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 the unsaved changes
currently in the working check-out.

If the "--from VERSION" or "-r VERSION" option is used it specifies
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.

If the "--to VERSION" option appears, it 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 "-i" command-line option forces the use of the 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 "-N" or "--new-file" option causes the complete text of added or
deleted files to be displayed.

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 external diff program.
This option overrides the "binary-glob" setting.

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
  --checkin VERSION          Show diff of all changes in VERSION
  --command PROG             External diff program - overrides "diff-command"
  --context|-c N             Use N lines of context
  --diff-binary BOOL         Include binary files when using external commands
  --exec-abs-paths           Force absolute path names with external commands.
  --exec-rel-paths           Force relative path names with external commands.
  --from|-r VERSION          Select VERSION as source for the diff
  --internal|-i              Use internal diff logic
  --new-file|-N              Show complete text of added and deleted files
  --numstat                  Show only the number of lines delete and added
  --side-by-side|-y          Side-by-side diff
  --strip-trailing-cr        Strip trailing CR
  --tclsh PATH               Tcl/Tk used for --tk (default: "tclsh")
  --tk                       Launch a Tcl/Tk GUI for display
  --to VERSION               Select VERSION as target for the diff
  --undo                     Diff against the "undo" buffer
  --unified                  Unified diff
  -v|--verbose               Output complete text of added or deleted files
  -w|--ignore-all-space      Ignore white space when comparing lines
  -W|--width <num>           Width of lines in side-by-side diff
  -Z|--ignore-trailing-space Ignore changes to end-of-line whitespace