The Doctor's boot care regimen.

Jul 14 2018

Boots: 14 hole Doc Martens, black, real leather.

Unlace.

Wipe down with damp paper towels.

Wipe down with dry paper towels.

Coat with Dr. Martens Wonder Balsam using included sponge.  Be sure to work balsam into stitches and exposed edges.  I ordinarily don't like to shill for particular products, but I started using this stuff to help break in my boots (it makes the leather softer, so it adapts to your feet more readily) and I was wearing them clubbing within a month of getting them (instead of six months to a year).  It's amazing stuff.

Wait half an hour.  Get some coffee, go for a run, something like that.

Buff balsam off with a clean, dry cloth.  I use a regular washcloth set aside for doing my boots.

Prep your boot polish.  I like Kiwi Shoe Polish Paste, just make sure it's the right color for your boots.  Pop the lid and set the polish on fire with a lighter or matches.  No, seriously, I mean set it on fire.  The polish will melt faster than it burns.  When at least half the polish is burning, drop the lid back on and make sure it closes completely.

Wait.  The flame will burn itself out because the oxygen inside the container (there isn't much) will be used up.  Wait for the pressure to build up inside the tin and pop the lid off with a festive "Poing!"

(If this doesn't happen inside of five minutes, just open the tin.  No big deal.)

The shoe polish is now a thick goop instead of a waxy mass.  Apply polish to your boots with a sponge.

Wait another half hour.

Buff dried polish off with a clean, dry cloth.  I usually flip the washcloth over and use that, but do whatever works.  Rub until the finish doesn't look smoky anymore.  Mine tend to look clean but a little on the dull side.  That goes away as I wear them for a while.

Re-lace and wear for an hour or two to take advantage of the new dose of balsam soaked into the leather making it a bit softer than usual.

Repeat every one or two months, or after cleaning them if they get dirty.

Setting random backgrounds in LXDE.

Jun 28 2018

So, here's the situation:

On Windbringer, I habitually run LXDE as my desktop environment because it's lightweight and does what I need: It manages windows, gives me a menu, and stays out of my way so I can do interesting things.  For years I've been using a utility called GKrellm to implement not only system monitoring on my desktop (because I like to know what's going on), but to set and change my desktop background every 24 hours.  However, GKrellm has gotten somewhat long in the tooth and I've started using something different for realtime monitoring (but that's not the point of this post).  So, the question is, how do I set my background now?  Conky doesn't have that capability.

I tried a few of the old standbys like feh and nitrogen, but they didn't seem to work.  The reason for this appears to be that PCmanFM, which is both the file manager and the desktop... stuff... of LXDE.  By this, I refer to the desktop icons as well as the background image.  As it turns out, nothing I tried to change the background worked, and that is due to the fact that PCmanFM is a jealous desktop module and doesn't let other tools frob the settings it's in charge of.  After some tinkering, here's how I did it:

Short form: pcmanfm -w `ls -d -1 /home/drwho/backgrounds/* | shuf -n 1`

Long form (from inside to outside):

  • ls -d -1 /home/drwho/backgrounds/* - List all of the files in /home/drwho/backgrounds.  Show the full path to each file.  List everything in a single column.
  • | - Feed the output of the last command to the input of the next command.
  • shuf -n 1 - shuf is a little-known GNU Coreutils tool which randomly shuffles whatever things you give it.  It only returns one line of output, a randomly chosen image file.
  • The output of the previous two commands (captured between back-ticks) is passed to...
  • pcmanfm -w - Set the current desktop background to whatever filename is passed on the command line as a free action.

To set an initial background when I log in, I added the following command to my ~/.config/lxsession/LXDE/autostart file: @pcmanfm -w `ls -d -1 /home/drwho/backgrounds/* | shuf -n 1`

This means that the command will run every time my desktop starts up.  The @ symbol tells lxsession to re-run the command if it ever crashes.  However, how do I change my background periodically?

