FindList
FindList["file","text"] gives a list of lines in the file that contain the specified string.
FindList["file",{"text1","text2",…}] gives a list of all lines that contain any of the specified strings.
FindList[{"file1",…},…] gives a list of lines containing the specified strings in any of the filei.
FindList[files,text,n] includes only the first n lines found.
Please visit the official Wolfram Language Reference for more details and examples on core symbols.