cmd

Documentation

Overview

Package cmd runs external commands with concurrent access to output and status. It wraps the Go standard library os/exec.Command to correctly handle reading output (STDOUT and STDERR) while a command is running and killing a command. All operations are safe to call from multiple goroutines.

A basic example that runs env and prints its output:

import (
    "fmt"
    "github.com/go-cmd/cmd"
)

func main() {
    // Create Cmd, buffered output
    envCmd := cmd.NewCmd("env")

    // Run and wait for Cmd to return Status
    status := <-envCmd.Start()

    // Print each line of STDOUT from Cmd
    for _, line := range status.Stdout {
        fmt.Println(line)
    }
}

Commands can be ran synchronously (blocking) or asynchronously (non-blocking):

envCmd := cmd.NewCmd("env") // create

status := <-envCmd.Start() // run blocking

statusChan := envCmd.Start() // run non-blocking
// Do other work while Cmd is running...
status <- statusChan // blocking

Start returns a channel to which the final Status is sent when the command finishes for any reason. The first example blocks receiving on the channel. The second example is non-blocking because it saves the channel and receives on it later. Only one final status is sent to the channel; use Done for multiple goroutines to wait for the command to finish, then call Status to get the final status.

Index

• Constants
• type Cmd
  • func NewCmd(name string, args ...string) *Cmd
  • func NewCmdOptions(options Options, name string, args ...string) *Cmd
  • func (c *Cmd) Done() <-chan struct{}
  • func (c *Cmd) Start() <-chan Status
  • func (c *Cmd) Status() Status
  • func (c *Cmd) Stop() error
• type ErrLineBufferOverflow
  • func (e ErrLineBufferOverflow) Error() string
• type Options
• type OutputBuffer
  • func NewOutputBuffer() *OutputBuffer
  • func (rw *OutputBuffer) Lines() []string
  • func (rw *OutputBuffer) Write(p []byte) (n int, err error)
• type OutputStream
  • func NewOutputStream(streamChan chan string) *OutputStream
  • func (rw *OutputStream) Lines() <-chan string
  • func (rw *OutputStream) SetLineBufferSize(n int)
  • func (rw *OutputStream) Write(p []byte) (n int, err error)
• type Status

Constants

const (
	// DEFAULT_LINE_BUFFER_SIZE is the default size of the OutputStream line buffer.
	// The default value is usually sufficient, but if ErrLineBufferOverflow errors
	// occur, try increasing the size by calling OutputBuffer.SetLineBufferSize.
	DEFAULT_LINE_BUFFER_SIZE = 16384

	// DEFAULT_STREAM_CHAN_SIZE is the default string channel size for a Cmd when
	// Options.Streaming is true. The string channel size can have a minor
	// performance impact if too small by causing OutputStream.Write to block
	// excessively.
	DEFAULT_STREAM_CHAN_SIZE = 1000
)

Types

type Cmd

type Cmd struct {
	Name   string
	Args   []string
	Env    []string
	Stdin  io.ReadCloser
	Stdout chan string // streaming STDOUT if enabled, else nil (see Options)
	Stderr chan string // streaming STDERR if enabled, else nil (see Options)
	*sync.Mutex
	// contains filtered or unexported fields
}

Cmd represents an external command, similar to the Go built-in os/exec.Cmd. A Cmd cannot be reused after calling Start. Exported fields are read-only and should not be modified, except Env which can be set before calling Start. To create a new Cmd, call NewCmd or NewCmdOptions.

func NewCmd

func NewCmd(name string, args ...string) *Cmd

NewCmd creates a new Cmd for the given command name and arguments. The command is not started until Start is called. Output buffering is on, streaming output is off. To control output, use NewCmdOptions instead.

func NewCmdOptions

func NewCmdOptions(options Options, name string, args ...string) *Cmd

NewCmdOptions creates a new Cmd with options. The command is not started until Start is called.

func (*Cmd) Done

func (c *Cmd) Done() <-chan struct{}

Done returns a channel that's closed when the command stops running. This method is useful for multiple goroutines to wait for the command to finish.Call Status after the command finishes to get its final status.

func (*Cmd) Start

func (c *Cmd) Start() <-chan Status

Start starts the command and immediately returns a channel that the caller can use to receive the final Status of the command when it ends. The caller can start the command and wait like,

status := <-myCmd.Start() // blocking

or start the command asynchronously and be notified later when it ends,

statusChan := myCmd.Start() // non-blocking
// Do other work while Cmd is running...
status := <-statusChan // blocking

Exactly one Status is sent on the channel when the command ends. The channel is not closed. Any Go error is set to Status.Error. Start is idempotent; it always returns the same channel.

func (*Cmd) Status

func (c *Cmd) Status() Status

Status returns the Status of the command at any time. It is safe to call concurrently by multiple goroutines.

With buffered output, Status.Stdout and Status.Stderr contain the full output as of the Status call time. For example, if the command counts to 3 and three calls are made between counts, Status.Stdout contains:

"1"
"1 2"
"1 2 3"

The caller is responsible for tailing the buffered output if needed. Else, consider using streaming output. When the command finishes, buffered output is complete and final.

