Mathematica allows text searching using regular expressions (based on the PCRE library). It would take some work to re-implement the whole grep functionality within Mathematica, but for your concrete example

grep -nr -C 2 <pattern> 

it is as easy as follows:

ClearAll[Grep] Grep[files_List, patt_, c_Integer: 0, style : {__} : {Red, Bold}] := Monitor[Do[Grep[files[[i]], patt, c, style], {i, Length[files]}], ProgressIndicator[i, {1, Length[files]}]]; Grep::noopen = "Can't open \"``\"."; Grep[file_, patt_, c_Integer: 0, style : {__} : {Red, Bold}] := Module[{lines, pos}, Quiet[Check[lines = ReadList[file, "String"], Return[Message[Grep::noopen, file], Module]], {ReadList::noopen, ReadList::stream}]; pos = Flatten@Position[StringContainsQ[lines, patt], True]; If[pos =!= {}, Echo@Grid[Prepend[{#, Column@StringReplace[lines[[Span[Max[# - c, 1], UpTo[# + c]]]], str : patt :> "\!\(\*StyleBox[\"" <> str <> "\"," <> StringRiffle[ToString /@ style, ", "] <> "]\)"]} & /@ pos, {file, SpanFromLeft}], Dividers -> All, Alignment -> Left]];]; 

where

• file or files is a file name/path or a list of them

• patt is a literal string, StringExpression or RegularExpression pattern to search for

• c is number of additional lines of leading and trailing output context

• style is a List of styling directives to be applied to the matching text (I use here great solution by halirutan from this answer); if you don't want to apply a style, put {{}} as the value for this option

For obtaining the complete listing of files in a directory and all its subdirectories at all levels one can use FileNames as Select[FileNames[All, dir, Infinity], Not@*DirectoryQ]. A very enlightening discussion of its usage for obtaining only specific filepaths can be found here.

Examples

Find lines containing the word "Welfare" and display them with 1 surrounding line of leading and trailing context:

Grep[FindFile@"ExampleData/USConstitution.txt", WordBoundary ~~ "Welfare" ~~ WordBoundary, 1] 

Search for word "eye" in all files in a directory and all its subdirectories:

dir = FileNameJoin[{$InstallationDirectory, "Documentation/English/System/ExamplePages"}]; files = Select[FileNames[All, dir, Infinity], Not@*DirectoryQ]; Grep[files, WordBoundary ~~ "eye" ~~ WordBoundary] 

(* during evaluation it displays ProgressIndicator *)

Update

As recommended in the comments by @b3m2a1, you can also use RunProcess as a simpler way to execute grep. You need to supply the command as a list of the command plus the space delimited arguments and set the ProcessDirectory. To do a recursive search for NotebookDirectory in notebook files enter the following:

cmd = "grep -RH \"NotebookDirectory\" --include=\"Int*.nb\" *"; RunProcess[{"bash", "-c", cmd}, "StandardOutput", ProcessDirectory -> NotebookDirectory[]] (* "Absorption/BakedSlider/InterphaseMassTransfer_slider.nb: \ RowBox[{\"NotebookDirectory\", \"[\", \"]\"}], \"]\"}], \";\"}], Absorption/BakedSlider/InterphaseMassTransfer_slider.nb:" *) 

Original Answer

Here is an example calling the system grep (using Cygwin and putting bash.exe in my path on Windows). Remember to escape special characters. The following does a recursive directory search on "NotebookDirectory" including Mathematica notebooks matching the pattern "Int*.nb".

SetDirectory[NotebookDirectory[]]; file = CreateFile[]; Run["grep -RH \"NotebookDirectory\" --include=\"Int*.nb\" * >>" <> file]; FilePrint[file] DeleteFile[file]; (*Absorption/BakedSlider/InterphaseMassTransfer_slider.nb: RowBox[{"NotebookDirectory", "[", "]"}], "]"}], ";"}], Absorption/BakedSlider/InterphaseMassTransfer_slider.nb: RowBox[{"NotebookDirectory", "[", "]"}], "]"}], ";"}], Absorption/BakedSlider/InterphaseMassTransfer_sliderb.nb: RowBox[{"NotebookDirectory", "[", "]"}], "]"}], ";"}], Absorption/BakedSlider/InterphaseMassTransfer_sliderb.nb: *)