grep-command

grep-command is a variable defined in `grep.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
The default grep command for M-x grep.
If the grep program used supports an option to always include file names
in its output (such as the `-H' option to GNU grep), it's a good idea to
include it when specifying `grep-command'.

In interactive usage, the actual value of this variable is set up
by `grep-compute-defaults'; to change the default value, use
Customize or call the function `grep-apply-setting'.

You can customize this variable.