(specification)=

# The Reproducible Execution Environment Specification

repo2docker scans a repository for particular {ref}`config-files`, such
as `requirements.txt` or `Project.toml`. The collection of files, their contents,
and the resulting actions that repo2docker takes is known
as the **Reproducible Execution Environment Specification** (or REES).

The goal of the REES is to automate and encourage existing community best practices
for reproducible computational environments. This includes installing packages using
community-standard specification files and their corresponding tools,
such as `requirements.txt` (with `pip`), `Project.toml` (with Julia), or
`apt.txt` (with `apt`). While repo2docker automates the
creation of the environment, a human should be able to look at a REES-compliant
repository and reproduce the environment using common, clear steps without
repo2docker software.

Currently, the definition of the REE Specification is the following:

> Any directory containing zero or more files from the {ref}`config-files` list is a
> valid reproducible execution environment as defined by the REES. The
> configuration files have to all be placed either in the root of the
> directory, in a `binder/` sub-directory or a `.binder/` sub-directory.

For example, the REES recognises `requirements.txt` as a valid config file.
The file format is as defined by the `requirements.txt` standard of the Python
community. A REES-compliant tool will install a Python interpreter (of unspecified version)
and perform the equivalent action of `pip install -r requirements.txt` so that the
user can afterwards run python and use the packages installed.