This package provides completion for the Fish shell to pcomplete (used by shell and Eshell). You can set it up globally with:

(when (and (executable-find "fish")
           (require 'fish-completion nil t))
  (global-fish-completion-mode))

Alternatively, you can call the fish-completion-mode manually or in shell/Eshell mode hook.

The package emacs-bash-completion is an optional dependency: if available, fish-completion-complete can be configured to fall back on bash to further try completing. See fish-completion-fallback-on-bash-p.