I don't see any point in this. Unless you need this there's no point in copying it from other packages.

This is necessary. The comment is just a hint to know where I have it from. Mcfly and fzf are both ctrl+r replacements and thus a file needs to be sourced by bash to use them. This additional command shows the path to this bash script.

I can delete the comment if you think it is confusing.

I was just about to complain how you can just discover these files by just inspecting the derivations (tree $(nix-build '<nixpkgs>' -A fzf)), but then I found #27709 which introduced this. And it's in the wiki too.

I personally really dislike this approach. Using binaries to find paths is really weird. How I'd think it should work without such a -share binary is to have packages install these sourceable scripts into a standard location (say $out/share/bash/sourceables) and then do

for p in $NIX_PROFILES; do
  source "$p/share/bash/sourceables/fzf"
  source "$p/share/bash/sourceables/autojump"
done

This is also how bash sources its completion scripts:

Alternatively (as just mentioned by @samueldr in #nixos-dev) I can imagine a nix-share binary usable like this (also requires standard locations for these files):

source $(nix-share fzf)
source $(nix-share autojump)

Maybe also combine these approaches and have a standard bash setup script that creates a function for the for p in $NIX_PROFILES thing.

There is also an entry in the nixpkgs manual. And weirdly enough autojump doesn't have this script anymore, 47aceb0 removed it.

Maybe also combine these approaches and have a standard bash setup script that creates a function for the for p in $NIX_PROFILES thing.

Creating a shell setup script makes sense. I just don't get exactly what you mean with ..that creates a function.... What kind of function do you mean, what should it do?

What I could think of is a script called something like nix-source which looks similar to this autojump script, except it also takes a $package argument like nix-source fzf to know which sourceable it should load and goes through a for p in $NIX_PROFILES loop to find the file.

I think it's best to just remove that convenience script for now, we can always make it more convenient later (but you can never take something convenient away without people complaining).

I've removed the part in question and as (ugly) workaround the following works for now:

source $(dirname $(which mcfly))/../share/mcfly/mcfly.bash