I needed to execute shell commands and keep the process alive to keep the changes made in the env variables, so I made a function that looks like this:

import { spawn } from 'child_process';

function exec(command) {

    const bash = spawn('/bin/bash', {
        detached: true
    });

    bash.stdout.setEncoding('utf-8');
    bash.stderr.setEncoding('utf-8');

    return new Promise(resolve => {
        const result = {
            stdout: '',
            stderr: '',
            code: 0
        };

        bash.stdout.on('data', data => {
            // Removes the last line break (i.e \n) from the data.
            result.stdout += data.substring(0, data.length - 1);

            // Gets the code by getting the number before the `:` on the last line
            result.code = Number(result.stdout.split('\n').pop().split(':')[0]);

            resolve(result);
        });

        bash.stderr.on('data', err => {
            result.stderr += err;

            resolve(result);
        });

        // Writes the passed command and another to check the code of the command
        bash.stdin.write(command + `; echo "$?:${ command }"\n`);
    });
}

The test is simple:

it('should run a command that outputs an error', async () => {
    // Notice the typo in `echoo`
    const res = await exec('echoo test');

    expect(res.code).toBe(127);
});

When running this test, it sometimes fails with res.code being 0 instead of 127.

When testing without jest, it works flawlessly 100% of the time.