add shell.nix and make it work on nixos

build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 set -e
 

default.nix

@@ -0,0 +1,19 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    coreutils posix_man_pages bash-completion less
+    gitFull diffutils
+    gnumake which
+    ncurses perl python2 python3
+
+    # keep this line if you use bash
+    bashInteractive
+  ];
+
+  shellHook =
+  ''
+    alias ..='cd ..'
+    alias ...='cd ../..'
+  '';
+}