streaming os/exec.Command

Question

I want to build a benchmarking tool similar to the unix tool time. What I currently have is this:

package main

import (
    "fmt"
    "os"
    "os/exec"
    "time"
)

func main() {
    command := os.Args[1]
    args := os.Args[2:]
    cmd := exec.Command(command, args...)
    start_time := time.Now().UnixNano()
    stdout, err := cmd.Output()

    if err != nil {
        println(err.Error())
        return
    }

    print(string(stdout))
    total_time := int64(time.Nanosecond) * (time.Now().UnixNano() - start_time) / int64(time.Millisecond)
    fmt.Println(total_time)
}

My problem with this is, that the output is not streaming, instead it is printed all at once, and weirdly enough for some programs not at all.

Stephen Weinberg · Accepted Answer · 2015-06-26T20:38:56.150

2

The best way is to avoid touching the stream altogether. You can pass your own file descriptors for stdout and stderr directly.

package main

import (
    "fmt"
    "os"
    "os/exec"
    "time"
)

func main() {
    command := os.Args[1]
    args := os.Args[2:]
    cmd := exec.Command(command, args...)
    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr
    start_time := time.Now()

    err := cmd.Run()
    if err != nil {
        println(err.Error())
        return
    }

    total_time := int64(time.Since(start_time) / time.Millisecond)
    fmt.Println(total_time)
}

Also, print() should never be used. Instead, use one of the fmt.Print* functions.

total_time := int64(time.Nanosecond) * (time.Now().UnixNano() - start_time) / int64(time.Millisecond)

This line is extremely confusing. int64(time.Nanosecond) * x resolves to 1 * x or just x.

edited Jun 26 '15 at 20:38

answered Jun 26 '15 at 20:32

Stephen Weinberg

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OP probably saw that time * x doesn't work when x is of type int. There's no reason for `total_time := int64(...`, – Danver Braganza Jun 26 '15 at 20:41
I am saying that multiplying it by Nanosecond is a no-op because Nanosecond = 1. – Stephen Weinberg Jun 26 '15 at 20:43
1

Thanks for all your suggestions, I'm very new to go and mostly copy and pasting things together from different sources. – brancz Jun 26 '15 at 20:44
The Nanosecond thing originated from comments in this thread http://stackoverflow.com/questions/24122821/go-golang-time-now-unixnano-convert-to-milliseconds – brancz Jun 26 '15 at 20:46
Actually, nvm. My previous suggestion was wrong for all sorts of reasons. – Danver Braganza Jun 26 '15 at 20:49

evanmcdonnal · Answer 2 · 2015-06-26T20:41:48.563

0

I'm not sure what you're attempting to do here with print(string(stdout)) but it's unnecessary and the problems you're facing are likely a side effect of some misuse there.

func main() {
    command := os.Args[1]
    args := os.Args[2:]
    cmd := exec.Command(command, args...)
    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr
    start_time := time.Now().UnixNano()

    if err != nil {
        fmt.Println(err.Error())
        return
    }

    total_time := int64(time.Nanosecond) * (time.Now().UnixNano() - start_time) / int64(time.Millisecond)
    fmt.Println(total_time)
}

edited Jun 26 '15 at 20:41

answered Jun 26 '15 at 20:34

evanmcdonnal

46,131
16
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The OP wants to see the output of command to be piped to stdout and stderr. – Danver Braganza Jun 26 '15 at 20:39
@DanverBraganza that's a simple change, I'll edit it. – evanmcdonnal Jun 26 '15 at 20:41

streaming os/exec.Command

2 Answers2