exec

Documentation

Overview

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.

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.

Index

• Variables
• func LookPath(file string) (string, error)
• type Cmd
  • func Command(name string, arg ...string) *Cmd
  • func CommandContext(ctx context.Context, name string, arg ...string) *Cmd
  • func (c *Cmd) CombinedOutput() ([]byte, error)
  • func (c *Cmd) Output() ([]byte, error)
  • func (c *Cmd) Run() error
  • func (c *Cmd) Start() error
  • func (c *Cmd) StderrPipe() (io.ReadCloser, error)
  • func (c *Cmd) StdinPipe() (io.WriteCloser, error)
  • func (c *Cmd) StdoutPipe() (io.ReadCloser, error)
  • func (c *Cmd) Wait() error
• type Error
  • func (e *Error) Error() string
• type ExitError
  • func (e *ExitError) Error() string

Examples

• Cmd.Output
• Cmd.Start
• Cmd.StdoutPipe
• Command
• LookPath

Variables

var ErrNotFound = errors.New("executable file not found in $PATH")

ErrNotFound is the error resulting if a path search failed to find an executable file.

Functions

func LookPath

func LookPath(file string) (string, error)

LookPath searches for an executable binary named file in the directories named by the PATH environment variable. If file contains a slash, it is tried directly and the PATH is not consulted. The result may be an absolute path or a path relative to the current directory.

Example

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)
}

Types

type Cmd

type Cmd struct {
	Path string

	Args []string

	Env []string

	Dir string

	Stdin io.Reader

	Stdout io.Writer
	Stderr io.Writer

	ExtraFiles []*os.File

	SysProcAttr *syscall.SysProcAttr

	Process *os.Process

	ProcessState *os.ProcessState
	// contains filtered or unexported fields
}

Cmd represents an external command being prepared or run.

A Cmd cannot be reused after calling its Run, Output or CombinedOutput methods.

func Command

func Command(name string, arg ...string) *Cmd

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 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")

Example

package main

import (
	"bytes"
	"fmt"
	"log"
	"os/exec"
	"strings"
)

func main() {
	cmd := exec.Command("tr", "a-z", "A-Z")
	cmd.Stdin = strings.NewReader("some input")
	var out bytes.Buffer
	cmd.Stdout = &out
	err := cmd.Run()
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("in all caps: %q\n", out.String())
}

func CommandContext

func CommandContext(ctx context.Context, name string, arg ...string) *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 (*Cmd) CombinedOutput

func (c *Cmd) CombinedOutput() ([]byte, error)

CombinedOutput runs the command and returns its combined standard output and standard error.

func (*Cmd) Output

func (c *Cmd) Output() ([]byte, error)

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.

Example

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)
}

func (*Cmd) Run

func (c *Cmd) Run() 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 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 (*Cmd) Start

func (c *Cmd) Start() error

Start starts the specified command but does not wait for it to complete.

The Wait method will return the exit code and release associated resources once the command exits.

Example

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)
}

func (*Cmd) StderrPipe

func (c *Cmd) StderrPipe() (io.ReadCloser, error)

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 (*Cmd) StdinPipe

func (c *Cmd) StdinPipe() (io.WriteCloser, error)

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 (*Cmd) StdoutPipe

func (c *Cmd) StdoutPipe() (io.ReadCloser, error)

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.

Example

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)
}

func (*Cmd) Wait

func (c *Cmd) Wait() error

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.

If c.Stdin is not an *os.File, Wait also waits for the I/O loop copying from c.Stdin into the process's standard input to complete.

Wait releases any resources associated with the Cmd.

type Error

type Error struct {
	Name string
	Err  error
}

Error records the name of a binary that failed to be executed and the reason it failed.

func (*Error) Error

func (e *Error) Error() string

type ExitError

type ExitError struct {
	*os.ProcessState

	Stderr []byte
}

An ExitError reports an unsuccessful exit by a command.

func (*ExitError) Error

func (e *ExitError) Error() string