AWS JavaScript SDK: Retrieving Shell Output from ECS Execute Command

I was able to accomplish this in Node using npm packages 'ssm-session' and 'ws'.

First, executing the command using ECS ExecuteCommandCommand:

    const { ECSClient, ListTasksCommand, ExecuteCommandCommand } = require("@aws-sdk/client-ecs");
    const ecs = new ECSClient({ region })
    const executeCommand = new ExecuteCommandCommand( {cluster, interactive: true, command, task})
    const response = await ecs.send(executeCommand)
    const { streamUrl, tokenValue } = response.session

Then, you can use the following snippet to log the output using the streamUrl and tokenValue connected above.

    const WebSocket = require("ws");
    const { ssm } = require("ssm-session");
    const util = require("util");

    const textDecoder = new util.TextDecoder();
    const textEncoder = new util.TextEncoder();

    const termOptions = {
        rows: 34,
        cols: 197,
    };

    const connection = new WebSocket(streamUrl);

    process.stdin.on("keypress", (str, key) => {
        if (connection.readyState === connection.OPEN) {
            ssm.sendText(connection, textEncoder.encode(str));
        }
    });

    connection.onopen = () => {
        ssm.init(connection, {
            token: tokenValue,
            termOptions: termOptions,
        });
    };

    connection.onerror = (error) => {
        console.log(`WebSocket error: ${error}`);
    };

    connection.onmessage = (event) => {
        var agentMessage = ssm.decode(event.data);
        ssm.sendACK(connection, agentMessage);
        if (agentMessage.payloadType === 1) {
            process.stdout.write(textDecoder.decode(agentMessage.payload));
        } else if (agentMessage.payloadType === 17) {
            ssm.sendInitMessage(connection, termOptions);
        }
    };

    connection.onclose = () => {
        console.log("websocket closed");
    };

I hope this helps!