Is there a word for "truncate to size zero", preferably one which could be used and understood in the following way:
"...ing the file" := "truncating the file to zero bytes"?
RECAP: The best answers so far seem to be "empty the file", "zero the file size" and "truncate to (size|length) zero". In a suitable context, "truncate" has "truncate to (size|length) zero" as the default meaning (esp. since it's the commonest specific length to which files are truncated) but it's by no means the only one