The usual convention is a single hyphen for short-form (single-letter) options, and a double hyphen for long-form options:

    > python -v

or

    > python —-version

It’s good practice to offer both. It should also be possible to set multiple options at once by appending one after another in short form following a single hyphen:

    > ls -alR

is the same as

    > ls -a -l -R

Long-form options are technically a GNU thing [0] and are not mentioned in the POSIX standard, but they’re conventional enough now that I think it’s good practice to include them in any CLI program.

There are also a number of looser conventions about the meaning of certain short-form options [1].

0. https://www.gnu.org/prep/standards/html_node/Command_002dLin...

1. http://www.catb.org/esr/writings/taoup/html/ch10s05.html