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+# SPDX-License-Identifier: Unlicense
+# https://raw.githubusercontent.com/lilyball/nix-env.fish/master/conf.d/nix-env.fish
+
+# Setup Nix
+
+# We need to distinguish between single-user and multi-user installs.
+# This is difficult because there's no official way to do this.
+# We could look for the presence of /nix/var/nix/daemon-socket/socket but this will fail if the
+# daemon hasn't started yet. /nix/var/nix/daemon-socket will exist if the daemon has ever run, but
+# I don't think there's any protection against accidentally running `nix-daemon` as a user.
+# We also can't just look for /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh because
+# older single-user installs used the default profile instead of a per-user profile.
+# We can still check for it first, because all multi-user installs should have it, and so if it's
+# not present that's a pretty big indicator that this is a single-user install. If it does exist,
+# we still need to verify the install type. To that end we'll look for a root owner and sticky bit
+# on /nix/store. Multi-user installs set both, single-user installs don't. It's certainly possible
+# someone could do a single-user install as root and then manually set the sticky bit but that
+# would be extremely unusual.
+
+set -l nix_profile_path /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh
+set -l single_user_profile_path ~/.nix-profile/etc/profile.d/nix.sh
+if test -e $nix_profile_path
+  # The path exists. Double-check that this is a multi-user install.
+  # We can't just check for ~/.nix-profile/… because this may be a single-user install running as
+  # the wrong user.
+
+  # stat is not portable. Splitting the output of ls -nd is reliable on most platforms.
+  set -l owner (string split -n ' ' (ls -nd /nix/store 2>/dev/null))[3]
+  if not test -k /nix/store -a $owner -eq 0
+    # /nix/store is either not owned by root or not sticky. Assume single-user.
+    set nix_profile_path $single_user_profile_path
+  end
+else
+  # The path doesn't exist. Assume single-user
+  set nix_profile_path $single_user_profile_path
+end
+
+if test -e $nix_profile_path
+  # Source the nix setup script
+  # We're going to run the regular Nix profile under bash and then print out a few variables
+  for line in (env -u BASH_ENV bash -c '. "$0"; for name in PATH "${!NIX_@}"; do printf "%s=%s\0" "$name" "${!name}"; done' $nix_profile_path | string split0)
+    set -xg (string split -m 1 = $line)
+  end
+
+  # Insert Nix's fish share directories into fish's special variables.
+  # nixpkgs-installed fish tries to set these up already if NIX_PROFILES is defined, which won't
+  # be the case when sourcing $__fish_data_dir/share/config.fish normally, but might be for a
+  # recursive invocation. To guard against that, we'll only insert paths that don't already exit.
+  # Furthermore, for the vendor_conf.d sourcing, we'll use the pre-existing presence of a path in
+  # $fish_function_path to determine whether we want to source the relevant vendor_conf.d folder.
+
+  # To start, let's locally define NIX_PROFILES if it doesn't already exist.
+  set -al NIX_PROFILES
+  if test (count $NIX_PROFILES) -eq 0
+    set -a NIX_PROFILES $HOME/.nix-profile
+  end
+  # Replicate the logic from nixpkgs version of $__fish_data_dir/__fish_build_paths.fish.
+  set -l __nix_profile_paths (string split ' ' -- $NIX_PROFILES)[-1..1]
+  set -l __extra_completionsdir \
+    $__nix_profile_paths/etc/fish/completions \
+    $__nix_profile_paths/share/fish/vendor_completions.d
+  set -l __extra_functionsdir \
+    $__nix_profile_paths/etc/fish/functions \
+    $__nix_profile_paths/share/fish/vendor_functions.d
+  set -l __extra_confdir \
+    $__nix_profile_paths/etc/fish/conf.d \
+    $__nix_profile_paths/share/fish/vendor_conf.d \
+
+  ### Configure fish_function_path ###
+  # Remove any of our extra paths that may already exist.
+  # Record the equivalent __extra_confdir path for any function path that exists.
+  set -l existing_conf_paths
+  for path in $__extra_functionsdir
+    if set -l idx (contains --index -- $path $fish_function_path)
+      set -e fish_function_path[$idx]
+      set -a existing_conf_paths $__extra_confdir[(contains --index -- $path $__extra_functionsdir)]
+    end
+  end
+  # Insert the paths before $__fish_data_dir.
+  if set -l idx (contains --index -- $__fish_data_dir/functions $fish_function_path)
+    # Fish has no way to simply insert into the middle of an array.
+    set -l new_path $fish_function_path[1..$idx]
+    set -e new_path[$idx]
+    set -a new_path $__extra_functionsdir
+    set fish_function_path $new_path $fish_function_path[$idx..-1]
+  else
+    set -a fish_function_path $__extra_functionsdir
+  end
+
+  ### Configure fish_complete_path ###
+  # Remove any of our extra paths that may already exist.
+  for path in $__extra_completionsdir
+    if set -l idx (contains --index -- $path $fish_complete_path)
+      set -e fish_complete_path[$idx]
+    end
+  end
+  # Insert the paths before $__fish_data_dir.
+  if set -l idx (contains --index -- $__fish_data_dir/completions $fish_complete_path)
+    set -l new_path $fish_complete_path[1..$idx]
+    set -e new_path[$idx]
+    set -a new_path $__extra_completionsdir
+    set fish_complete_path $new_path $fish_complete_path[$idx..-1]
+  else
+    set -a fish_complete_path $__extra_completionsdir
+  end
+
+  ### Source conf directories ###
+  # The built-in directories were already sourced during shell initialization.
+  # Any __extra_confdir that came from $__fish_data_dir/__fish_build_paths.fish was also sourced.
+  # As explained above, we're using the presence of pre-existing paths in $fish_function_path as a
+  # signal that the corresponding conf dir has also already been sourced.
+  # In order to simulate this, we'll run through the same algorithm as found in
+  # $__fish_data_dir/config.fish except we'll avoid sourcing the file if it comes from an
+  # already-sourced location.
+  # Caveats:
+  # * Files will be sourced in a different order than we'd ideally do (because we're coming in
+  #   after the fact to source them).
+  # * If there are existing extra conf paths, files in them may have been sourced that should have
+  #   been suppressed by paths we're inserting in front.
+  # * Similarly any files in $__fish_data_dir/vendor_conf.d that should have been suppressed won't
+  #   have been.
+  set -l sourcelist
+  for file in $__fish_config_dir/conf.d/*.fish $__fish_sysconf_dir/conf.d/*.fish
+    # We know these paths were sourced already. Just record them.
+    set -l basename (string replace -r '^.*/' '' -- $file)
+    contains -- $basename $sourcelist
+    or set -a sourcelist $basename
+  end
+  for root in $__extra_confdir
+    for file in $root/*.fish
+      set -l basename (string replace -r '^.*/' '' -- $file)
+      contains -- $basename $sourcelist
+      and continue
+      set -a sourcelist $basename
+      contains -- $root $existing_conf_paths
+      and continue # this is a pre-existing path, it will have been sourced already
+      [ -f $file -a -r $file ]
+      and source $file
+    end
+  end
+end