Man page says
-d ACTION, --directories=ACTION
If an input file is a directory, use ACTION to process it. By
default, ACTION is read, which
means that directories are read just as if they were ordinary
files. If ACTION is skip,
directories are silently skipped. If ACTION is recurse,
grep reads all files under each
directory, recursively; this is equivalent to the -r option.
Add an example of when ACTION=read of ever finding anything, or admit
$ grep -c . /
0
$ grep -vc . /
0
else people will wonder what you are talking about.
Also add an ACTION=warn, which acts like
$ wc /
wc: /: Is a directory
0 0 0 /
Of course don't warn if they are recursing.
