we have a new way to install these completion files #68329 that you could try here.

we have a new way to install these completion files #68329 that you could try here.

I wanted it to be simple to follow the change (for the reviewer) and to stay with the existing style of the postInstall phase. Bringing in a shell helper function for one of several files installed would look a little out of place.

But I can revise the commit if desired. In that case, I would also suggest using install -D instead of mkdir followed by ln and to not install git-prompt.sh into bash_completion.d. I believe the latter is done to automatically source it, but that cannot be relied upon, nor should it really be automatically sourced, and the file is already copied to share/git.

In my case (by default?) __load_completion does not include the profile-based directory ~/.nix-profile/share/bash-completion/completions where git completion is put. So I had to add BASH_COMPLETION_USER_DIR=~/.nix-profile/share/bash-completion to my ~/.profile.

In my case (by default?) __load_completion does not include the profile-based directory ~/.nix-profile/share/bash-completion/completions where git completion is put

Try add this to your shell startup (prior to sourcing bash_completion.sh):

if [[ -d "$HOME/.nix-profile/share" ]]; then
    XDG_DATA_DIRS="$HOME/.nix-profile/share${XDG_DATA_DIRS:+:$XDG_DATA_DIRS}"
fi

@sorbits Yes, that works and I thought to use that variable but it was not defined in my environment and that's probably the default state. As __load_completion uses for dir in ${XDG_DATA_DIRS:-/usr/local/share:/usr/share}; if I set it to only my value, I'll lose completions loaded from the base OS (I'm on MacOS) if there are any.

There might be some other use cases for that variable I'm not aware of, so I think it would be better to either populate XDG_DATA_DIRS fully in ~/.nix-profile/etc/profile.d/nix.sh, or to not touch it at all and to use some other variable only for completions.

if I set it to only my value, I'll lose completions loaded from the base OS (I'm on MacOS) if there are any.

You can include the default values like this:

if [[ -d "$HOME/.nix-profile/share" ]]; then
    XDG_DATA_DIRS="$HOME/.nix-profile/share:${XDG_DATA_DIRS:-/usr/local/share:/usr/share}"
fi

These defaults are from the XDG Base Directory Specification so they will not change in bash_completion.sh (i.e. it is safe to copy/paste them to your profile).

However, I do not think it makes sense to source completions from those locations when you use nixpkgs, only directories controlled by the nix package manager should be referenced by your system to ensure everything is in a known state / reproductible.

There might be some other use cases for that variable I'm not aware of, so I think it would be better to either populate XDG_DATA_DIRS fully in ~/.nix-profile/etc/profile.d/nix.sh, or to not touch it at all and to use some other variable only for completions.

I think it would be ideal if it was setup in ~/.nix-profile/etc/profile.d/nix.sh so that completions would work out-of-the-box.

But lack of default setup just means that we must do it manually in our custom shell startup scripts, not that we should avoid touching the variable.

I strongly encourage you to look into this and possibly submit a pull request to change this or at least get a conversation started somewhere where those more knowledgeable about this can weigh in on the pros and cons.

If you google for XDG_DATA_DIRS and NixOS, this is not the only issue caused by lack of useful default value, e.g. the Nix Cookbook also mentions that users must setup XDG_DATA_DIRS for the Desktop Environment to find .desktop files.