What does 2>&1 actually mean?

The quick answer

File descriptor 1 is stdout and 2 is stderr. The expression 2>&1 duplicates the current destination of stdout onto stderr.

command >out.log 2>&1

What's going on?

File descriptor 1 is stdout and 2 is stderr. The expression 2>&1 duplicates the current destination of stdout onto stderr.

Understanding the underlying mechanism makes troubleshooting much easier than memorising a command in isolation.

Dig a little deeper

Redirections are processed left to right. command >out 2>&1 sends both streams to out; command 2>&1 >out leaves stderr pointing at the original stdout.

Hands-on commands

Try these in a suitable test environment. Read each command before running it, especially if you have elevated privileges.

printf 'hello\n' >out.log 2>&1
ls /no-such-path >out.log 2>&1
cat out.log