example3.go

cmd := exec.Command("/bin/sh", "-c", "watch date > date.txt")
cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
start := time.Now()
time.AfterFunc(3*time.Second, func() {
	syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
})
err := cmd.Run()
fmt.Printf("pid=%d duration=%s err=%s\n", cmd.Process.Pid, time.Since(start), err)

output.txt

$ go run example3.go
pid=41352 duration=3.001809822s err=signal: killed
$ ps -j
USER    PID  PPID  PGID   SESS JOBC STAT   TT       TIME COMMAND
felix 41008 41002 41008      0    0 Ss   s000    0:00.14 -/bin/bash