Note that in most (if not all) shells, using two greater-than signs (e.g. >>filename) will append the output to the specified file (or create it if it does not yet exist), whereas using only one such character will replace it.

In Unix-ish environments, all programs have one “standard input” file (STDIN), one “standard output” (STDOUT), and one “ standard error-output” (STDERR). The default is that STDIN reads from the terminal (or from an input-pipe), and both STDOUT and STDERR write to the terminal.