Fossil `grep` doesn’t support any of the GNU and BSD `grep` extensions.
For instance, it doesn’t support the common `-R` extension to POSIX,
which would presumably search a subtree of managed files. If Fossil does
one day get this feature, it would have a different option letter, since
`-R` in Fossil has a different meaning, by convention. Until then, you
can get the same effect on systems with a POSIX shell like so:

      $ fossil grep COMMAND: $(fossil ls src)
    $ fossil grep COMMAND: $(fossil ls src)

If you run that in a check-out of the [Fossil self-hosting source
repository][fshsr], that returns the first line of the built-in
documentation for each Fossil command, across all historical verisons.

Fossil `grep` has extensions relative to these other `grep` standards,
such as `--verbose` to print each checkin ID considered, regardless of