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README.rst

BitShuffle README

Build Status

TravisCI:

https://travis-ci.org/charlesdaniels/bitshuffle.svg?branch=master

AppVeyor:

https://ci.appveyor.com/api/projects/status/h7h2a8ltcxkk4926?svg=true

Introduction

What is it?

BitShuffle is a program for encoding and decoding arbitrary binary data into printable ASCII characters for transfer over arbitrary media. In many respects, it can fill the same purpose as base64 or uudecode / uuencode, however it is more sophisticated than these tools. Some key features that BitShuffle offers include:

  • Automatic chunking of data into arbitrary sizes.
  • Automatic checksumming of data
  • Automatic compression of data (bzip and gzip are both support, see #2)
  • Support for both Python 2 and 3

Example Use-Cases

  • The use case which spawned the project in the first place; copying small files over an existing interactive ssh session without needing to re-authenticate when using scp.
  • Transferring arbitrary files over chat programs which either don't allow attachments, or which restrict what file types are allowed. For example sending a small script to a friend over GroupMe.
  • Embedding arbitrary binary data in program logs (as an example, one of BitShuffle's authors once used a spiritual precursor to BitShuffle to pickle and embed live Python objects into a program's debug log for later interactive debugging).
  • Sending e-mail attachments across e-mail servers that don't allow certain file types/extensions (email attachments are really just base64 encoded data anyway, but BitShuffle would avoid inspection by most mail services).

FAQ

Why Not Use Dropbox/Google Drive/MediaFire/Etc?

These services are inconvenient to use for very small or transient files; i.e. "let me show you this cool shell script I wrote", or "here look at this 10 line long log file".

Why Not Use PasteBin/HasteBin/Sprunge/Etc?

These services are designed specifically for transferring plain text data, and often mangle binary data. They usually have size limitation as well.

Is This Really Useful?

The authors of BitShuffle find it useful. Maybe you will too. Maybe not.

Why so Much CI / Testing?

The amount of automated tests may seem high for a project as small as BitShuffle is. However, BitShuffle is intended to be a tool used on a daily basis (as it is by its authors), inside of pipelines, and possibly inside of other automation. It is critical thus that it not break or behave in strange or unusual ways for the same reason ls needs to not break on weird edge cases - it's used too frequently.

Can I Embed BitShuffle in my Project?

Yes, but please wait until we have a stable release. The data packet format may change without warning until there is at least one stable release.

Does BitShuffle Have a Stable API?

Not at this time, but it will in the future as the project matures a bit. Until then use BitShuffle as a Python module at your own risk.

Installation

Dependencies

To install/run BitShuffle:

  • POSIX-ish operating system, or Windows (as of #38).
  • Python (>= 2.7)

To run BitShuffle's automated tests locally:

  • POSIX sh compliant shell interpreter
  • uuidgen
  • travis (hint: gem install travis)
  • bc
  • /tmp must exist and be write-able
  • pycodestyle

Installing with setup.py

Simply run python ./setup.py install. (Note: this assumes which python is identical to python)

Installing Manually

If you are only going to be using BitShuffle as a script, not as a python module, you can also just drop bitshuffle/bitshuffle.py into $PATH (I suggest symlinking to ~/bin/bitshuffle).

Installing a Binary Release

Binary releases for various platforms are available via the GitHub releases page. At present, builds are available for Linux and Windows as static binaries, which can be dropped anywhere in $PATH without requiring Python to be installed.

macOS ships with Python installed in the default install, and the version available thus is sufficient to run BitShuffle. Consequentially, no static build is provided for macOS at this time.

Contributing

Contributions are welcome! Simply open a GitHub pull request. All contributions need to pass the automated TravisCI checks, most of which are available as a script (I recommend symlinking scripts/pre-commit to .git/hooks/).

If you would like to contribute by sending patches over e-mail, that is fine too, just get in touch with @charlesdaniels.

Technical Details

BitShuffle Data Packet Specification (compatibilty level 1)

A BitShuffle data packet is a sequence of ASCII text. A data packet may be arbitrarily long. A data backed may contain arbitrary whitespace, which is stripped during processing.

A BitShuffle packet is surrounded by special sigil characters:

  • It is preceeded by the string literal ((<< (opening token)
  • It is succeeded by the string literal >>)) (closing token)

These string literals are deliberately selected to avoid common markup characters, such as #, @, and *, which are frequency used by messaging services to denote special formatting for messages.

The data packed is comprised of several segments. A segment begins with either the opening token or the | character. A segment ends with either the closing token or a | character. A segment may contain only the characters a-zA-Z0-9, as well as =, :, /, +, -. Again, keep in mind that whitespace is ignored entirely.

The data packed contains the following segments, in order:

  • Message indicating that this a BitShuffle data packet, with a link to download BitShuffle. Note that the decoder does not support line breaks in this segment (see #10).
  • BitShuffle data packet format compatibility level (currently 1).
  • BitShuffle data encoding format (current base64).
  • BitShuffle data compression type (currently either bz2 or gzip).
  • BitShuffle packet sequence number (i.e. 23).
  • BitShuffle packet sequence end (the number of packets in the message).
  • BitShuffle data checksum (encoded)
  • BitShuffle data chunk (encoded)

Segments marked as encoded indicate their contents is arbitrary data which has been compressed with the specified compression type, and encoded with the specified encoding format.

Note that the data packet spec is liable to change without warning in non-release versions of BitShuffle. Any changes made since the last release will result in a compatibility level bump at time of release. Use non-release versions at your own risk.

BitShuffle Automated Testing Strategy

BitShuffle is tested automatically by multiple CI systems (AppVeyor and TravisCI), executing a large battery of tests to ensure it is functioning correctly. These scripts are implemented in POSIX sh, and are stored int the scripts/ directory. A subset of these tests that are safe to run locally (do not modify the disk or require sudo) can be executed with the script scripts/pre_commit_check.sh. For convenience, only one version of python is tested locally. Contributors should not open PRs for code that does not pass this script.

Note that Windows support is tested via a PowerShell script, which is intended to run only on AppVeyor. It executes only a few very simple smoke tests that ensure the program can run successfully on Windows, but does not exhaustively test every feature.

Most of BitShuffle's tests are end-to-end/blackbox tests that aim to validate real-world use cases. At this time, BitShuffle is too small and monolithic for actual unit tests to be of value. In the future, a stable public API will be defined, at which time comprehensive unit tests will need to be written to avoid regressions (see #39, #5).

In addition to automated functionality tests, we also adhere strictly to PEP8, which is enforced by pycodestyle.

Version Number Conventions

BitShuffle loosely follows Semantic Versioning. The following suffixes are used:

  • No suffix - implies this is a stable release.
  • -git - this version is from the BitShuffle git repository, and probably has not been tested.
  • -RCX - the is the Xth release candidate for the relevant version.
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