linux - How to redirect and append both stdout and stderr to a file with Bash?

To redirect stdout to a truncated file in Bash, I know to use:

cmd > file.txt

To redirect stdout in Bash, appending to a file, I know to use:

cmd >> file.txt

To redirect both stdout and stderr to a truncated file, I know to use:

cmd &> file.txt

How do I redirect both stdout and stderr appending to a file? cmd &>> file.txt did not work for me.

Answer

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.