comint-input-filter

comint-input-filter is a variable defined in `comint.el'.
Its value is

#[257 "\300\301\"?\207"
      [string-match "\\`\\s *\\'"]
      4 "\n\n(fn STR)"]

Documentation:
Predicate for filtering additions to input history.
Takes one argument, the input. If non-nil, the input may be saved on the input
history list. Default is to save anything that isn't all whitespace.