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This package is a small but very useful wrapper aroundos/exec.Cmd that makes it safe and simple to run external commands in highly concurrent, asynchronous, real-time applications. It works on Linux, macOS, and Windows. Here's the basic usage:

import ("fmt""time""github.com/go-cmd/cmd")funcmain() {// Start a long-running process, capture stdout and stderrfindCmd:=cmd.NewCmd("find","/","--name","needle")statusChan:=findCmd.Start()// non-blockingticker:=time.NewTicker(2*time.Second)// Print last line of stdout every 2sgofunc() {forrangeticker.C {status:=findCmd.Status()n:=len(status.Stdout)fmt.Println(status.Stdout[n-1])}}()// Stop command after 1 hourgofunc() {<-time.After(1*time.Hour)findCmd.Stop()}()// Check if command is doneselect {casefinalStatus:=<-statusChan:// donedefault:// no, still running}// Block waiting for command to exit, be stopped, or be killedfinalStatus:=<-statusChan}

That's it, only three methods:Start,Stop, andStatus. When possible, it's better to usego-cmd/Cmd thanos/exec.Cmd becausego-cmd/Cmd provides:

1. Channel-based fire and forget
2. Real-time stdout and stderr
3. Real-time status
4. Complete and consolidated return
5. Proper process termination
6. 100% test coverage, no race conditions

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 exitc:=cmd.NewCmd("foo")s:=<-c.Start()

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

It's common to want to read stdout or stderrwhile the command is running. The common approach is to callStdoutPipe and read from the providedio.ReadCloser. This works but it's wrong because it causes a race condition (thatgo test -race detects) and the docs say it's wrong:

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.

The proper solution is to set theio.Writer ofStdout. 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.

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:

typeStatusstruct {CmdstringPIDintCompleteboolExitintErrorerrorRuntimefloat64// secondsStdout   []stringStderr   []string}

Speaking of that struct above, Go built-inCmd 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 theStatus struct above to convey all information about the command. Even when the command finishes, callingStatus returns the final status, the same final status sent to the status channel returned by the call toStart.

os/exec/Cmd.Wait can block even after the command is killed. That can be surprising and cause problems. Butgo-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.

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

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

MIT © go-Cmd.