If you want to get both the output and errors from a command, you might try it this way at first:
$ grep pattern myfile /nonexistent > matches 2> matches
However, this does not do what you might expect; you might get only the output, or only the errors, or nothing at all, depending on the system. The syntax you want is to specify a file descriptor as the redirection target, using the ampersand (&), syntax to specify 1 (the standard output stream's descriptor):
$ grep pattern myfile /nonexistent > matches 2>&1
You can read the last word of this command line as "redirect standard error to the same place standard output is already going."
Note that the order matters here; you can't put 2>&1 before > matches to accomplish ...
