Create gzip compressed tar file - write to stdout

I try to create a gzip compress tar file. The problem, the output is not a valid gzip compress file.

When i do a file logfiles.tgz it just say: logfiles.tgz: data and it shows no files/nothing when i open it.

const { exec } = require("child_process");
const fs = require("fs");


const outputStream = fs.createWriteStream("../logfiles.tar.gz");


const child = exec(`tar -cz *`, {
    cwd: "./logs"
}, (err) => {
    if (err) {
        console.error(`Could not create archive: ${err}`);
    } else {
        console.log(`Archive created`);
    }
});

child.stdout.pipe(outputStream);

outputStream.on("error", (err) => {
    console.error(`Could not write: ${err}`);
});

outputStream.on("finish", () => {
    console.log("Write Done");
});

When i let tar write directly to the filesystem it works just fine:

exec("tar -czf ../logfiles.tar.gz *", {
    cwd: "./logs"
}, (err) => {
    if (err) {
        console.error(`Could not create archive: ${err}`);
    } else {
        console.log(`Archive created`);
    }
});

As soon as i want the output via stdout as stream, i get a corrupt/invalid file. I want to send the tar output over http without the need to first write to the filesystem.

When i do a gunzip: logfiles.tar.gz it says gzip: logfiles.tar.gz: not in gzip format, which is not really a surprise when file ... says "data" instead of "gzip".

Why is this not a valid gzip compressed file when i use streams/stdout?

When i compare both files, the stream file has (nearly) twice the size:

-rw-rw-r--   1 marc marc 117936 Apr 26 11:58 logfiles-direct.tar.gz
-rw-rw-r--   1 marc marc 213813 Apr 26 11:58 logfiles-stream.tar.gz

With the help of Jochaim Sauer's comment i got it working.

The problem was the encoding option for "exec". Setting it to "buffer" resolved the problem.

    const child = exec(`tar -cz *`, {
        cwd: "./logs",
        encoding: "buffer"
    });

Works like expected.