exec

package
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 Go to latest Published: Sep 26, 2023 License: MIT Imports: 5 Imported by: 0

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.

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.

Index

Examples

Constants

This section is empty.

Variables

View Source 
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 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 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.

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())
}
Example (Environment) 
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)
	}
}

func CommandContext added in v1.7.0 

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.

Example 
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.
	}
}

func (*Cmd) CombinedOutput 

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

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

Example 
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)
}

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 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.

Example 
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)
}

func (*Cmd) Start 

func (c *Cmd) Start() error

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.

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. 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.

Example 
package main

import (
	"fmt"
	"io/ioutil"
	"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, _ := ioutil.ReadAll(stderr)
	fmt.Printf("%s\n", slurp)

	if err := cmd.Wait(); err != nil {
		log.Fatal(err)
	}
}

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.

Example 
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)
}

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. 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.

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) String added in v1.13.0 

func (c *Cmd) String() string

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 (*Cmd) Wait 

func (c *Cmd) Wait() error

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.

type Error 

type Error struct {
	Name string

	Err error
}

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

func (*Error) Error 

func (e *Error) Error() string

func (*Error) Unwrap added in v1.13.0 

func (e *Error) Unwrap() error

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

Source Files

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