uutils/coreutils

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

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#10,095 opened on Jan 7, 2026

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U - fmtgood first issue

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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

Research direction
Inspect the test cases in the issue and trace the line wrapping logic in src/uu/fmt/src/linebreak.rs to understand where the break points differ. Create a test case that matches GNU fmt output and adjust the algorithm accordingly.
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.
2
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
gitrust
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75