The easiest way to set that up was to set a cron job that runs every day.  Every user gets their own set of cron jobs (called a crontab) so you don't need any particular privileges to do this (unless your machine's really locked down).  If you've never set a cronjob before, the command I used was this: crontab -e

My cronjob looks like this: 00 10 * * * pcmanfm -w `ls -d -1 /home/drwho/backgrounds/* | shuf -n 1`

"At 10:00 hours every day, run the following command..."

And there you have it.  One randomly set desktop background in LXDE.

Incidentally, if you're curious about all the nifty things you can do with cron, I recommend playing around at crontab.guru, it's an online editor for crontab settings.  It's good for experimenting in such a way that you don't have to worry about messing up your system, and it's also handy for figuring out particularly arcane cronjobs.

Algorithm for implementing a dead man's switch.

Mar 04 2018

So, you're probably wondering why I'm posting this, because it's a bit off of my usual fare.  The reason is I think it would be useful to make available a fairly simple algorithm for implementing a general purpose dead man's switch in whatever language you want, which is to say a DMS that could conceivably do just about anything if it activated.

But what's a dead man's switch?  Ultimately, it's a mechanism that has to be manually engaged at all times if you want something to happen, and if that switch turns off for some reason, something else happens (like a failsafe).  A good example of this is the bar on the handle of a power lawnmower you have to hold down so it'll move while the engine's running.  If you let go of the bar the engine keeps running but the lawnmower doesn't keep rolling forward.  Another example can be found in locomotives; the conductor has to hold down a switch or lever so the engine will pull the train, and if that lever is ever let go (say the engineer has a heart attack or is otherwise incapacitated) the throttle closes and the train will grind to a halt.  More along the lines of what I'll be talking about are the watchdogs found in industrial controllers and realtime operating systems.  While running normally a software process inside the device flips a bit somehow - say, writing a 0 into a certain device node.  If the underlying hardware ever finds that the bit didn't get flipped within a certain period of time it reacts somehow to fix things (for example, it might reboot in an attempt to un-stick the gizmo).

Making offline backups of a Linux machine using Backblaze.

Jan 14 2018

As frequent readers may or may not remember, I rebuilt my primary server last year, and in the process set up a fairly hefty RAID-5 array (24 terabytes) to store data.  As one might reasonably expect, backing all of that stuff up is fairly difficult.  I'd need to buy enough external hard drives to fit a copy of everything on there, plus extra space to store incremental backups for some length of time.  Another problem is that both Leandra and the backup drives would be in the same place at the same time, so if anything happened at the house I'd not only not have access to Leandra anymore, but there's an excellent chance that the backups would be wrecked, leaving me doubly screwed.

Here are the requirements I had for making offsite backups:

  • Backups of Leandra had to be offsite, i.e., not in the same state, ideally not on the same coast.
  • Reasonably low cost.  I ran the numbers on a couple of providers and paying a couple of hundred dollars a month to back up one server was just too expensive.
  • Linux friendly.
  • My data gets encrypted with a key only I know before it gets sent to the backup provider.
  • A number of different backup applications had to support the provider, in case one was no longer supported.
  • Easy to restore data from backup.

After a week or two of research and experimentation, as well as pinging various people to get their informed opinions, I decided to go with Backblaze as my offsite backup provider, and Duplicity as my backup software.  Here's how I went about it, as well as a few gotchas I ran into along the way.

Quick and dirty copies of website with wget.

Jan 14 2018

Let's say there's a website that you want to make a local mirror of.  This means that you can refer to it offline, and you can make offline backups of it for archival.  Let's further state that you have access to some server someplace with enough disk space to hold the copy, and that you can start a task, disconnect, and let it run to completion some time later, with GNU Screen for example.  Let's further state that you want the local copy of the site to not be broken when you load it in a browser; all the links should work, all the images should load, and so forth.  One of the quickest and easiest ways to do this is with the wget utility.