
(If the command exits with a non-success status, or Cancel
returns an error that wraps os.ErrProcessDone, Wait and similar methods
continue to return the command's usual exit status.)
If Cancel is set to nil, nothing will happen immediately when the command's
Context is done, but a nonzero WaitDelay will still take effect. That may
be useful, for example, to work around deadlocks in commands that do not
support shutdown signals but are expected to always finish quickly.
Cancel will not be called if Start returns a non-nil error. 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.
On Unix systems, the value of Dir also determines the
child process's PWD environment variable if not otherwise
specified. A Unix process represents its working directory
not by name but as an implicit reference to a node in the
file tree. So, if the child process obtains its working
directory by calling a function such as C's getcwd, which
computes the canonical name by walking up the file tree, it
will not recover the original value of Dir if that value
was an alias involving symbolic links. However, if the
child process calls Go's [os.Getwd] or GNU C's
get_current_dir_name, and the value of PWD is an alias for
the current directory, those functions will return the
value of PWD, which matches the value of Dir. 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.
See also the Dir field, which may set PWD in the environment. // LookPath error, if any. 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. 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. Process is the underlying process, once started. ProcessState contains information about an exited process.
If the process was started successfully, Wait or Run will
populate its ProcessState when the command completes.Stderrio.Writer 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,
or because a nonzero WaitDelay was set and expired. 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 or a nonzero WaitDelay
expires.
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. SysProcAttr holds optional, operating system-specific attributes.
Run passes it to os.StartProcess as the os.ProcAttr's Sys field. If WaitDelay is non-zero, it bounds the time spent waiting on two sources
of unexpected delay in Wait: a child process that fails to exit after the
associated Context is canceled, and a child process that exits but leaves
its I/O pipes unclosed.
The WaitDelay timer starts when either the associated Context is done or a
call to Wait observes that the child process has exited, whichever occurs
first. When the delay has elapsed, the command shuts down the child process
and/or its I/O pipes.
If the child process has failed to exit — perhaps because it ignored or
failed to receive a shutdown signal from a Cancel function, or because no
Cancel function was set — then it will be terminated using os.Process.Kill.
Then, if the I/O pipes communicating with the child process are still open,
those pipes are closed in order to unblock any goroutines currently blocked
on Read or Write calls.
If pipes are closed due to WaitDelay, no Cancel call has occurred,
and the command has otherwise exited with a successful status, Wait and
similar methods will return ErrWaitDelay instead of nil.
If WaitDelay is zero (the default), I/O pipes will be read until EOF,
which might not occur until orphaned subprocesses of the command have
also closed their descriptors for the pipes. CombinedOutput runs the command and returns its combined standard
output and standard error. Environ returns a copy of the environment in which the command would be run
as it is currently configured. Output runs the command and returns its standard output.
Any returned error will usually be of type [*ExitError].
If c.Stderr was nil and the returned error is of type
[*ExitError], Output populates the Stderr field of the
returned error. 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. 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 [Cmd.Wait] method must be called in
order to release associated system resources. StderrPipe returns a pipe that will be connected to the command's
standard error when the command starts.
[Cmd.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 [Cmd.Run] when using StderrPipe.
See the StdoutPipe example for idiomatic usage. 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 [Cmd.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. StdoutPipe returns a pipe that will be connected to the command's
standard output when the command starts.
[Cmd.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 [Cmd.Run] when using StdoutPipe.
See the example for idiomatic usage. 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. 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 [Cmd.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].
*Cmd : expvar.Var
*Cmd : fmt.Stringer
func Command(name string, arg ...string) *Cmd
func CommandContext(ctx context.Context, name string, arg ...string) *Cmd
func internal/testenv.CleanCmdEnv(cmd *Cmd) *Cmd
func internal/testenv.Command(t testing.TB, name string, args ...string) *Cmd
func internal/testenv.CommandContext(t testing.TB, ctx context.Context, name string, args ...string) *Cmd
func internal/testenv.CleanCmdEnv(cmd *Cmd) *Cmd
Error is returned by [LookPath] when it fails to classify a file as an
executable. Err is the underlying error. Name is the file name for which the error occurred.(*Error) Error() string(*Error) Unwrap() error
*Error : error
An ExitError reports an unsuccessful exit by a command.ProcessState*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.(*ExitError) Error() string ExitCode returns the exit code of the exited process, or -1
if the process hasn't exited or was terminated by a signal. Exited reports whether the program has exited.
On Unix systems this reports true if the program exited due to calling exit,
but false if the program terminated due to a signal. Pid returns the process id of the exited process.( ExitError) String() string Success reports whether the program exited successfully,
such as with exit status 0 on Unix. Sys returns system-dependent exit information about
the process. Convert it to the appropriate underlying
type, such as [syscall.WaitStatus] on Unix, to access its contents. SysUsage returns system-dependent resource usage information about
the exited process. Convert it to the appropriate underlying
type, such as [*syscall.Rusage] on Unix, to access its contents.
(On Unix, *syscall.Rusage matches struct rusage as defined in the
getrusage(2) manual page.) SystemTime returns the system CPU time of the exited process and its children. UserTime returns the user CPU time of the exited process and its children.
*ExitError : error
ExitError : expvar.Var
ExitError : fmt.Stringer
Package-Level Functions (total 3)
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.
CommandContext is like [Command] but includes a context.
The provided context is used to interrupt the process
(by calling cmd.Cancel or [os.Process.Kill])
if the context becomes done before the command completes on its own.
CommandContext sets the command's Cancel function to invoke the Kill method
on its Process, and leaves its WaitDelay unset. The caller may change the
cancellation behavior by modifying those fields before starting the command.
LookPath searches for an executable 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.
Otherwise, on success, the result is an absolute path.
In older versions of Go, LookPath could return a path relative to the current directory.
As of Go 1.19, LookPath will instead return that path along with an error satisfying
[errors.Is](err, [ErrDot]). See the package documentation for more details.
Package-Level Variables (total 3)
ErrDot indicates that a path lookup resolved to an executable
in the current directory due to ‘.’ 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.
ErrNotFound is the error resulting if a path search failed to find an executable file.
ErrWaitDelay is returned by [Cmd.Wait] if the process exits with a
successful status code but its output pipes are not closed before the
command's WaitDelay expires.
The pages are generated with Goldsv0.7.7-preview. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
Please follow @zigo_101 (reachable from the left QR code) to get the latest news of Golds.
