uutils/coreutils

fmt: line-wrapping differs from gnu implementation in some situations

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#10095 opened on Jan 7, 2026

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U - fmtgood first issue

Description

Description

When trying to come across edge cases in the non utf-8 fmt PR I came across that some outputs to fmt were not matching due to the line breaks being inserted in different places

Examples:


  $ printf 'aa bb cc dd ee' | /usr/bin/fmt -w 8
GNU
  aa
  bb cc
  dd ee

uutils
  aa bb cc
  dd ee


  $ printf '漢 字 test 日 本 語' | /usr/bin/fmt -w 10
GNU
  漢 字
  test 日
  本 語

uutils
  漢 字 test
  日 本 語


  $ printf '漢字 test 日本語' | /usr/bin/fmt -w 15
GNU
  漢字 test
  日本語

uutils
  $ printf '漢字 test 日本語' | uutils fmt -w 15
  漢字 test 日本語

Where to fix:

src/uu/fmt/src/linebreak.rs is the file that has the logic

Contributor guide

Tech stack
rust
Domain
cli
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rustbasic understanding of text formatting
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Examine the line breaking logic in src/uu/fmt/src/linebreak.rs. The provided examples show discrepancies in how words are grouped and line breaks inserted, particularly with Unicode characters. Compare the current algorithm to GNU fmt's behavior for these edge cases. Focus on the handling of word boundaries and line width evaluation. Look for comments or discussions in the issue to understand expected behavior.