Week 5

Regular Expressions

+ An expression needs to be evaluated and describes a result

+ A regular expression is a shorthand for abbreviating patterns of text. 

+ Regular expressions are set of rules, used to specify one or more items
  in a single character string (a sequence of characters)

+ A regular expression describes a pattern , a particular sequence of 
  characters. It does not necessarily specify a single exact sequence

+ A regular expression is a way of describing a pattern of characters to a
  program so that it can either display or modify the occurrences of that
  pattern.

+ Regular expressions are a very powerful tool for searching and processing
  text

+ Regular expressions are case sensitive

+ A regular expression is made up of the two elements

	o	a value expressed as a literal or a variable
	o	An operator

+ Regular expressions are used to write simple to complex descriptions of
  patterns


Cmds that use RG

	grep(1), egrep(1), sed(1), awk(1), perl(1) and shell.

Shell Metacharacters

	? * [] 

? means match any single character
[] means match any single character specified as a character or as a range 
inside the sq. brackets
* means match all


I/O redirection.
----------------

ps -ef > ps.out

cat /etc/passwd >> cat.out 

ps -ef

cat < /etc/group

ps -ef > ps.out 2>ps.err

ps -ef > ps.out 2>&1

ps -ef 2>ps.err 1>&2

Exec
----

exec < /etc/passwd
exec > /tmp/output

exec 3<>/dev/console
exec 4</etc/passwd
exec 5>/tmp/opt

ps -ef > &5
ls >&5

cat < &4