Opinionated cli wrapper for nix expressions

datakurre-nix-exec.nix

###
# Simple 'nix-exec' shebang wrapper for using nix-expression as 'standalone'
# execs (outside NixOS, e.g. on OSX) without installing them with 'nix-env'.
#
# The original use case was to replace Python virtualenvs with nix expressions
# in Python development.
#
# Install:
#
# $ nix-env -i -f datakurre-nix-exec.nix
#
# Usage:
#
# $ echo "!/usr/bin/env nix-exec
# with import <nixpkgs> { };
#
# buildEnv {
#  name = "python27";
#  paths = [ python27 ];
# }" > python
# $ chmod u+x python
#
# $ ./python
# ➜ /nix/store/0gnln0ks3js26zc9p0575pvhnmpchywv-python27/bin/python

with import <nixpkgs> { };

stdenv.mkDerivation {
  name = "datakurre-nix-exec-1.2.1";

  builder = builtins.toFile "builder.sh" "
    source $stdenv/setup
    mkdir -p $out/bin
    echo \"#!/bin/bash
build=\\`nix-build --no-out-link \\$1\\`
if [ \\$build ]; then

  MY_TZ=\\\"\\$TZ\\\"
  MY_PATH=\\\"\\$build/bin:\\$build/sbin:\\$build/libexec:\\$PATH\\\"
  MY_http_proxy=\\\"\\$http_proxy\\\"
  MY_ftp_proxy=\\\"\\$http_proxy\\\"

  if [ -d \\$build/dev-envs ]; then
    source \\\"\\$build/dev-envs/\\\"*

    export TZ=\\\"\\$MY_TZ\\\"
    export PATH=\\\"\\$PATH:\\$MY_PATH\\\"
    export http_proxy=\\\"\\$MY_http_proxy\\\"
    export ftp_proxy=\\\"\\$MY_ftp_proxy\\\"

    export CFLAGS=\\`echo \\$NIX_CFLAGS_COMPILE|sed 's/-isystem /-I/g'\\`
    export LDFLAGS=\\$NIX_LDFLAGS
  else
    export PATH=\\\"\\$MY_PATH\\\"
  fi

  cmd=\\$\{1##*/\}; cmd=\\$\{cmd%%@*\}; cmd=\\$\{cmd%.nix\}
  paths=(\\\"\\$build/bin\\\" \\\"\\$build/sbin\\\" \\\"\\$build/libexec\\\")
  for path in \\\"\\$\{paths[@]\}\\\"; do
    if [ -f \\\"\\$path/\\$\{cmd\}\\\" ]; then
      cmd=\\\"\\$path/\\$\{cmd\}\\\"
      break
    fi
  done

  if [ -t 1 ]; then echo \\\"➜\\\" \\$cmd \\\"\\$\{@:2\}\\\"; fi
  \\\"\\$cmd\\\" \\\"\\$\{@:2\}\\\"
fi
\" > $out/bin/nix-exec
    chmod a+x $out/bin/nix-exec
  ";
}