The "diff" command:

Usage: fossil diff|gdiff ?OPTIONS? ?FILE1? ?FILE2 ...?

Show the difference between the current version of each of the FILEs specified (as they exist on disk) and that same file as it was checked- out. Or if the FILE arguments are omitted, show all unsaved changes currently in the working check-out. The "gdiff" variant means to use a GUI diff.

The default output format is a "unified patch" (the same as the output of "diff -u" on most unix systems). Many alternative formats are available. A few of the more useful alternatives:

--tk
Pop up a Tcl/Tk-based GUI to show the diff
--by
Show a side-by-side diff in the default web browser
-b
Show a linear diff in the default web browser
-y
Show a text side-by-side diff
--webpage
Format output as HTML
--webpage -y
HTML output in the side-by-side format

The "--from VERSION" option is used to specify the source check-in for the diff operation. If not specified, the source check-in is the base check-in for the current check-out. Similarly, the "--to VERSION" option specifies the check-in from which the second version of the file or files is taken. If there is no "--to" option then the (possibly edited) files in the current check-out are used. The "--checkin VERSION" option shows the changes made by check-in VERSION relative to its primary parent. The "--branch BRANCHNAME" shows all the changes on the branch BRANCHNAME.

With the "--from VERSION" option, if VERSION is actually a directory name (not a tag or check-in hash) then the files under that directory are used as the baseline for the diff.

The "-i" command-line option forces the use of Fossil's own internal diff logic rather than any external diff program that might be configured using the "setting" command. If no external diff program is configured, then the "-i" option is a no-op. The "-i" option converts "gdiff" into "diff".

The "--diff-binary" option enables or disables the inclusion of binary files when using an external diff program.

The "--binary" option causes files matching the glob PATTERN to be treated as binary when considering if they should be used with the external diff program. This option overrides the "binary-glob" setting.

These commands show differences between managed files. Use the "fossil xdiff" command to see differences in unmanaged files.

Options:

--binary PATTERN
Treat files that match the glob PATTERN as binary
--branch BRANCH
Show diff of all changes on BRANCH
--brief
Show filenames only
-b|--browser
Show the diff output in a web-browser
--by
Shorthand for "--browser -y"
-ci|--checkin VERSION
Show diff of all changes in VERSION
--command PROG
External diff program. Overrides "diff-command"
-c|--context N
Show N lines of context around each change, with negative N meaning show all content
--dark
Use dark mode for the Tcl/Tk-based GUI and HTML
--diff-binary BOOL
Include binary files with external commands
--exec-abs-paths
Force absolute path names on external commands
--exec-rel-paths
Force relative path names on external commands
-r|--from VERSION
Use VERSION as the baseline for the diff, or if VERSION is a directory name, use files in that directory as the baseline.
-w|--ignore-all-space
Ignore white space when comparing lines
-i|--internal
Use internal diff logic
--invert
Invert the diff
--json
Output formatted as JSON
-n|--linenum
Show line numbers
-N|--new-file
Alias for --verbose
-s|--numstat
Show the number of added and deleted lines per file, omitting the diff. When combined with --brief, show only the total row.
-y|--side-by-side
Side-by-side diff
--strip-trailing-cr
Strip trailing CR
--tcl
Tcl-formatted output used internally by --tk
--tclsh PATH
Tcl/Tk shell used for --tk (default: "tclsh")
--tk
Launch a Tcl/Tk GUI for display
--to VERSION
Select VERSION as target for the diff
--undo
Use the undo buffer as the baseline
--unified
Unified diff
-v|--verbose
Output complete text of added or deleted files
-h|--versions
Show compared versions in the diff header
--webpage
Format output as a stand-alone HTML webpage
-W|--width N
Width of lines in side-by-side diff
-Z|--ignore-trailing-space
Ignore changes to end-of-line whitespace