man grep

    grep  searches the named input FILEs (or standard input if no files are   named, or if a single hyphen-minus (-) is given as file name) for lines containing  a  match to the given PATTERN.  By default, grep prints thematching lines.

       In  addition,  three  variant  programs  egrep,  fgrep  and  rgrep  areavailable.   egrep  is  the  same  as  grep -E.   fgrep  is the same asgrep -F.  rgrep is the same as grep -r.  Direct  invocation  as  either egrep  or  fgrep  is  deprecated,  but  is provided to allow historical applications that rely on them to run unmodified.

 Examples: ifconfig | grep -i packets

Grep第一课(基本)