5.1 Jekyll

For Jekyll (https://jekyllrb.com) users, I have prepared a minimal example in the GitHub repository yihui/blogdown-jekyll. If you clone or download this repository and open blogdown-jekyll.Rproj in RStudio, you can still use all addins mentioned in Section 1.3, such as “New Post,” “Serve Site,” and “Update Metadata,” but it is Jekyll instead of Hugo that builds the website behind the scenes now.

I assume you are familiar with Jekyll, and I’m not going to introduce the basics of Jekyll in this section. For example, you should know what the _posts/ and _site/ directories mean.

The key pieces of this blogdown-jekyll project are the files .Rprofile, R/build.R, and R/build_one.R. I have set some global R options for this project in .Rprofile:42

options(
  blogdown.generator = "jekyll",
  blogdown.method = "custom",
  blogdown.subdir = "_posts"
)

First, the website generator was set to jekyll using the option blogdown.generator, so that blogdown knows that it should use Jekyll to build the site. Second, the build method blogdown.method was set to custom, so that we can define our custom R script R/build.R to build the Rmd files (I will explain the reason later). Third, the default subdirectory for new posts was set to _posts, which is Jekyll’s convention. After you set this option, the “New Post” addin will create new posts under the _posts/ directory.

When the option blogdown.method is custom, blogdown will call the R script R/build.R to build the site. You have full freedom to do whatever you want in this script. Below is an example script:

build_one = function(io) {
  # if output is not older than input, skip the compilation
  if (!blogdown:::require_rebuild(io[2], io[1])) return()

  message('* knitting ', io[1])
  if (xfun::Rscript(shQuote(c('R/build_one.R', io))) != 0) {
    unlink(io[2])
    stop('Failed to compile ', io[1], ' to ', io[2])
  }
}

# Rmd files under the root directory
rmds = list.files('.', '[.]Rmd$', recursive = T, full.names = T)
files = cbind(rmds, xfun::with_ext(rmds, '.md'))

for (i in seq_len(nrow(files))) build_one(files[i, ])

system2('jekyll', 'build')

  • Basically it contains a function build_one() that takes an argument io, which is a character vector of length 2. The first element is the input (Rmd) filename, and the second element is the output filename.

  • Then we search for all Rmd files under the current directory, prepare the output filenames by substituting the Rmd file extensions with .md, and build the Rmd files one by one. Note there is a caching mechanism in build_one() that makes use of an internal blogdown function require_rebuild(). This function returns FALSE if the output file is not older than the input file in terms of the modification time. This can save you some time because those Rmd files that have been compiled before will not be compiled again every time. The key step in build_one() is to run the R script R/build_one.R, which we will explain later.

  • Lastly, we build the website through a system call of the command jekyll build.

The script R/build_one.R looks like this (I have omitted some non-essential settings for simplicity):

local({
  # fall back on "/" if baseurl is not specified
  baseurl = blogdown:::get_config2("baseurl", default = "/")
  knitr::opts_knit$set(base.url = baseurl)
  knitr::render_jekyll()  # set output hooks

  # input/output filenames as two arguments to Rscript
  a = commandArgs(TRUE)
  d = gsub("^_|[.][a-zA-Z]+$", "", a[1])
  knitr::opts_chunk$set(
    fig.path   = sprintf("figure/%s/", d),
    cache.path = sprintf("cache/%s/", d)
  )
  knitr::knit(
    a[1], a[2], quiet = TRUE, encoding = "UTF-8",
    envir = globalenv()
  )
})

  • The script is wrapped in local() so that an Rmd file is knitted in a clean global environment, and the variables such as baseurl, a, and d will not be created in the global environment, i.e., globalenv() used by knitr::knit() below.

  • The knitr package option base.url is a URL to be prepended to figure paths. We need to set this option to make sure figures generated from R code chunks can be found when they are displayed on a web page. A normal figure path is often like figure/foo.png, and it may not work when the image is rendered to an HTML file, because figure/foo.png is a relative path, and there is no guarantee that this image file will be copied to the directory of the final HTML file. For example, for an Rmd source file _posts/2015-07-23-hello.Rmd that generates figure/foo.png (under _posts/), the final HTML file may be _site/2015/07/23/hello/index.html. Jekyll knows how to render an HTML file to this location, but it does not understand the image dependency and will not copy the image file to this location. To solve this issue, we render figures at the root directory /figure/, which will be copied to _site/ by Jekyll. To refer to an image under _site/figure/, we need the leading slash (baseurl), e.g., <img src="/figure/foo.png">. This is an absolute path, so no matter where the HTML is rendered, this path always works.

  • What knitr::render_jekyll() does is mainly to set up some knitr output hooks so that source code and text output from R code chunks will be wrapped in Liquid tags {% highlight %} and {% end highlight %}.

  • Remember in build.R, we passed the variable io to the Rscript call xfun::Rscript(). Here in build_one.R, we can receive them from commandArgs(TRUE). The variable a contains an .Rmd and an .md file path. We removed the possible leading underscore (^_) and the extension ([.][a-zA-Z]$) in the path. Next we set figure and cache paths using this string. For example, for a post _posts/foo.Rmd, its figures will be written to figure/foo/ and its cache databases (if there are any) will be stored under cache/foo/. Both directories are under the root directory of the project.

  • Lastly, we call knitr::knit() to knit the Rmd file to a Markdown output file, which will be processed by Jekyll later.

A small caveat is that since we have both .Rmd and .md files, Jekyll will treat both types of files as Markdown files by default. You have to ask Jekyll to ignore .Rmd files and only build .md files. You can set the option exclude in _config.yml:

exclude: ['*.Rmd']

Compared to the Hugo support in blogdown, this approach is limited in a few aspects:

  1. It does not support Pandoc, so you cannot use Pandoc’s Markdown. Since it uses the knitr package instead of rmarkdown, you cannot use any of bookdown’s Markdown features, either. You are at the mercy of the Markdown renderers supported by Jekyll.

  2. Without rmarkdown, you cannot use HTML widgets. Basically, all you can have are dynamic text output and R graphics output from R code chunks. They may or may not suffice, depending on your specific use cases.

It may be possible for us to remove these limitations in a future version of blogdown, if there are enough happy Jekyll users in the R community.

  1. If you are not familiar with this file, please read Section 1.4.↩︎