Getting one garbage character from container.exec()

[jamesmurdza]

I've written two helper functions using Dockerode. The idea is to run a command synchronously and then return the results as a string.

It works well, except each chunk that is returned by stream.on('data') starts with a garbage character, like "X", "*", "!", etc. I can't figure out why.

This happens for multiple commands, such as "bash" and "cat".

Help is much appreciated!

My code below:

async function waitForStreamEnd(stream: NodeJS.ReadableStream): Promise<void> {
  return new Promise<void>((resolve, reject) => {
    try {
      stream.on('end', async () => {
        resolve();
      });
    } catch (err) {
      reject(err);
    }
  });
}

async function runCommandInContainer(container: Docker.Container, command: string[]): Promise<string> {
  const exec = await container.exec({
    Cmd: command,
    AttachStdout: true,
    AttachStderr: true,
  });
  const stream = await exec.start({ hijack: true, stdin: true });
  let output = "";
  stream.on('data', (data) => {
    output += data;
  });
  await waitForStreamEnd(stream);
  return output;
}

[Akimyou]

const readStream = (stream: stream.Duplex) => {
  let output: string = "";
  return new Promise<string>((resolve, reject) => {
    stream.on("data", (chunk: string) => {
      output += chunk;
    });

    stream.on("end", () => {
     // handle garbage
      resolve(output.trim().split("\n").map(processString).join("\n"));
    });
  });
  
  function processString(str: string): string {
    const out = Buffer.from(str, "binary");
    if (out.readUInt8(0) === 1) {
      return out.toString("utf8", 8);
    } else {
      return out.toString("utf8", 0);
    }
  }
};

May be this function can help you.

[jamesmurdza]

Interesting, I will try this. Any explanation why this would work?

[Fhwang0926]

add more comment
i also has it issue but i guess it is docker' api problem, is not?

for example, exec with ping

i just console.log of exec result

[jamesmurdza]

@Fhwang0926 Were you able to figure out any solution?

[ewanmellor]

When you do not use the tty flag, Docker sends data with the first four bytes set to tell you whether the data is from stdout or stderr, and its length. This is the "garbage" that you're seeing at the start of the line.

See "Stream format" within https://docs.docker.com/reference/api/engine/version/v1.43/#tag/Container/operation/ContainerAttach