How to append the output to a file?

Yeah.

command >> file to redirect just stdout of command.

command >> file 2>&1 to redirect stdout and stderr to the file (works in bash, zsh)

And if you need to use sudo, remember that just

sudo command >> /file/requiring/sudo/privileges does not work, as privilege elevation applies to command but not shell redirection part. However, simply using tee solves the problem:

command | sudo tee -a /file/requiring/sudo/privileges

Use >> to append:

command >> file