grep-find-ignored-files

grep-find-ignored-files is a variable defined in `grep.el'.
Its value is

(".#*" "*.o" "*~" "*.bin" "*.lbin" "*.so" "*.a" "*.ln" "*.blg" "*.bbl" "*.elc" "*.lof" "*.glo" "*.idx" "*.lot" "*.fmt" "*.tfm" "*.class" "*.fas" "*.lib" "*.mem" "*.x86f" "*.sparcf" "*.dfsl" "*.pfsl" "*.d64fsl" "*.p64fsl" "*.lx64fsl" "*.lx32fsl" "*.dx64fsl" "*.dx32fsl" "*.fx64fsl" "*.fx32fsl" "*.sx64fsl" "*.sx32fsl" "*.wx64fsl" "*.wx32fsl" "*.fasl" "*.ufsl" "*.fsl" "*.dxl" "*.lo" "*.la" "*.gmo" "*.mo" "*.toc" "*.aux" "*.cp" "*.fn" "*.ky" "*.pg" "*.tp" "*.vr" "*.cps" "*.fns" "*.kys" "*.pgs" "*.tps" "*.vrs" "*.pyc" "*.pyo")

Original value was

(".#*" "*.o" "*~" "*.bin" "*.lbin" "*.so" "*.a" "*.ln" "*.blg" "*.bbl" "*.elc" "*.lof" "*.glo" "*.idx" "*.lot" "*.fmt" "*.tfm" "*.class" "*.fas" "*.lib" "*.mem" "*.x86f" "*.sparcf" "*.dfsl" "*.pfsl" "*.d64fsl" "*.p64fsl" "*.lx64fsl" "*.lx32fsl" "*.dx64fsl" "*.dx32fsl" "*.fx64fsl" "*.fx32fsl" "*.sx64fsl" "*.sx32fsl" "*.wx64fsl" "*.wx32fsl" "*.fasl" "*.ufsl" "*.fsl" "*.dxl" "*.lo" "*.la" "*.gmo" "*.mo" "*.toc" "*.aux" "*.cp" "*.fn" "*.ky" "*.pg" "*.tp" "*.cps" "*.fns" "*.kys" "*.pgs" "*.tps" "*.vrs" "*.pyc" "*.pyo")

Documentation:
List of file names which `rgrep' and `lgrep' shall exclude.
If an element is a cons cell, the car is called on the search directory
to determine whether cdr should not be excluded.

You can customize this variable.