The Golang flag [0] package uses a single dash for long options.
The flags that a UNIX command uses can sometimes used to carbon-date that command. For example, dd(1) doesn't use a dash at all, uses long args, and uses an equals-sign to connect a flag with its argument (e.g. `dd if=/dev/null`). It's because dd(1) was written in the dawn of UNIX, before a convention was established for flags.
I believe the first step in creating a convention was the introduction of the getopt(3) library, which introduced single dash, single character options (e.g. `tar -x`). Commands that only have short options tend to be old commands.
Later, GNU introduced getopt_long(3), which allowed for long options and double-dashes to distinguish them from the short ones. This made commands more readable.
There have also been cases where the `-` is optional. `tar xvf` for example. And with the `ps` command, the `-` is used to distinguish BSD flags from System V flags (e.g. `ps auxwww` vs. `ps -elf`).
There has also been a movement for subcommands rather than options (which bring to mind the Cisco CLI), of which `ip` is an excellent example: `ip route show`. But subcommands are not a substitute for options; sometimes you need both (e.g. `ip -6 route show`).
All this goes to show that, although long options are quite common, they are not only game in town, and UNIX commands have a rich and varied set of conventions.
[0] https://golang.org/pkg/flag/