当缓冲区接收数据时，将exec.Command输出复制到文件

Question

I have a script that dumps quite a bit of text into STDOUT when run. I'm trying to execute this script and write the output to a file without holding the entire buffer in memory at one time. (We're talking many megabytes of text that this script outputs at one time.)

The following works, but because I'm doing this across several goroutines, my memory consumption shoots up to > 5GB which I would really like to avoid:

var out bytes.Buffer
cmd := exec.Command("/path/to/script/binary", "arg1", "arg2")
cmd.Stdout = &out
err := cmd.Run()
if err != nil {
    log.Fatal(err)
}
out.WriteTo(io) // io is the writer connected to the new file

Ideally as out fills up, I want to be emptying it into my target file to keep memory usage low. I've tried changing this to:

cmd := exec.Command("/path/to/script/binary", "arg1", "arg2")
cmd.Start()
stdout, _ := cmd.StdoutPipe()
r := *bufio.NewReader(stdout)
r.WriteTo(io)
cmd.Wait()

However when I print out these variables stdout is <nil>, r is {[0 0 0 0 0...]}, and r.WriteTo panics: invalid memory address or nil pointer dereference.

Is it possible to write the output of cmd as it is generated to keep memory usage down? Thanks!

Answer 1

Why don't you just write to a file directly?

file, _ := os.Create("/some/file")
cmd.Stdout = file

Or use your io thing (that's a terrible name for a variable, by the way, since it's a) the name of a standard library package, b) ambiguous--what does it mean?)

cmd.Stdout = io