How to show the modification date of a file in Jekyll?

Question

I know that I can specify a variable date in the YAML frontmatter of a file and access its value with {{ page.date }}. This is cumbersome since one easily forgets to change the date when a file is updated. So how can I access a file's modification date?

Answer 1

This is a relatively new plugin that does what you're looking for:

https://github.com/gjtorikian/jekyll-last-modified-at

(found it while searching Google for alternatives to the other answers in this thread)

Answer 2

From Jekyll 1.x there is a page.path that gives you the filename of the post or page being currently processed. Add the following filter (place e.g. in _plugins/myfilters.rb) to get the modification time of a given filename:

module Jekyll
  module MyFilters
    def file_date(input)
      File.mtime(input)
    end
  end
end

Liquid::Template.register_filter(Jekyll::MyFilters)

So now you can render the date on your posts and pages, e.g., as

{{ page.path | file_date | date_to_string }}

Answer 3

Based on krlmlr's post, I wrote a pre-commit script to update modification time of the date: field in YAML front matter of the modified files.

#!/bin/sh
# Contents of .git/hooks/pre-commit

git diff --cached --name-status | grep "^M" | while read a b; do
  cat $b | sed "/---.*/,/---.*/s/^date:.*$/date: $(date -u "+%Y-%m-%d %T %Z")/" > tmp
  mv tmp $b
  git add $b
done

It is working fine for me. I actually use update: field to indicate the modified time, and date: field to indicate the creation time.

Answer 4

I don't think it's possible to do this without using the YAML frontmatter. Note, however, that specifying date in the frontmatter actually overrides the date given in the post's filename.

So if you have a file 2013-02-19-my-post.md and in its frontmatter you have date: 2013-02-20, the frontmatter takes precedence, and the post's date is 2013-02-20.

If you want to keep track of when a post is modified, I'm afraid there's no way to do this without using a custom field in your frontmatter. Call it modified-date or something. Yes, it is a bit tedious. I have a TextExpander snippet that automatically outputs current date/time in the proper format, which is handy.

Answer 5

If you're using Git, you can install a pre-commit hook that automatically updates the date field in the front matter of modified files. I was very surprised to learn that a pre-commit hook can indeed change the contents of the commit! You only have to manually install a pre-commit hook into each clone of the repository -- the following rough draft works for me for updating the overall modification time of the entire website:

#!/bin/sh
# Contents of .git/hooks/pre-commit

set -e
set -x

sed -i "s/^date:.*$/date: $(TZ=UTC date "+%Y-%m-%d %H:%M:%S %Z")/" _config.yml
git add _config.yml

The _config.yml is expected to have a field "date", its contents will be replaced by each commit. It can be then accessed via {{ site.date }}. Example:

date: 2015-04-10 10:51:37 UTC

The script can be generalized to update only those pages or posts that have changed (query via git diff-index --cached) -- I don't have the time to implement this but I'd be interested to learn if a neater solution pops out eventually.