The oldest and most widely compatible method to get colorized diffs in Fossil is to use its web UI:

fossil ui --page '/vdiff?from=2024-04-01&to=trunk'

That syntax is admittedly awkward, and it doesn’t work where “from” is the current checkout. Fortunately, there are many other methods to get colorized diff output from Fossil.

fossil diff -b

This produces a graphical diff in HTML format and sends it to the user’s default web browser for viewing.

fossil diff -tk

You may be surprised to learn that the prior feature doesn’t use any of the skinning or chrome from Fossil UI. This is because it is meant as a functional replacement for an older method of getting colorized diffs, “fossil diff -tk”. The feature was added after Apple stopped shipping Tcl/Tk in macOS, and the third-party replacements often failed to work correctly. It’s useful on other platforms as well.

Delegate to Git

It may be considered sacrilege by some, but the most direct method for those who want Git-like diff behavior may be to delegate diff behavior to Git:

fossil set --global diff-command 'git diff --no-index'

The flag permits it to diff files that aren’t inside a Git repository.

GNU Diffutils

If your system is from 2016 or later, it may include GNU Diffutils 3.4 or newer, which lets you say:

fossil set --global diff-command 'diff -dwu --color=always'

You might think you could give --color=auto, but that fails with commands like “fossil diff | less” since the pipe turns the output non-interactive from the perspective of the underlying diff instance.

This use of unconditional colorization means you will then have to remember to add the -i option to fossil diff commands when producing patch(1) files or piping diff output to another command that doesn’t understand ANSI escape sequences, such as diffstat.

Bat, the Cat with Wings

We can work around the --color=auto problem by switching from GNU less as our pager to bat, as it can detect GNU diff output and colorize it for you:

fossil set --global diff-command 'diff -dwu --color=auto'
fossil diff | bat

In this author’s experience, that works a lot more reliably than GNU less’s ANSI color escape code handling, even when you set LESS=-R in your environment.

The reason we don’t leave the diff-command unset in this case is that Fossil produces additional lines at the start which confuse the diff format detection in bat. Forcing output through an external diff command solves that. It also means that if you forget to pipe the output through bat, you still get colorized output from GNU diff.

Colordiff

A method that works on systems predating GNU diffutils 3.4 or the widespread availability of bat is to install colordiff, as it is included in many package systems, including ones for outdated OSes. That then lets you say:

fossil set --global diff-command 'colordiff -dwu'

The main reason we list this alternative last is that it has the same limitation of unconditional color as above.