Grep第一课(基本)

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