← Back | Deepak Cherian | 19 Jan 2017

A setup for reproducible research


André Palóczy has implemented some of these ideas in Python.


  • Version control system to track content. I use git.
  • exiftool to read and write metadata to images
  • I have some code below for MATLAB but the principles could be extended to any other software package.


The approach is simple: use git to track files that generate output.

git automatically assigns a unique 40 character alphanumeric string (a “hash”) that identifies the state of a repository.

By saving the value of the hash when a certain output file is created, we know what code created the output.

With data files, it is simple to add an extra variable containing the hash.

With figures, I use the metadata fields to save the hash value.

Getting the current hash in MATLAB

The following MATLAB function githash will return the hash of the last commit that modified the file in fname. If not provided with fname it returns the hash of the last commit in the repository.

   function [hash] = githash(fname, gitdir)

       if ~exist('fname', 'var')
           fname = '.';

       if ~exist('gitdir', 'var')
           gitdir = '';
           gitdir = ['--git-dir=' gitdir];

       [~, hashout] = system(['TERM=xterm git ' gitdir ...
                           ' log -n 1 --no-color --pretty=format:''%H'' ''' ...
                           fname ''' < /dev/null']);

       % remove bash escape characters
       hash = hashout(9:48)

Using it in a MATLAB script requires the incantation

  hash = githash([mfilename('fullpath') '.m']);

This provides githash with the path to the current mfile that is calling githash.

Quite frequently, I calculate diagnostics that take a while which means that rerunning them every time I make an image is not feasible. I save the hash variable to the file containing diagnostic output. This lets me know what version of the code created that version of the saved output.

Using the hash

MATLAB’s FileExchange has a couple of useful scripts insertAnnotation & getAnnotation that insert and recover metadata in MATLAB figure windows.

An obvious choice is to save the hash. More importantly, one can save the exact function call that generated a figure. Then, you know two things:

  1. the version of the code that created the figure, and
  2. all parameters provided to the code;

both of which are saved in the metadata of the figure itself.

getAnnotation can then recover the saved metadata when saving a figure to file.

Saving the hash in an image file

Once you have a hash value, or any metadata in general, it needs to be saved when the image is saved. I have modified export_fig (original, my fork) to do this for me.

In general, all you need is a line that looks like

  system(['exiftool -overwrite_original -Producer=' ...
          hash ' ' pdf_nam]);

The above tells exiftool to save the contents of variable hash in the metadata field Producer of the file named pdf_nam. The slight complication here is that the metadata field names are not standardized among different image formats.

Table 1: Metadata fields for various image formats that I use to save hashes.
format pdf, eps png jpg tif
metadata field Producer Software Comment Description

exiftool is only required to modify the metadata fields of PDF and EPS files. MATLAB’s imwrite can write metadata to bitmap files (e.g. PNG).

Searching for hash in my fork of export_fig.m will show you how imwrite can be used.

Extracting commit hash from image metadata

To recover the recorded hash, it suffices to call exiftool FILENAME which will print all metadata stored in the image; not just the hash. grep can then find the recorded hash:

  # displays saved git hash of a provided file using exiftool

  hash=$(exiftool $file | grep -i "hash:")

  echo $hash
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