git diff pdfs – Minding Broken Clocks

Binary files and git repositories can be a pain, especially when looking at the git diff output. Here are two helpful things to do with git to make working with .pdfs less painful.

The standard output from git diff when a tracked .pdf has changed is

diff --git a/dissertation.pdf b/dissertation.pdf
index 2a9ec08..d3aaa79 100644
Binary files a/dissertation.pdf and b/dissertation.pdf differ

Not the most helpful output. The two things I have found to be helpful are

A git difftool to call diffpdf.
Have the differences in the pdfinfo returned when calling git diff.

Note, I’m writing this on a Debian machine. Set up for other Linux distributions, Winblows and Mac may differ.

Setting up git to use diffpdf is simple. In either you project or global config file add the following lines

[difftool "diffpdf"]
  cmd = diffpdf \"$LOCAL\" \"$REMOTE\"

From the command line

git difftool --no-prompt --tool=diffpdf dissertation.pdf

will open the diffpdf gui and show the differences, changes, in the dissertation.pdf file. I find this to be the most helpful when comparing different versions of the .pdf file with/for non-git users. For example, version-0.9.1 versus the current version

git difftool --no-prompt --tool=diffpdf version-0.9.1:dissertation.pdf dissertation.pdf

Now, from time to time, I know that the .pdf changes are limited, or can easily be assess by looking at the metadata. To have the output of git diff show the changes in the pdf metadata do the following.

Add, or add to, .gitattributes file. Setting one up for all your repos is done by adding the following to you global git config file

git config --global core.attributesfile ~/.gitattributes
echo "*.pdf diff=diffpdfinfo" >> ~/.gitattributes

If you only want this to work in one particular repository you need only create the .gitattributes file in the project root directory.

We need to write the diffpdfinfo script and save it somewhere in your PATH.

#!/bin/bash

pdfinfo $1 > .localpdfmetadata
pdfinfo $2 > .remotepdfmetadata

diff .localpdfmetadata .remotepdfmetadata

Remember to made the script executable via chmod a+x.

Now git diff has meaningful output for the .pdf file. For example, a quick recompile of the .pdf document results in the diffpdf tool telling me the two files are the same. However, git diff tells me they are different, specifically in that the creation date and modification dates have change.

$ git diff

diff --git a/dissertation.pdf b/dissertation.pdf
index 2a9ec08..edd66d8 100644
--- a/dissertation.pdf
+++ b/dissertation.pdf
@@ -4,8 +4,8 @@ Keywords:
 Author:
 Creator:        LaTeX with hyperref package
 Producer:       pdfTeX-1.40.15
-CreationDate:   Wed Mar 29 14:55:27 2017
-ModDate:        Wed Mar 29 14:55:27 2017
+CreationDate:   Mon Apr  3 00:41:05 2017
+ModDate:        Mon Apr  3 00:41:05 2017
 Tagged:         no
 UserProperties: no
 Suspects:       no

Diffing a draft version to the current version there should be some changes in the metadata. The creation and modification dates have changed, but so has the number of pages and the file size.

$ git diff version-0.9.1:dissertation.pdf dissertation.pdf

diff --git a/dissertation.pdf b/dissertation.pdf
index bac8a42..edd66d8 100644
--- a/dissertation.pdf
+++ b/dissertation.pdf
@@ -4,17 +4,17 @@ Keywords:
 Author:
 Creator:        LaTeX with hyperref package
 Producer:       pdfTeX-1.40.15
-CreationDate:   Sat Mar 18 16:03:34 2017
-ModDate:        Sat Mar 18 16:03:34 2017
+CreationDate:   Mon Apr  3 00:41:05 2017
+ModDate:        Mon Apr  3 00:41:05 2017
 Tagged:         no
 UserProperties: no
 Suspects:       no
 Form:           none
 JavaScript:     no
-Pages:          130
+Pages:          140
 Encrypted:      no
 Page size:      612 x 792 pts (letter)
 Page rot:       0
-File size:      13127074 bytes
+File size:      23775044 bytes
 Optimized:      no
 PDF version:    1.5