A Weird Imagination

Timezones and scheduling tasks with at

The problem

My system for automatically posting future-dated blog posts mysteriously stopped working recently. The posts would appear if I manually published the blog, but not with the automatic scheduling mechanism.

The solution

In schedule_publish.sh, I changed the line

echo "$0" | at -q g $time

to

if [ "$(date -d "$time PST" +'%s')" -ge "$now" ]
then
    echo "$0" | at -q g -t "$(date +'%Y%m%d%H%M' -d "$time PST")"
fi

(where "PST" is the timezone of this blog; adjust as appropriate for your blog). $now is initialized with

now="$(date +'%s')"

before the call to make publish to avoid a race condition.

The details

Read more…

Speeding up Pelican's regenerate

Posted in

Pelican's default Makefile includes an option make regenerate which uses Pelican's -r/--autoreload option to regenerate the site whenever a file is modified. Combined with the Firefox extension Auto Reload, this makes it easy to keep an eye on how a blog post will be rendered as you author it and to quickly preview theme changes.

The problem

With just thirty articles, Pelican already takes several seconds to regenerate the site. For publishing a site, this is plenty fast, but for tweaking formatting in a blog post or theme, this is too slow.

The quick solution

Pelican has an option, --write-selected, which makes it only write out the files listed. Writing just one file takes about half a second on my computer, even though it still has to do some processing for all of the files in order to determine what to write. To use --write-selected, you have to determine the output filename of the article you are editing:

$ pelican -r content -o output -s pelicanconf.py \
    --relative-urls \
    --write-selected output/draft/in-progress-article.html

The right solution

Optimally, we wouldn't have to tell Pelican which file to output; instead, it would figure out which files could be affected by a change and regenerate only those files.

Read more…

Changing Pelican URL scheme

Posted in

The problem

I changed the URI scheme of this blog recently from /posts/YYYY/MM/slug/ to /YYYY/MM/DD/slug/. The latter looks better and makes the actual day of the post more visible.

But I already had posts using the old scheme and cool URIs don't change. Luckily, someone wrote a Pelican plugin called pelican-alias which allows articles to be tagged with additional URIs to redirect to their canonical location. All I had to do was add an Alias: /posts/2015/02/... line to the top of each of the posts I had already written and the plugin would take care of the rest.

Automating the aliasing

The non-trivial part of automating this is that the URIs include the article's slug, which may have been generated by Pelican from the title, so Pelican has to be involved in generating the correct redirects.

There are two ways I could have automated this process:

  1. Modify the plugin to add a redirect from the old scheme to the new scheme for every article. Unless somehow controlled, this would result in creating redirects for new articles which do not need them.
  2. Write a one-off script to get the slugs out of Pelican and write the Alias: lines into the blog posts.

I took the latter approach because it was simpler and involved no new code to maintain.

Read more…

My first Pelican plugin

The problem

My previous blog post has a footnote in the first sentence. Due to the way footnotes are handled, the footnote reference is a link to #fn:prg, which works fine if the footnote is actually on the page, but on the blog main page (or any other listing of multiple articles) the footnote is not present because it's after the Read more… link. The result is that on those pages, all footnote references are broken links. These broken links should either be repaired such that they point to the article page or removed.

First attempt

Unable to find an existing solution, I decided to write my own plugin, summary_footnotes. I started by finding another plugin, clean_summary that modifies summary and based my code off of it. That plugin uses Beautiful Soup to parse the summary and rewrite it. A quick look at the docs and I was able to figure out how to select the footnote links and rewrite them, which got me this version of the plugin.

Read more…

Future-dating static blog content

Posted in

The problem

Static site generators are great. But so are blog posts that automatically appear on schedule. How do we reconcile the two? There are solutions involving checking for updates on a schedule like every hour or every day, but that seems unsatisfying: if the posts have already been written, the blog should only need to be regenerated exactly when there is new content to publish.

The solution

(These instructions are specifically for Pelican as that is what this blog uses, a similar method should work for other static blogging engines.)

Use Pelican's WITH_FUTURE_DATES setting to make future dated posts not appear as part of the blog, but only as drafts. Add the following to the article template in order to include the future publication dates in an easy to parse format:

{% if article.status == "draft" %}
    <!-- Post at datetime {{ article.date|strftime("%H:%M %Y-%m-%d") }} -->
{% endif %}

Then the following script schedule_publish.sh uses those comments to schedule rerunning itself using at:

#!/bin/sh

# Pelican publish
make publish

# Clear old queue entries if they call this script.
for q in `atq -q g | cut -f1`
do
    if [ `at -c $q | tail -2 | head -1` = "$0" ]
    then
        atrm $q
    fi
done

# Check newly published drafts for when they should be published.
# Not using for because output lines have spaces.
grep -F -- '<!-- Post at datetime ' output/drafts/* | cut -d' ' -f5-6 | while read time
do
    # Schedule running this script for that time.
    echo "$0" | at -q g $time
done

Last, follow the instructions in this blog post and run that script as the deployment task.

The details

Read more…

Hello World!

Posted in

Welcome to my blog. I am Daniel Perelman; I am presently a computer science graduate student at the University of Washington.

Much of my time is spent writing programs and using various highly-configurable tools like Bash, Vim, and LaTeX. I, like many other users of these tools, find myself often performing web searches for help on how to use these tools. Thanks to StackExchange and myriad technical blogs, the answers I am looking for are often close at hand. But not always. Sometimes I end up piecing together a solution from many sources. This blog is a place for me to save others time by sharing that knowledge.