Is there some way to get zsh to complete long flag names on the command line?
$ command --reall<tab>
$ command --really-long-flag-name
Seems like a zshy thing to do.
The short answer is yes, or course it can.
To turn on zsh’s completion system, you need to do the following – probably in a startup file like your ~/.zshrc:
autoload -U compinit && compinit
On most modern Unix-like systems, once you do that you ought to find that many commands already have their flags and parameters completed, because zsh ships with a library of completion functions for common Unix commands and utilities. This library ought to be installed in a location like /usr/local/share/zsh/function (or similar, depending on your system) and consists of a bunch of scripts with filenames starting in a _ character, each of which defines the completion for a specific command.
If a command or utility you’re interested in is not yet completed correctly by zsh, you have several options:
Look into the zsh-completions package. (It may well be installable by your operating system or distribution’s package manager.)
Read the documentation for the tool you wish to have completion. Many Unix utilities ship with completion scripts for bash and/or zsh, or with some way of generating completion scripts.
If all else fails, read the documentation on zsh’s completion system (or find a good book or online tutorial) and write it yourself. This can — obviously — be non-trivial!
Reading that zsh documentation might also show you how to do other things that you may not even know yet that you want, like turning on menu-based completion.