Packer
======

Packer is an utility file that creates an executable binary "bundle"
out of a tree of files and an executable. When the bundle is executed,
the tree is extracted to a temporary directory, the inner binary
binary is executed, and the temporary directory is deleted when it
terminates.

For example, suppose you have the following tree:

myproject/
  project_executable
  data/
    foo.dat
    bar.dat
  images/
    splash.png

The project executable requires the data files and images to be
present in order to function, but distributing a tree is a little
cumbersome.

Instead create a packfile:

$ cd myproject
$ /path/to/packer packed_project project_executable data/*.dat images

As you can see, the first argument to the packer is the output
filename, and the second is the executable in the tree. All subsequent
arguments are file patterns for files to include in the packed file.

When you execute packed_project, it will extract the pack into a
temporary tree, for example:

/tmp
  pack_4492_4952052859802/
    project_executable
    data/
      foo.dat
      bar.dat
    images/
      splash.png

Once extracted, the unpacker will execute project_executable from
inside the extracted tree.

Now, how do you get to your files? The current working directory is
set to the directory from which packed_project was executed, to
maintain the illusion for the user that the packed file is a normal
executable. So, to get to the data files packed alongside your binary,
the packer sets environment variables for you before it invokes your
executable:

  - UNPACK_DIR is set to the location of the unpacked tree.
  - The following variables are prepended with UNPACK_DIR:
    - PATH
    - PYTHONPATH
    - RUBYPATH
    - LD_LIBRARY_PATH

If the files associated with the binary are other binaries, shared
libraries, python modules or ruby scripts, the integration is pretty
much automagic. For all other uses the program can examine UNPACK_DIR
to find its files.

Once your main program terminates, the unpacker will delete the
temporary directory. Make sure that anything you want to keep between
executions is moved somewhere else.