mongosh exits when it encounters the end of its input and no more commands are running. However, this is an issue when Node.js builtin async APIs are used; for example,

cursor = db.getCollection("accounts").aggregate([{$match:{}}])
cursorAsStream = stream.Readable.from(cursor.map(entry => EJSON.stringify(entry) + '\n'));
cursorAsStream.pipe(fs.createWriteStream('accounts'));

will truncate the resulting file when mongosh exits after the .pipe() call.

As a workaround, I suggested on slack to add while (!dest.writableFinished) sleep(10); in this scenario, but that’s obviously not something that we really want to recommend as the official solution to this issue.

We should think of a better way of solving this. It would be relatively easy to add a waitFor(promise) shell API, however, this would essentially mean committing to support for top-level await in mongosh scripts.