Echo into a file with root permissions
echo will fail since the redirection is executed by the shell, not the command you run.
$ sudo echo "foo" >> /etc/bar bash: /etc/bar: Permission denied
Shortly, the above is running
echo as root, but
>> will be run by the surrounding shell.
To overcome this, you can use
$ echo "foo" | sudo tee -a /etc/bar/
To overwrite the file, rather than append to it, omit