print lines matching a pattern
    -R, -r, --recursive
           Read all files under each directory, recursively; this is equivalent to the -d recurse option.

Other Options
           Use line buffering on output.  This can cause a performance penalty.
-i, --ignore-case
       Ignore case distinctions in both the PATTERN and the input files.  (-i is specified by POSIX.)
-n, --line-number
       Prefix  each  line of output with the 1-based line number within its input file.  (-n is specified
       by POSIX.)
source manpages: rgrep