I’m learning a little Python for a side project. I wrote a SELECT statement that would produce NULLs and rather than use COALESCE to prevent returning NULLs, I wanted the application to be able to detect and then replace the value with a default.

In Perl, this is a no-brainer to me - the NULL value would be ‘undef’, and I could write something like:

$value = $default if (! defined $value);

So, after a bit of struggling, I found out that the ‘None’ object is returned when a value is NULL in python.

To determine whether you’ve got a NULL, you write something like:

if $value is None:
    $value = $default

Generally speaking, Python considers an undefined object to be a rare occurrence.

Here are the slides from my PDX.pm talk this week. A link to the accompanying podcast will be along soon.

Other fun things we discussed at the meeting:
Cisco::Reconfig
trapgen
logger
logwatch

Thanks for the lively discussion!

[edit] podcast!

[edit] Clarification of two items from the podcast:
- multiline messages do indeed come in multiple packets. There is a message counter that increments for each message, so you could use the host name + message counter to match up multi-line messages. For what I’m doing, the important part is in that first line, so the payoff isn’t worth the investment.
- re hypens in the mnemonic field of the system message: I went back through and wasn’t able to find any examples of this, so I retract my statement. (I do have examples of system messages with hyphens in the facility field.)

:bd - closes the current buffer window

CTRL-F - page down; CTRL-B - page up; CTRL-D - put current line in the center of your screen

CTRL-R - Redo (plus COUNT for redoing a number of changes); ‘u‘ is for undo; ‘U‘ is for undoing all changes on a particular line

:set undolevels=NUM - number of changes saved in memory (can set to negative number if running out of memory

:split - split your current window into two; switch between windows with CTRL-W CTRL-W

!{cmd} - run a shell command, shows you the output and prompts you before returning to your current buffer; ‘:!‘ by itself runs the last external command; ‘:!!‘ repeats the last command; :silent !{cmd} eliminates the need to hit enter after the command is done; ‘:r !{cmd}‘ puts the output of $cmd into the current buffer.

I’m learning the PostgreSQL system catalog tables right now, and was wishing that I had some catalog flashcards.  I made my wish ‘aloud’ on IRC today, and @davidfetter managed to blow my mind with this simple command-line switch for psql: -E. Now, every time I use a command, the tool tells me what the underlying SQL query is. Great for jogging the memory, and I’ll be putting off making my catalog flashcards for a few more days. Now I wonder what other revealing command-line switches I’ve overlooked!

See screenshot:

Poking around on CPAN a week or so ago, I stumbled across Cisco::Reconfig.

It looked pretty interesting, and turned out to be the work of David Muir Sharnoff. He’s responsible for Net::Netmask, which is one of my five favorites, so I had to check it out.

Sample router config (not all lines shown):

interface Loopback0
 ip address 10.254.254.1 255.255.255.255
!
interface Ethernet0/0
 description Admin LAN
 ip address 10.10.1.1 255.255.255.0
 duplex auto
!
interface Serial0/0
 description to Internet ID W065432
 ip address 1.1.1.1 255.255.255.252
!
interface Ethernet0/1
 no ip address
 shutdown
!
interface Ethernet1/0
 description User LAN
 ip address 10.10.2.1 255.255.254.0
 duplex auto
!
interface Ethernet1/1
 ip address 10.10.4.1 255.255.254.0
 duplex auto
!

Let’s write a quick & dirty script^W program to look for blank interface descriptions, because that’s something I find really annoying:

#!/usr/local/bin/perl

use strict;
use Cisco::Reconfig;

my $host        = "gabrielle";
my $host_config = "$host.confg";
print ("Checking: $host\n");
my $config  = readconfig("$host_config");

for my $intf ( $config->get('interface') ) {
    next if ( $intf->get('shutdown') );	#ignore, don't care
    next if ( $intf =~ /Loopback/ );	#idc

    my $descr   = $intf->get('description');
    chomp ($intf, $descr);  	# hm, kinda feel like I shouldn't have to do this?
    print ("$intf: $descr\n");	# just to be chatty
    if (! $descr) {
        print ("$host: $intf: Description is blank!\n");
    }
}

exit 0;

:::–>./sample.pl
Checking: gabrielle
interface Ethernet0/0: description Admin LAN
interface Serial0/0: description to Internet ID W065432
interface Ethernet1/0: description User LAN
interface Ethernet1/1:
gabrielle: interface Ethernet1/1: Description is blank!

I’m primarily interested in it for checking compliance of our configurations with our business standards. No need to chunk through a config line-by-line with regexps; in a lot of cases, a simple ‘get’ will tell me if something’s configured or not (eg “snmp server-location”.)

Specific features that look really intriguing:
- you can generate the commands you need to “fix” your config
- the “context” method allows you to draw out the surrounding lines

I can’t wait to mess around with this some more.

Specifically for the management of IP networks & equipment - routers, switches, etc

1. Connection automation:

Because it’s really handy to update, say, the passwords on all 1000 of your network devices in a couple of hours by kicking off a single script. It’s faster and more reliable. Of course, if you fat-finger the password in the script, you’ve just fat-fingered it on your entire network, so please test it first.

Net::Telnet::Cisco - this is really cool, but doesn’t work on older equipment, due to Cisco’s lack of a standardized interface. So, we roll our own with Expect.pm.

—–
2. IP Addressing:

Socket.pm of course, though not being a C programmer I have a hard time remembering the syntax off the top of my head so I keep a couple of examples around.

my $ip = $ARGV[0];
my @addr = split(/\./, $ip);
my $addr = pack(’C4′, @addr);
my $name = gethostbyaddr($addr, AF_INET);
print (”Name: $name\n”);

my $host = $ARGV[0];
my $ip = scalar gethostbyname($host);
my @ip = unpack(”C4″,$ip);
my $ip = join(”.”,@ip);
print (”IP: $ip\n”);
—–
3. Subnetting:

Net::Netmask, baby! For figuring out network & bcast addresses, accept no substitutes.

my $ip = “10.1.1.1″;
my $netmask = “255.255.255.0″;
my $block = new Net::Netmask($ip, $netmask);
my $bcast_addr = $block->broadcast();
print (”Broadcast address: $bcast_addr\n”);

—–
4. SNMP. Y’all knew I would have to talk about this.

Several options, two I have actual experience with.

A lot of people use Net::SNMP I use SNMP.pm (which comes with Net-SNMP - confused?* ) It’s probably the most complex, but also (to me) the most useful. I can get pretty much any data I want with this module.

SNMP-Simple - much more user-friendly than SNMP.pm. Lighter, faster, but you can only get *values* back from this, you can not get the OIDs, so it requires some pre-knowledge of what you want to monitor, which isn’t always possible.

—–
5. RRD.pm - perl module for Tobi Oetiker’s RRDTool. Many network performance tools are based on RRDTool: mrtg, cacti, orca, NMIS. It’s indispensable if you’re going to write your own monitoring app, or tinker with one of the aforementioned tools.

—–
6. Date::Format and Date::Manip. I use Date::Format because it’s so easy to create timestamps for log files & reports - it follows the Unix strftime format. Date::Manip is huge and slow, but it’s the only thing I found that could handle some complex user time reporting I was doing a while back, and I love it.

—–
7. Net::Mac. I haven’t had time to experiment with this to the extent I’d like, but I have a feeling it’s going to be as useful as Net::Netmask. I’ll let you know in a couple of weeks.

*One of my favorite jokes, just for me.