grep, egrep and fgrep from manpage

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 the matching lines.

egrep
-E, --extended-regexp
Interpret PATTERN as an extended regular expression (ERE, see below).

fgrep
-F, --fixed-strings
Interpret PATTERN as a list of fixed strings, separated by newlines, any of which is to be matched.

Important Options with grep :
-R, -r, --recursive
Read all files under each directory, recursively; this is equivalent to the -d recurse option.
--exclude-dir=DIR
Exclude directories matching the pattern DIR from recursive searches.
-A NUM, --after-context=NUM
Print NUM lines of trailing context after matching lines. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.
-B NUM, --before-context=NUM
Print NUM lines of leading context before matching lines. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.
-n, --line-number
Prefix each line of output with the 1-based line number within its input file.
-c, --count
Suppress normal output; instead print a count of matching lines for each input file.
-i, --ignore-case
Ignore case distinctions in both the PATTERN and the input files.


egrep is the same as grep -E.
fgrep is the same as grep -F.
rgrep is the same as grep -r.

grep Repetition
A regular expression may be followed by one of several repetition operators:
? The preceding item is optional and matched at most once.
* The preceding item will be matched zero or more times.
+ The preceding item will be matched one or more times.
{n} The preceding item is matched exactly n times.
{n,} The preceding item is matched n or more times.
{,m} The preceding item is matched at most m times.
{n,m} The preceding item is matched at least n times, but not more than m times.