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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
kernel-2 …
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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 extension on the output_thumbnail …

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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 …

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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.

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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 …
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I switched my XFCE machines over to use Compton for window compositing today - and it’s a noticeable improvement.

A compositor glues your stacks of windows together to form the final image that you see on screen. It’s responsible for any fancy effects like drop-shadows, as well drawing windows while dragging, resizing and minimizing or maximizing them. [1]

Compton does this beautifully. It does one thing and it does it well. It provides glassy smooth, tear free compositing and supports a few tasteful effects - drop shadows, fades and transparency.

I stumbled across this while searching for something else and found a great guide in the NeoWin forums [2], where most of this info comes from, so thanks ViperAFK.

Switch off the existing compositor

Screenshot of the XFCE Applications menu

You’ll need to switch of any existing compositing you’ve got running, otherwise this won …

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Ever tried to copy something onto a USB flash drive, only to discover that the file was too big to copy?

This is because most USB Flash drives are formatted using the FAT32[1] filesystem - which only supports individual files up to 4 GB in size, no matter how much free space you’ve got. It also only supports drives up to 2 TB, can’t store symbolic links, can’t store files with these characters in the name: "*/:<>?\| – and is generally pretty crappy.

The solution is to use a better filesystem

There are many filesystems to choose from - but there are a few criteria that a good USB flash drive filesystem needs to meet which cut down the choices quite a bit:

  • Compatible with every computer - just plug it in, and it should work
  • Has no permissions, or optional …
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I recently needed to convert some Apple Lossless music files to FLAC. Here’s how to do it:

If you don’t already have ffmpeg or libav-tools installed, do this:

$ sudo apt-get install libav-tools

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

$ for f in *.m4a; do avconv -i "$f" "${f%.m4a}.flac"; done

And that’s it - it will convert all the .m4a files in that folder to .flac files, preserving the metadata. You can now delete the .m4a files if you want:

$ rm ./*.m4a
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Blueprint style diagram showing the compose key sequence for the Euro currency symbol.
Figure 1. Just hold down your chosen compose key, then press the other keys in turn: [compose key] + e + = gets you a Euro symbol.

The compose key on Linux is incredibly useful, but not configured by default - and on XFCE there’s currently no graphical UI to change it. However, it’s pretty simple to change…​ here’s how to make the Caps Lock key your compose key:

In the file /etc/default/keyboard, change XKBOPTIONS to look like this:

XKBOPTIONS="compose:caps"

Save the file. You can now either reboot, restart X, or see Activating your change without rebooting, below.

If you don’t want to use Caps Lock as your compose key, there are some other options to choose from in this file: /usr/share/X11/xkb/rules/xorg.lst - search for ‘compose’, they’re listed towards the …

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