Bash too can do that. It does by default in Debian for instance, it also does cool things like autocompleting filenames based on which command is already typed (so it won't autocomplete to some pdf filename if the command is mplayer), autocompletion depending on config files (alias from ~/.ssh/config and /etc/hosts, remotes from the git config, package names when the command is apt-*…).

About fish, for the history based autocompletion of commands, how is that different from Bash's C-r?

Bash has completion, of course, but does it have autocompletion? Big difference as getting completions without hitting the tab key changes how you interact with the line editor.

Bash's auto completion is a child's toy compared to Zsh's.

People say this kind of thing a lot, but I’m pretty sure it’s simply not true. bash-completion handles pretty much everything I can imagine already, and can be extended to handle anything you can express programmatically.

Care to back that FUD up with some proof?

And when it cannot complete arguments, just hit `ESC h` or Alt-h to get help on the current command you're typing. (See also: `ESC q` or Alt-q)