duncanlock.net

We use CentOS VMs at work to emulate our production environment - and it took me a while to figure out how to get the VirtualBox Guest Additions to build reliably on CentOS 6.4/5. This is what I’ve currently settled on as a reliable method.

First, make sure that you’ve got the kernel headers and tools installed that you need to build stuff:

$ sudo yum update -y
$ sudo yum install gcc kernel-devel kernel-headers dkms make bzip2 perl

Make sure that you’ve only got the current set of kernel headers installed - the one for the kernel you’re actually running. Having more than one set installed prevents this working properly. Running this should show you one version of each kernel package:

$ rpm -qa | grep kernel | sort

It should look something like this:

dracut-kernel-004-336.el6_5.2.noarch …
Continue reading “Reliably Building VirtualBox Guest Additions on CentOS 6.x”

To create image thumbnails from a PDF document, run this in a terminal window:

$ convert -thumbnail x300 -background white -alpha remove input_file.pdf[0] output_thumbnail.png

The parameters to convert do the following things:

ParameterEffect
-thumbnailSimilar to -resize, but optimized for speed and strips metadata.
x300Make the thumbnail 300px tall, and whatever width maintains the aspect ratio.
-background whiteSets the thumbnail background to white.
-alpha removeRemoves the alpha channel from the thumbnail output.
input_file.pdfThe PDF file to use as input.
[0]The page number of the input file to use for the thumbnail.
output_thumbnail.pngThe output thumbnail file to create.

If you want larger thumbnails, just change the x300 parameter to match. If you want to output .jpg’s (or anything else, like .gif), just change the file …

Continue reading “How to create thumbnails for PDFs with ImageMagick on Linux”

A dummy placeholder image
Figure 1. This figure has automatic figure numbering.

I had a feature request for automatic figure numbering, like latex. I was revamping this plugin for Pelican 3.3 anyway - and this didn’t seem too hard - so I decided to add it.

So, the Better Figures & Images plugin now supports automatic figure numbering. To enable this for all posts, just add this to your config file:

FIGURE_NUMBERS = True

If you want to enable this per post, just add this to the metadata at the top of the post:

for restructuredText add this:

:figure_numbers: true

and for Markdown add this:

figure_numbers: true
Caution:

Can you have Figures in Markdown?

I use reStructuredText for this site, and I’m not sure if you can even have Figures in Markdown documents - and I haven’t tested it, so caveat emptor …

Continue reading “The Better Figures & Images Pelican plugin now supports Figure Numbering”

There are quite a few changes in Pelican 3.3 - most of them minor, but a few which might mean making some changes to your site in order to upgrade. This is what I did to move my site from Pelican 3.2 to 3.3.

The change that had the biggest impact and took the most work was around image linking - caused by a combination of things. I think I was doing it wrong before and things changed in a way that meant this no longer worked. I also had to update my Better Figures & Images plugin to take this into account.

Previously, I’d been linking to my images like this, both in my theme:

/* Theme CSS Image Link */
background: #2C71B8 url(/static/images/blueprint-background.png) repeat;
{# Theme HTML/Jinja image link #}
<meta name="twitter:image" content …
Continue reading “How I upgraded this website to Pelican 3.3”

https://github.com/kragen/xcompose

Continue reading “How to add new Compose Key combinations on Linux”

Flag of Brazil

I’ve recently been corresponding with Denny Dias, from Brazil, who’s converted his blog over to Pelican - and we’ve been helping each other out a bit with building themes and whatnot.

He’s written up his conversion & theme building process - and was generous enough to credit me after he borrowed some of my theme’s logic, from this blog’s GitHub repo.

As he’s such a nice guy - and as I’ve just borrowed his tagsort jinja filter for my blog’s tag page, I though I’d return the favour - cheers Deny! :)

Continue reading “Obrigado Deny!”

I’m going to build on Jamie Zawinski’s excellent advice about backups, which you should read first. This is basically that, but with some extra bits. If this seems too complex, then just do what he says.

The plan is to use Backupninja to backup everything to an external USB drive – and also to Amazon S3 or Dropbox, depending on what it is. Backupninja provides a centralized way to configure and schedule many different backup utilities, just by dropping a few simple configuration files into /etc/backup.d/.

I have a multiple hard disk setup for my desktop Linux box - my /home folders live on one disk and / lives on another one. I don’t want to backup everything from the system disk - I can re-install it in 10 mins, and I don’t really want to complicate this …

Continue reading “Comprehensive Linux Backups with etckeeper & backupninja”

I recently needed to convert some FLAC music files from the increasingly common 48 bit encoding, down to 16 bit at 44100 kHz, so that they’ll play on my Sonos. Here’s how to do it:

If you don’t already have sox installed, do this to install it:

$ sudo apt-get install sox

Then run this to do the conversion, in the folder with music in:

$ mkdir resampled
$ for flac in *.flac; do sox -S "${flac}" -r 44100 -b 16 ./resampled/"${flac}"; done

And that’s it - it will convert all the .flac files in that folder to 16 bit at 44100 kHz and put the result into the ./resampled subfolder, preserving the metadata.

Continue reading “How to convert FLAC files from 24/48 bit to 16 bit on Ubuntu Linux”

Compressing Limited Colour PNG images

Most of the .PNG files on this site are the ‘blueprint’ style diagrams, like this one:

Blueprint style diagram showing an optical microscope. The sample inside is magnified in a bubble to 2500 times
Figure 1. This illustration is a large 5.6MB SVG file, mostly because of the very detailed paisley pattern that I used. Exported to PNG - and then compressed using the process below, you can get this down to 118.5kB.

I create these in Inkscape as vector .SVG files & export them to bitmap .PNG files. I then re-compress them, to ensure that the image files that are used on the live website are as small & quick to load as possible.

As these diagrams have a fairly limited colour palette, I can get lots of extra compression by reducing the colour depth of the final .PNG files from the default 32bit (millions of colours) to 8bit (256 colours) - without any …

Continue reading “How I compress PNG files on this website”

Thunar's icon

Thunar - XFCE & XUbuntu’s small but perfectly formed file manager - has a simple mechanism that allows you to easily add new commands to the right click menu of files and folders. These are called Custom Actions and are easy to create…​ here’s how to do it.

Click the Edit menu, then click ‘Configure custom actions…​‘. This will take you to the Custom Actions Manager, where you can create, edit or delete your custom actions.

You can enter anything into the command box, including complex bash scripts, names of scripts or executables on the PATH, or the full path and filename of the command you want to run.

thunar custom actions edit 1

On the ‘Appearance Conditions‘ tab, you tell Thunar when you want your item to appear in the right click menu:

thunar custom actions edit 3
Figure 1. Now, when I right click on a text file, I …
Continue reading “Useful Thunar Custom Actions”