Status.Runtime is updated while the command is running and final when it finishes.

func (*Cmd) Stop

func (c *Cmd) Stop() error

Stop stops the command by sending its process group a SIGTERM signal. Stop is idempotent. An error should only be returned in the rare case that Stop is called immediately after the command ends but before Start can update its internal state.

type ErrLineBufferOverflow

type ErrLineBufferOverflow struct {
	Line       string // Unterminated line that caused the error
	BufferSize int    // Internal line buffer size
	BufferFree int    // Free bytes in line buffer
}

ErrLineBufferOverflow is returned by OutputStream.Write when the internal line buffer is filled before a newline character is written to terminate a line. Increasing the line buffer size by calling OutputStream.SetLineBufferSize can help prevent this error.

func (ErrLineBufferOverflow) Error

func (e ErrLineBufferOverflow) Error() string

type Options

type Options struct {
	// If Buffered is true, STDOUT and STDERR are written to Status.Stdout and
	// Status.Stderr. The caller can call Cmd.Status to read output at intervals.
	// See Cmd.Status for more info.
	Buffered bool

	// If Streaming is true, Cmd.Stdout and Cmd.Stderr channels are created and
	// STDOUT and STDERR output lines are written them in real time. This is
	// faster and more efficient than polling Cmd.Status. The caller must read both
	// streaming channels, else lines are dropped silently.
	Streaming bool
}

Options represents customizations for NewCmdOptions.

type OutputBuffer

type OutputBuffer struct {
	*sync.Mutex
	// contains filtered or unexported fields
}

OutputBuffer represents command output that is saved, line by line, in an unbounded buffer. It is safe for multiple goroutines to read while the command is running and after it has finished. If output is small (a few megabytes) and not read frequently, an output buffer is a good solution.

A Cmd in this package uses an OutputBuffer for both STDOUT and STDERR by default when created by calling NewCmd. To use OutputBuffer directly with a Go standard library os/exec.Command:

import "os/exec"
import "github.com/go-cmd/cmd"
runnableCmd := exec.Command(...)
stdout := cmd.NewOutputBuffer()
runnableCmd.Stdout = stdout

While runnableCmd is running, call stdout.Lines() to read all output currently written.

func NewOutputBuffer

func NewOutputBuffer() *OutputBuffer

NewOutputBuffer creates a new output buffer. The buffer is unbounded and safe for multiple goroutines to read while the command is running by calling Lines.

func (*OutputBuffer) Lines

func (rw *OutputBuffer) Lines() []string

Lines returns lines of output written by the Cmd. It is safe to call while the Cmd is running and after it has finished. Subsequent calls returns more lines, if more lines were written. "\r\n" are stripped from the lines.

func (*OutputBuffer) Write

func (rw *OutputBuffer) Write(p []byte) (n int, err error)

Write makes OutputBuffer implement the io.Writer interface. Do not call this function directly.

type OutputStream

type OutputStream struct {
	// contains filtered or unexported fields
}

OutputStream represents real time, line by line output from a running Cmd. Lines are terminated by a single newline preceded by an optional carriage return. Both newline and carriage return are stripped from the line when sent to a caller-provided channel.

The caller must begin receiving before starting the Cmd. Write blocks on the channel; the caller must always read the channel. The channel is not closed by the OutputStream.

A Cmd in this package uses an OutputStream for both STDOUT and STDERR when created by calling NewCmdOptions and Options.Streaming is true. To use OutputStream directly with a Go standard library os/exec.Command:

import "os/exec"
import "github.com/go-cmd/cmd"

stdoutChan := make(chan string, 100)
go func() {
    for line := range stdoutChan {
        // Do something with the line
    }
}()

runnableCmd := exec.Command(...)
stdout := cmd.NewOutputStream(stdoutChan)
runnableCmd.Stdout = stdout

While runnableCmd is running, lines are sent to the channel as soon as they are written and newline-terminated by the command. After the command finishes, the caller should wait for the last lines to be sent:

for len(stdoutChan) > 0 {
    time.Sleep(10 * time.Millisecond)
}

Since the channel is not closed by the OutputStream, the two indications that all lines have been sent and received are the command finishing and the channel size being zero.

func NewOutputStream

func NewOutputStream(streamChan chan string) *OutputStream

NewOutputStream creates a new streaming output on the given channel. The caller must begin receiving on the channel before the command is started. The OutputStream never closes the channel.

func (*OutputStream) Lines

func (rw *OutputStream) Lines() <-chan string

Lines returns the channel to which lines are sent. This is the same channel passed to NewOutputStream.

func (*OutputStream) SetLineBufferSize

func (rw *OutputStream) SetLineBufferSize(n int)

SetLineBufferSize sets the internal line buffer size. The default is DEFAULT_LINE_BUFFER_SIZE. This function must be called immediately after NewOutputStream, and it is not safe to call by multiple goroutines.

Increasing the line buffer size can help reduce ErrLineBufferOverflow errors.

func (*OutputStream) Write

func (rw *OutputStream) Write(p []byte) (n int, err error)

Write makes OutputStream implement the io.Writer interface. Do not call this function directly.

type Status

type Status struct {
	Cmd      string
	PID      int
	Complete bool     // false if stopped or signaled
	Exit     int      // exit code of process
	Error    error    // Go error
	StartTs  int64    // Unix ts (nanoseconds), zero if Cmd not started
	StopTs   int64    // Unix ts (nanoseconds), zero if Cmd not started or running
	Runtime  float64  // seconds, zero if Cmd not started
	Stdout   []string // buffered STDOUT; see Cmd.Status for more info
	Stderr   []string // buffered STDERR; see Cmd.Status for more info
}

Status represents the running status and consolidated return of a Cmd. It can be obtained any time by calling Cmd.Status. If StartTs > 0, the command has started. If StopTs > 0, the command has stopped. After the command finishes for any reason, this combination of values indicates success (presuming the command only exits zero on success):

Exit     = 0
Error    = nil
Complete = true

Error is a Go error from the underlying os/exec.Cmd.Start or os/exec.Cmd.Wait. If not nil, the command either failed to start (it never ran) or it started but was terminated unexpectedly (probably signaled). In either case, the command failed. Callers should check Error first. If nil, then check Exit and Complete.