ucs-detect 1.0.3

Installation & Usage

To install:

$ pip install -U ucs-detect

UNICODE_VERSION

The environment variable, UNICODE_VERSION is used by the python wcwidth library, which contains every past unicode table version, to determine how dependent python programs, such as IPython render wide and zero-width characters.

Create sh-compatible line for export:

$ ucs-detect --shell --quick
UNICODE_VERSION=15.0.0; export UNICODE_VERSION

$ eval "$(ucs-detect --quick --shell)"
$ echo $UNICODE_VERSION
15.0.0

Problem

Chinese, Japanese, Korean, and Emoticon characters are “double-wide”, occupying 2 cells, instead of 1, and some other special characters are “zero-width”, which do not occopy any cells at all, or modify the previous cell as a “combining” character.

Any terminal application that formats and displays these characters may have trouble determining how it will be displayed to the end-user. Here is one such example, from vercel/hyper terminal:

This problem happens often, because the Unicode Consortium releases new versions of the Unicode Standard periodically, but the source code of libraries and applications are not updated at the same time, or at all!

Many languages and libraries continue to conform only to Unicode 5.0, which is the last version of wcwidth.c released by Markus Kuhn in 2007.

Solution

The most important factor is to determine: What version of unicode is the Terminal using?

This program, ucs-detect, is able to automatically detect the version of unicode that the connecting Terminal supports. The python wcwidth library supports all Unicode versions, 4.1.0 through 12.1.0 at time of this writing, and so it is able to select and match the correct width, by selecting for the given value of the UNICODE_VERSION environment variable.

How it works

The unicode version is determined using the Query Cursor Position terminal sequence, which asks, “where is the cursor?” using a special sequence, and conforming terminals reply.

By displaying a series of Wide Unicode characters for each Unicode version expected to advance the cursor by 2 cells, the very last version that successfully advances 2 cells determines the version of Unicode supported by the Terminal.

This solution of using Query Cursor Position and exporting an sh variable is precisely the same solution used by the resize(1) program distributed with X11, which determines the terminal size over transports that are not capable of communicating or forwarding it (such as over a serial line).

Further

I hope that this CLI tool is provisional! I’d like to see all Terminals automatically export the environment variable, UNICODE_VERSION and that this tool would not be required.

If you would like to read more about this tool and the related problems I hope to address with the UNICODE_VERSION environment variable, have a look at this companion article, https://jeffquast.com/post/terminal_wcwidth_solution/