I write all my documents in Markdown and pipe them through Pandoc to make PDFs. Typing pandoc myfile.md -o myfile.md.pdf over and over at the command line gets tedious, especially if the Pandoc command requires options for BibTeX and CSL files.

My solution is to use GNU Make (helpful introduction here). Make lets you define a series of rules to compile files. You define ’targets’ (the files to be made), ‘dependencies’ (the source files), and the commands that express the relation between the two. A typical rule using Pandoc might look like this:

lesson.01.md.pdf : lesson.01.md
    pandoc lesson.01.md -o lesson 01.md.pdf

But in larger projects, such as the class that I’m teaching, every time I added a Markdown file I also had to add a new rule to the Makefile. Besides the time this took, that process was error-prone, especially if I ever changed the name of the file. The Makefile for the class was over 60 lines long—and that’s at the start of the semester.

So this afternoon I took some time to figure out how to write a Makefile which will run Pandoc on any Markdown file in a directory. I’ve put the file up as a Gist on GitHub. Here is how it works.

First we define a variable with the list of all the files we want to make. We get that list by using the wildcard function to find all the *.md files, then by using the patsubst function to append .pdf to those file names.

PDFS := $(patsubst %.md,%.md.pdf,$(wildcard *.md))

Next we define a generic target, all, using that list.

all : $(PDFS)

We need a rule for making PDFs. Using Make’s string replacement macros, we define a generic rule:

%.md.pdf : %.md
    pandoc $< -o $@

Now running make creates or updates all the PDFs in the project. Four lines of code that are portable to any project beat 60+ lines that are specific to the project.

Here is the whole Makefile as a Gist (with a few additional rules):

# Produce PDFs from all Markdown files in a directory

# List files to be made by finding all *.md files and appending .pdf
PDFS := $(patsubst %.md,%.md.pdf,$(wildcard *.md))

# The all rule makes all the PDF files listed
all : $(PDFS)
# This generic rule accepts PDF targets with corresponding Markdown 
# source, and makes them using pandoc
%.md.pdf : %.md
        pandoc $< -o $@

# Remove all PDF outputs
clean :
        rm $(PDFS)

# Remove all PDF outputs then build them again
rebuild : clean all

The filenames of my PDFs are a little peculiar: if the source file is handout.md, I name the PDF handout.md.pdf instead of handout.pdf. That’s a deliberate choice, so that when my shell completes filenames I get the full name of the Markdown file instead of having to make a choice between .md and .pdf. Nine times out of ten I want to edit the Markdown file instead of doing something with the PDF.