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Overlays allow you to modify the contents of the release, you may add/symlink files, create directories, and generate files based on templates.

You may add overlays via your release configuration, e.g:

release :myapp do
  set overlays: [
    {:mkdir, "configs"},
    {:template, "priv/templates/myconfig.eex", "configs/<%= release_name %>.config"}

Overlay paths are templated via EEx, with the contents of Distillery-provided overlay vars, including those you provide via the overlay_vars setting. The template overlay type assumes the source file is an EEx template, and generates it using the same overlay vars mentioned above, then copies it to the destination path provided.

All source paths (i.e. those in copy, link, and template overlays) are relative to the project root. All destination paths (mkdir is just a destination path) are relative to the release output directory, which will be rel/<release_name>.

Currently, the following overlay vars are provided out of the box by Distillery:

  • release_name - Name of the release being built

  • release_version - Version of the release being built

  • is_upgrade - Is this release an upgrade release

  • upgrade_from - The version of the release being upgraded from, nil if not an upgrade.

  • dev_mode - Is this release being built in dev mode

  • include_erts - Is ERTS being included in the release

  • include_src - Is source code being included in the release

  • include_system_libs - Are system libraries being included in the release

  • erl_opts - The string of options which will be passed to erl when running the release

  • run_erl_env - The string of environment variable assignments which will be applied to run_erl when running the release

  • erts_vsn - The current ERTS version

  • output_dir - The release output directory

You may add your own to this list by setting overlay_vars to a keyword list of names to values you wish to make available to templates.