fix: tail -n counts lines correctly when input lacks trailing newline (fixes #1787)

[gnodet]

Summary

• Fix off-by-one error in PosixCommands.tail where -n N returned one extra line when input doesn't end with a newline character
• The implementation counted newline characters (\n) rather than lines, so the final unterminated line wasn't counted, causing the window to include one extra line
• Add tests for both trailing-newline and no-trailing-newline cases

Fixes #1787

Summary by CodeRabbit

• Bug Fixes
  • Fixed the tail command to properly handle files without trailing newlines. Previously, when displaying the last N lines of a file that doesn't end with a newline, the final line could be incorrectly excluded from the output. The command now correctly counts and displays all lines.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 20aab721-20a1-49d1-a815-7b4b9dc7a05f

📥 Commits

Reviewing files that changed from the base of the PR and between 79b0f73 and 43f2a4f.

📒 Files selected for processing (2)

• builtins/src/main/java/org/jline/builtins/PosixCommands.java
• builtins/src/test/java/org/jline/builtins/PosixCommandsTest.java

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📝 Walkthrough

Walkthrough

The tail() command's line-counting logic is corrected to treat a non-empty buffer ending without a newline as containing an additional line, fixing undercounting. Test cases validate correct behavior with and without trailing newlines.

Changes

Cohort / File(s)	Summary
Line Counting Bug Fix builtins/src/main/java/org/jline/builtins/PosixCommands.java	Modified tail() implementation to account for the final line when buffer doesn't end with '\n', preventing undercounting of lines in files without trailing newlines.
Test Coverage builtins/src/test/java/org/jline/builtins/PosixCommandsTest.java	Added two test methods covering tail -n 2 behavior: one with input lacking trailing newline (testTailNoTrailingNewline) and one with trailing newline (testTailWithTrailingNewline).

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A tail that counts with care,
No more lines lost in the air,
The final line, though bare,
Now gets its rightful share! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 16.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main fix: correcting the line-counting behavior in tail when input lacks a trailing newline, with a reference to the issue number.
Linked Issues check	✅ Passed	The PR correctly implements the fix for issue #1787 by adjusting the tail() line-counting logic to treat non-empty buffers without trailing newlines as containing an additional line, and adds comprehensive tests for both scenarios.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the tail command's line-counting bug: the implementation fix and two new test cases for trailing-newline scenarios.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud