Always uses dashes in filenames

In these days of cloud storage, file synchronisation, and sharing on social media it's more important than ever to properly name files.

Spaces make sense in written language but computers don't like them. Try to use a space in a filename in a URL and it'll replace it with %20. Try to do anything on the command line and you'll have to wrap the filename in double quotes.

The two common conventions are underscores (_) and hyphens (-). One convention used commonly in Unix/Linux filenames is to use underscores to separate words in the file title, and hyphens to separate the title from version number. For example Apache modules take the form of file_name-version.extension, eg. mod_ftp-0.9.6.tar.gz.

Turns out if your filename is ever to be processed by a regular expressions (such as by the Google indexer!) the underscore will be removed and filename concatenated. When using the the \w operator to find words the underscore is ignored, but the hypen is not.

Always use dashes in filenames instead of spaces or underscores.

Screen On The Green - A Raspberry Pi Cinema

screenshot of screen on the green website

I have the pleasure of being one of a small group who run a cinema night once a month in the small town of Writtle, Essex. It's my job to take care of anything remotely technical.

We didn't have any money when we started so had to go cap-in-hand to the parish council who were great and bought the screen and the projector, we use an old Hifi system I had for sound.

We didn't want to use DVDs as we wanted a pre-show slideshow, trailers and an intermission which would've been impractical using DVDs, especially since we would need Blu-ray. Another option was to use a laptop which would've worked fine, but the Raspberry Pi with it's h.264 capable GPU seemed a perfect fit.

Initially we used a Raspberry Pi 3b+ but with the recent release of the A+ we've switched due it's lower power consumption.

In order to use the hardware h.264 decoding we had to use Omxplayer which unfortunately doesn't have playlist support so I had to use a little bash magic.

First we need a playlist file containing a list of video files in the order we want them to play:

cards/30min-preroll.mp4  
trailers/theapartment.mp4  
trailers/12angrymen.mp4  
trailers/incredibleshrinkingman.mp4  
cards/comingsoon.mp4  
cards/feature.mp4  
cards/intro.mp4  
film/amelie-pt1.mp4  
cards/intermission-start.mp4  
ads/ads.mp4  
cards/intermission-end.mp4  
film/amelie-pt2.mp4  
cards/thanks-for-coming.mp4  

A simple bash script to read the playlist and line-by-line and invoke Omxplayer. The -o both parameter sets the output of audio to both the HDMI cable and phones out. -b blanks the screen first.

#!/bin/bash

PLAYER=(/usr/bin/omxplayer -o both -b)  
PLAYLIST="sotg.pls"

clear  # clear the screen

if [ -e "$PLAYLIST" ]  
then  
    IFS=$'\012'  # Set the line ending
    for file in $(cat "$PLAYLIST")
    do
        ${PLAYER[@]} "$file"
    done
fi  

In order to split the film in two for the intermission I find a suitable spot, note the time, and convert it to seconds. For example, 45m 34s in would be (45 * 60) + 34 which is 2734. I then use this in a couple of FFMpeg commands to trim the single film file in to two parts:

#!/bin/bash

FN="amelie"  
TIME=2734

ffmpeg -ss 0 -t $TIME -i $FN.mp4 -strict -2 -af "volume=12dB" $FN-pt1.mp4  
ffmpeg -ss $TIME -i $FN.mp4 -strict -2 -af "volume=12dB" $FN-pt2.mp4  

-ss sets the start time for the trimmed file, and -t sets the end time, which we can omit on the second command as we want to trim to the end. -af "volume=12dB" boosts the volume while we're at it.

CSS Selectors

I can never remember the syntax for this stuff.

Select by attribute presence

<p>This paragraph has a <a href='#'>link</a> in it.</p>  
[href] { background : cyan; }

This paragraph has a link in it.


Combine with element

<p title='wigwam'>This paragraph has a title attribute.</p>  
p[title] { background : cyan; }  

This paragraph has a title attribute.

Putting a space between p and [title] would specify the children of paragraph with title attributes.

<p>This paragraph has a couple of <span title='wigwam'>spans</span> with a <span title='wigwam'>title</span> attributes.</p>  
p [title] { background : cyan; }  

This paragraph has a couple of spans with a title attributes.


Target an attribute and its contents

The content of an attribute can also be specified by having the attribute equal something

<p>This paragraph has a couple of <span title='t1'>spans</span> with different <span  
title='t2'>title</span> attributes.</p>  
[title="t1"] { background : cyan; }
[title=t2] { background : yellow; }

This paragraph has a couple of spans with different title attributes.


Target a specific word in the attribute

Using the tilde (~) it is possible to target by a single string in a space separated list.

<p>This paragraph has a couple of <span title='one two three'>spans</span> with different <span title='one three four'>title</span> attributes.</p>  
[title~="one"] { font-weight : bold; }<br/>
[title~="two"] { background : cyan; }

This paragraph has a couple of spans with different title attributes.

It is also possible to target a substring at the end using the dollar ($) sign.

<p>This paragraph has a couple of <span title='twotone'>spans</span> with different <span title='donedown'>title</span> attributes.</p>  
span[title$="one"] { background : cyan; }  

This paragraph has a couple of spans with different title attributes.

To target a substring at the beginning use the caret (^) sign.

<p>This paragraph has a couple of <span title='twotone'>spans</span> with different <span title='onedown'>title</span> attributes.</p>  
span[title^="one"] { background : cyan; }  

This paragraph has a couple of spans with different title attributes.

It is also possible to target an exact match followed by anything separated with a dash using the pipe (|) symbol.

<p>This paragraph has a couple of <span title='one-up'>spans</span> with different <span title='one-down'>title</span> attributes.</p>  
span[title|="one"] { background : cyan; }  

This paragraph has a couple of spans with different title attributes.

Lastly we can do a full text search using the asterisk (*)

<p>This paragraph has a couple of <span title='donedown'>spans</span> with different <span title='doneup'>title</span> attributes.</p>  
span[title*="one"] { background : cyan; }  

This paragraph has a couple of spans with different title attributes.


Attribute stacking

Attribute selection can also be stacked to give even finer control of selection.

<p>This paragraph has a couple of <span class='onetwothree' title='onetwothree'>spans</span> with different <span class='twothreefour' title='onetwothree'>title</span> attributes.</p>  
span[class~='one'][title*="two"] { background : cyan; }  

This paragraph has a couple of spans with different title attributes.


Outputting attributes

An element's attribute can also be used in pseudo-elements for output.

<p>This paragraph has a <a href='https://lewiswalsh.com'>link</a> with a dynamically printed href.</p>  
a:after { content: " (" attr(href) ")"; }  

This paragraph has a link with a dynamically printed href.


Case sensitivity

By default all CSS attribute selectors are case-sensitive. To make the selection case-insensitive add an 'i' to the end of the selection.

[title*="one" i] { }

Raspberry Pi Dumb Terminal And Then Some

As someone whose interest in computing struck in the early 1980s I've always loved small self-contained computers. In my teens I discovered dumb terminals. However, it wasn't until the last ten years when I found myself SSHing in to remote servers that the idea I might have a use for one made any sense.

I Googled, I pored over oldcomputers.com and terminals-wiki.org, I watched old films. I kept coming back to the DEC VT100. I scoured eBay and other sites but they rarely come up and when they do are out of my price range.

Picture of a VT100 terminal

It dawned on me, "heck! I have a 3D printer, a competency with CAD, Linux, and basic electronics, and oh, the Raspberry Pi would be perfect for this."

First things first was to source the parts so I could design my VT100 around it.

The guts would be a Raspberry Pi Zero W since all I intended to use it for would be connecting to other systems.

The screen was straightforward enough on AliExpress. A nice 8", 4:3 screen with driver board, HDMI, VGA, and AV inputs was only ~£26. It's bright and sharp and only requires 12v at 2-4amp.

8

With a big enough power supply I could power the Pi and the screen from one input. I bought a simple step-down board to convert 12v to 5v with a USB output.

12v to 5v converter

The keyboard was a bit of a headscratcher. I wasn't about to 3D print a chassis and full keyboard so I was forced to compromise on the VT100 authenticity. I bought a cheap small USB keyboard. A future upgrade might be to make a housing this 3rd party keyboard would fit in to.

I leapt in with both feet (and a set of calipers) and designed a small VT100 which would fit the screen I had nicely.

Screenshot of VT100 design

The further I got in to designing it I realized how difficult it would be to print on my printer (the case itself would require six or seven parts glued together) and that it would take up a lot of precious desk space. I decided version one should be a working machine, not a faithful reconstruction. With a coffee and a notebook I started with some sketches.

Sketches

With 3d printing in mind I selected one design and created it in CAD, but I found it wanting. Instead of a side-by-side arrangement for the circuit boards I came up with a design where the boards are stacked.

Abandoned design Abandoned design

Final design Final design

Circuit board arrangement Circuit board arrangement

In all I think it took about twenty-six hours to print. I'm happy to say it fits together nicely.

Photo of working terminal 1 Photo of working terminal 2

I realised that this was wasted as just a terminal so I put a Pi 3b+ in there and installed the desktop environment. I put a ZX Spectrum emulator on and realised I'd neglected to a put a speaker in! Something for v2. For version two I'll probably make some of the corners softer by rounding them and make it snap together tighter.

Preventing SYSLOG spam

One of my Raspberry Pis has a problem. When connected to WIFI every few seconds two lines are added to my /var/syslog:

Mar 6 10:13:15 Pi dhcpcd[764]: eth0: Router Advertisement from...  

In this particular example I could fix this by disabling the IPv6 DHCP servive in my router, or disabling IPv6 on the Pi, but for the moment I want to simply keep these messages out of my syslog.

Happily, there's a nice syntax which allows us to add lines to the /etc/rsyslog.conf to filter lines sent to the syslog and prohibit them from being added.

In the case of the example above I added the following line near the top of the configuration file:

:msg, contains, "Router Advertisement from" stop

Then restart with sudo systemctl restart rsyslog and no more junk in the syslog!