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.