go-cmd/Cmd

This package is a small but very useful wrapper around os/exec.Cmd for Linux and macOS that makes it safe and simple to run external commands in highly concurrent, asynchronous, real-time applications. Here's the look and feel:

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

// Start a long-running process, capture stdout and stderr
findCmd := cmd.NewCmd("find", "/", "--name" "needle")
statusChan := findCmd.Start() // non-blocking

ticker := time.NewTicker(2 * time.Second)

// Print last line of stdout every 2s
go func() {
  for range ticker.C {
    status := c.Status()
    n := len(status.Stdout)
    fmt.Println(status.Stdout[n - 1])
  }
}()

// Stop command after 1 hour
go func() {
  <-time.After(1 * time.Hour)
  findCmd.Stop()
}()

// Check if command is done
select {
case finalStatus := <-statusChan:
  // done
default:
  // no, still running
}

// Block waiting for command to exit, be stopped, or be killed
finalStatus := <-statusChan

That's it, only three methods: Start, Stop, and Status. When possible, it's better to use go-cmd/Cmd than os/exec.Cmd because go-cmd/Cmd provides:

Channel-based fire and forget
Real-time stdout and stderr
Real-time status
Complete and consolidated return
Proper process termination
100% test coverage, no race conditions

Channel-based fire and forget

As the example above shows, starting a command immediately returns a channel to which the final status is sent when the command exits for any reason. So by default commands run asynchronously, but running synchronously is possible and easy, too:

// Run foo and block waiting for it to exit
c := cmd.NewCmd("foo")
s := <-c.Start()

To achieve similar with os/exec.Cmd requires everything this package already does.

Real-time stdout and stderr

It's common to want to read stdout or stderr while the command is running. The common approach is to call StdoutPipe and read from the provided io.ReadCloser. This works but it's wrong because it causes a race condition (that go test -race detects) and the docs say it's wrong: "it is incorrect to call Wait before all reads from the pipe have completed. [...] it is incorrect to call Run when using StdoutPipe".

The proper solution is to set the io.Writer of Stdout. To be thread-safe and non-racey, this requires further work to write while possibly N-many goroutines read. go-cmd/Cmd has done this work.

Real-time status

Similar to real-time stdout and stderr, it's nice to see, for example, elapsed runtime. This package allows that: Status can be called any time by any goroutine, and it returns this struct:

type Status struct {
    Cmd      string
    PID      int
    Complete bool
    Exit     int
    Error    error
    Runtime  float64 // seconds
    Stdout   []string
    Stderr   []string
}

Complete and consolidated return

Speaking of that struct above, Go built-in Cmd does not put all the return information in one place, which is fine because Go is awesome! But to save some time, go-cmd/Cmd uses the Status struct above to convey all information about the command. Even when the command finishes, calling Status returns the final status, the same final status sent to the status channel returned by the call to Start.

Proper process termination

os/exec/Cmd.Wait can block even after the command is killed. That can be surprising and cause problems. But go-cmd/Cmd.Stop reliably terminates the command, no surprises. The issue has to do with process group IDs. It's common to kill the command PID, but usually one needs to kill its process group ID instead. go-cmd/Cmd.Stop implements the necessary low-level magic to make this happen.

100% test coverage, no race conditions

In addition to 100% test coverage and no race conditions, this package is actively used in production environments.

Acknowledgements

Brian Ip wrote the original code to get the exit status. Strangely, Go doesn't just provide this, it requires magic like exiterr.Sys().(syscall.WaitStatus) and more.

tkschmidt / cmd