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.

Index

• Variables
• func LookPath(file string) (string, error)
• type Cmd
  • func Command(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
• Bugs

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 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.
	// 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).
	// 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 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(rsc): On OS X 10.6, child processes may sometimes inherit unwanted fds.
	// https://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
	// 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 (*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.

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
}

An ExitError reports an unsuccessful exit by a command.

func (*ExitError) Error

func (e *ExitError) Error() string

Notes

Bugs

• On OS X 10.6, child processes may sometimes inherit unwanted fds. https://golang.org/issue/2603