feat: initial commit; NixOS Era

pkgs/fish/functions/nix-shell.fish

@@ -0,0 +1,45 @@
+function nix-shell
+	set -l _argv (string split ' ' -- $argv);
+
+	if contains -- '-q' $_argv; or contains -- '--query' $_argv
+		for pkg in (string split ' ' -- $FISH_NIX_PACKAGES)
+			echo $pkg
+		end
+		return 0;
+	end
+	
+	function package_flag -a arg
+		contains -- '-p' $arg
+		or contains -- '--packages' $arg
+	end
+
+	if not command -q nix-shell; or not package_flag $argv
+		command nix-shell --run fish $argv;
+		return $status;
+	end
+
+	set -l packages
+	set -l reading_packages 0
+	for arg in $_argv
+		if [ $reading_packages -ne 0 ]
+			if string match -r '^-'
+				set reading_packages 0
+			else
+				not contains -- $arg (string split ' ' -- $FISH_NIX_PACKAGES); 
+				and set packages $packages $arg
+			end
+		else
+			package_flag $arg; and set reading_packages 1
+		end
+	end
+
+	set -l prev_packages $FISH_NIX_PACKAGES;
+	set -x FISH_NIX_PACKAGES $prev_packages $packages;
+
+	command nix-shell --run fish $argv;
+	set -l nix_status $status;
+
+	set -x FISH_NIX_PACKAGES $prev_packages;
+
+	return $nix_status
+end