gettime.pl

#!/usr/local/bin/perl5 -w
#
#	gettime.pl
#
#	The #!.../perl lets the operating system know what program
#	to use to process this program.  This type of statement
#	is also used to start up shell scripts.
#
#	Any line starting with a "#" indicates that the line is a
#	comment.  In general, the "#" can be used to mark the rest
#	of the line as a comment as well.
#
#	Get the time out of server machine.
#
#	This is a quick hack given that the network time protocol
#	exists.  However, it does work, and is reasonably effective.
#

###
###	Configuration
###
#	I tend to put configuration information towards the start
#	of my programs.  This means that I only need to look in one
#	place when I need to change those items that I realized
#	beforehand were configuration issues.
#
#	Variables in Perl are prefixed by special characters.
#	The "$" character indicates that the variable is a "scalar"
#	variable.  It will contain a single value.  That value
#	can be numeric or a string.
#
#	All statements in Perl must be terminated with a
#	semicolon.
$host = 'kira.peak.org';

###
###	Query the time server
###
###	Many Internet machines have a time service.  We will be
###	using this service.
###
#	The localtime function (which is described in the book)
#	returns an array.  This next statement calls the localtime
#	function, selects the 8th entry (the timezone), and stores
#	it in a variable.
$dst = (localtime())[8];

#	Like "C", Perl has a ternary operator.  If the expression
#	is True, the result is the expression after the "?",
#	otherwise the result is the expression after the ":".
#
#	You will find all of the "C" operators in Perl.  You will
#	also find many of the operators from shell programming
#	as well.  Perl treats strings as a data type, and there
#	are a number of operators involving strings as well.
$tz = ($dst) ? "PDT" : "PST";

#	The "system" function sends the command in quotes to the
#	operating system to be executed, much as the "C" function
#	of the same name.
system("date > /dev/null");	# Load the date pgm into the cache

#	The backquotes (watch the direction carefully) also sends
#	a command to the system.  This syntax comes from shell
#	programming (and you can see that Perl is an interesting
#	combination of "C", shell programming, and later you'll
#	see awk, sed, and tr.)
#
#	This command starts up telnet to talk to the specified host
#	on port 13, which simply returns the time as a text string
#	along with information about telnet starting up and shutting
#	down.  The pipe through grep looks for the line containing
#	the two colons from the time, so we get only the line
#	containing the time.
#
#	$_ is a special variable.  Many functions (or commands)
#	will operate on this variable if no other variable is
#	specified.  The "chomp" deletes the last character of the
#	line, which is the newline character.
#
#	The grep command uses a "regular expression".  It says
#	look for two colons separated by exactly two characters,
#	whose contents we don't care about.  While this regular
#	expression is for the grep command, you will find that
#	they play an important part of Perl.  The set of regular
#	expressions is more fancy than most other Unix utilities
#	use, so pay attention to that sections of whatever book
#	you use.
$_ = `/usr/net/bin/telnet $host 13 | grep ":..:"`;
chomp;

###
###	Set the local time
###	Avoid doing things which take time between getting the time
###	from kira and setting the time locally.
###

#	The "if" statement can be a little different from most
#	computer languages (although a little closer to human
#	language).  The "length" is just checking the number of
#	characters in the time string to make sure that we received
#	something which could possibly be a time.  If not, we exit.
exit if length($_) < 5;

#	Strings in Perl are a bunch of character enclosed in quote.
#	marks.  However, variables are expanded in the string.
#	So the "$_" in the string printed below gets expanded into
#	the string which the time server sent back.  It doesn't
#	matter if the variable is a string variable or a numeric
#	variable; it gets expanded into a string as you
#	normally want.
print "Time from kira is $_\n";

$oldtime = time;
print "New time is:      ";
#	The backslashes before the quote marks indicate that these
#	characters don't terminate the string, but are literal
#	characters which in this case are sent to the system.
system("date -s \"$_ $tz\"");
$newtime = time;
if ($oldtime < $newtime)
    {
    print "Time has been advanced by ",
    		$newtime - $oldtime, " seconds\n";
    }
#	Note that Perl uses an "elsif" statement instead of
#	"else if" as would be used in "C".  Also, blocks of
#	statements must be enclosed in a pair of braces.
elsif ($oldtime > $newtime)
    {
    print "Time has been retarded by ",
    		$oldtime - $newtime, " seconds\n";
    }

Last modified 27 May 2006
Dave Regan
http://www.peak.org/~regan/
Resume / Biography