Forward STDOUT/STDERR

[alixaxel]

With a simple PHP script that writes to both STDOUT and STDERR:

fwrite(STDOUT, "stdout\n");
fwrite(STDERR, "stderr\n");

The output streams work as expected when executed with PHP directly:

alix@ThinkPad:~/dexec_1.0.2_linux_386$ php streams.php 1>/dev/null
stderr
alix@ThinkPad:~/dexec_1.0.2_linux_386$ php streams.php 2>/dev/null
stdout

However, when running via dexec, STDERR is treated as if it was STDOUT:

alix@ThinkPad:~/dexec_1.0.2_linux_386$ dexec streams.php 1>/dev/null
alix@ThinkPad:~/dexec_1.0.2_linux_386$ dexec streams.php 2>/dev/null
stdout
stderr

---

The exact same thing happens with a Node.js script:

console.log("stdout");
console.error("stderr");

alix@ThinkPad:~/dexec_1.0.2_linux_386$ node streams.js 1>/dev/null
stderr
alix@ThinkPad:~/dexec_1.0.2_linux_386$ node streams.js 2>/dev/null
stdout

alix@ThinkPad:~/dexec_1.0.2_linux_386$ dexec streams.js 1>/dev/null
alix@ThinkPad:~/dexec_1.0.2_linux_386$ dexec streams.js 2>/dev/null
stdout
stderr

---

Is there any option to make dexec respect and forward STDOUT and STDERR?

[andystanton]

Hi @alixaxel

There is not an option for this at the moment, but there should be! I'll add this in for the next release.

[alixaxel]

@andystanton Thanks for getting back to me. Yeah, I think that this feature is a important one to have.

If I use the following code as it's being used by RunAnonymousContainer, everything works as expected.

package main

import (
    "os"
    "os/exec"
)

func main() {
    cmd := exec.Command("node", "/home/alix/streams.js")

    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr

    cmd.Run()
}

So my assumption is that Docker itself is not forwarding the streams properly.

[alixaxel]

I found this old related issue (moby/moby#725), but it was fixed a long time ago.

So I did a couple of tests running the Docker container directly:

alix@900X4C:~/$ docker run --rm -v $(pwd -P)/streams.js:/tmp/dexec/build/streams.js -t dexec/lang-node streams.js
stdout
stderr
alix@900X4C:~/$ docker run --rm -v $(pwd -P)/streams.js:/tmp/dexec/build/streams.js -t dexec/lang-node streams.js 1>/dev/null
alix@900X4C:~/$ docker run --rm -v $(pwd -P)/streams.js:/tmp/dexec/build/streams.js -t dexec/lang-node streams.js 2>/dev/null
stdout
stderr

Same behavior. However, if you remove the -t flag:

alix@900X4C:~/$ docker run --rm -v $(pwd -P)/streams.js:/tmp/dexec/build/streams.js dexec/lang-node streams.js
stderr
stdout
alix@900X4C:~/$ docker run --rm -v $(pwd -P)/streams.js:/tmp/dexec/build/streams.js dexec/lang-node streams.js 1>/dev/null
stderr
alix@900X4C:~/$ docker run --rm -v $(pwd -P)/streams.js:/tmp/dexec/build/streams.js dexec/lang-node streams.js 2>/dev/null
stdout

It works out nicely. The flag -t allocates a new TTY but I'm not sure why we even need this in dexec.

  -t, --tty=false            Allocate a pseudo-TTY

Would you accept a PR that removes it or is this critical for something else?

[andystanton]

Thanks for the fix @alixaxel I've accepted the PR and will make a 1.0.3 patch release containing this fix.

Ultimately I want to move away from shelling out commands to the Docker CLI. I created #35 to address this and have done some work, but I think a better approach would be to make dexec use one of the existing Go libraries: https://docs.docker.com/engine/reference/api/remote_api_client_libraries/