cut -DF

[Rob Landley]

Around 5 years ago toybox added the -D, -F, and -O options to cut:

    -D  Don't sort/collate selections or match -fF lines without delimiter
    -F  Select fields separated by DELIM regex
    -O  Output delimiter (default one space for -F, input delim for -f)

This lets you do:

  $ echo one two three four five six seven eight nine | cut -DF 7,1-3,2
  seven one two three two

-F is a regex version of -f (defaulting to "match a run of whitespace")

-D says to show the raw matches in the order requested (and ONLY those matches,
it doesn't pass through lines with no matches)

-O is -d for output.

You need all three because -F is useful by itself, and -F needs -O because when
you're matching a regex it's not clear what to output. (Does
"echo -e one\ttwo three' cut | -DF 3,1" glue them together with what's before
match 1 (nothing), what's after match 3 (nothing), or an arbitrarily chosen one
of the two different splits in between?)

Elliott Hughes (the Android base OS maintainer) asked if I could get the feature
more widely adopted:

  http://lists.landley.net/pipermail/toybox-landley.net/2021-June/012453.html

> your non-POSIX cut(1) extension covers 80% of the in-the-wild use of awk
> anyway :-) if you still talk to any of the busybox folks, we should suggest
> they copy that --- it would be nice for it to be a de facto standard so we
> can get it into POSIX sometime around the 2040s... (and have made lives
> better for the folks who don't care about standards and just want to "get
> things done" in the intervening decades!)

So I offered to implement it in busybox:

  http://lists.busybox.net/pipermail/busybox/2021-June/088886.html

And the busybox maintainer merged it here:

  https://git.busybox.net/busybox/commit/?id=0068ce2fa0e3

This is working and in use in Android, and now in busybox, and it would simplify
my regression test suite if coreutils was in sync, so I thought I'd ask if you
were interested.

Thanks,

Rob

P.S. Somebody submitted a proposal to do this to posix way back when (see end of
rationale at https://pubs.opengroup.org/onlinepubs/9699919799/utilities/cut.html
-- they replied that they only standardize existing features, not take
suggestions for new ones nobody's implemented yet). If I'd noticed I'd have used
-o instead of -D at the time, but whoever suggested it apparently didn't try to
make it actually work because -F is useful without -D, and -F without -O isn't
well-defined.

P.P.S. -D implying -F doesn't help because -F is the one that takes arguments,
analogous to -f.