2 minute read

Updated 2019-07-31: I've released a tool which does this and more. Added some caveats and notes.

The LaTeX community is a bit different to other software communities in that it mostly consists of grey-bearded academics with work to do and PhD students procrastinating from writing their theses (ahem). As such, LaTeX packages and tools fall into one of two categories: relics of a bygone age that nevertheless do basically everything you want, and trendy projects on GitHub that die off after their authors graduate.

An example of the former is latexmk, the most popular LaTeX build tool. It's a single 10,000-line Perl script maintained by a semi-retired particle physics wizard1 which still contains bits of the original 1992-era go-make, but the only thing it's really missing is coloured output.

Colouring latexmk run messages

To colour the "Run number 1 of rule 'pdflatex'" messages, add this to latexmkrc:

    no warnings 'redefine';
    use Term::ANSIColor;
    my $old_warn_running = \&main::warn_running;
    sub color_warn_running {
        print STDERR color('green');
        print STDERR color('reset');
    if (-t STDERR) {
        # Only use color if a terminal is attached
        *main::warn_running = \&color_warn_running;

This wraps the subroutine latexmk uses to print important messages with print statements which change the terminal colour to green and back, as long as stderr is going to a terminal 2. The print statements have to be to stderr for two reasons. Firstly, warn_running prints to stderr and print is line buffered, so print color('green') wouldn't take effect until something else prints a line to stdout (or STDOUT->flush() is called). More importantly, for the colours to still work with stdout redirected (latexmk > log) the color escape sequences have to go to stderr anyway.

Colouring and filtering pdflatex output

There are many tools for doing all manner of clever things to the output of pdflatex, but I just want to hide the flood of files being loaded so I can see the errors and warnings. This Python script does the bare minimum:

  • Hides file loading messages
  • Colours warnings and errors
  • Keeps the output coming line-by-line
  • Doesn't mess with anything it doesn't understand
#!/usr/bin/env python3
"""Filter output of pdflatex.

Usage: filter.py <latex engine> <options> <file>
import os
import subprocess
import sys

import colorama
from colorama import Fore, Style

def main(cmd):
    # Disable pdflatex line wrap (where possible)
    env = dict(os.environ, max_print_line="1000000000")

    # Run pdflatex and filter/colour output
    pdflatex = subprocess.Popen(
        cmd, env=env, 
        stdout=subprocess.PIPE, stderr=subprocess.STDOUT
    for line in iter(pdflatex.stdout.readline, b''):
        line = line.decode('utf8').strip()
        if line.startswith('(/') or line.startswith('(./'):
            # Start loading file
        elif line.startswith(')'):
            # Finish loading file
        elif line.startswith('!'):
            # Error
            print(Fore.RED + line + Style.RESET_ALL)
        elif line.startswith('Overfull') or \
                line.startswith('Underfull') or \
                'warning' in line.lower() or \
                'missing' in line.lower() or \
                'undefined' in line.lower():
            # Warning
            print(Fore.YELLOW + line + Style.RESET_ALL)

if __name__ == '__main__':
    assert len(sys.argv) > 1

To use it, put it in the same directory as your TeX files and add $pdflatex = "./filter.py pdflatex %O %S"; to your latexmkrc. Note that the script doesn't handle TeX input prompts, so you should use -interaction=nonstopmode to avoid them (see my next post for more).

  1. Naturally, his personal website has a picture of him as a young man! 

  2. I got the idea for this from this Stack Overflow answer