How to redirect and append both standard output and standard error to a file with Bash

cmd >>file.txt 2>&1

Bash executes the redirects from left to right as follows:

1. >>file.txt: Open file.txt in append mode and redirect stdout there.
2. 2>&1: Redirect stderr to “where stdout is currently going”. In this case, that is a file opened in append mode. In other words, the &1 reuses the file descriptor which stdout currently uses.