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

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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

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