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
原文：http://blog.csdn.net/christprince007/article/details/39546415
