[BUG] # Issue Report: NUL File Creation on Windows

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Issue Report: NUL File Creation on Windows

Problem Description

When executing Windows BAT scripts that contain output redirection to NUL (e.g., >nul, 2>nul), Claude Code's Bash tool creates a literal file named "NUL" in the repository instead of properly redirecting to the Windows null device.

Environment

• Platform: Windows 10/11
• Tool: Claude Code Bash tool
• Shell Context: Git Bash / Unix-like shell on Windows
• Repository: Local Git repository

Steps to Reproduce

1. Create a BAT script with null redirection:

``batch
@echo off
call some_command.bat 2>nul
``

1. Execute the BAT script using Claude Code's Bash tool:

``bash
"path\to\script.bat"
``

1. Observe that a file named NUL (0 bytes) is created in the working directory

Expected Behavior

Output should be redirected to the Windows null device (NUL) without creating a physical file. This is the standard behavior when executing BAT scripts from cmd.exe or PowerShell.

Actual Behavior

A literal file named NUL is created in the current working directory. This file:

• Is 0 bytes
• Appears as an untracked file in git
• Gets recreated every time BAT scripts with >nul are executed
• Pollutes the repository

Impact

• Severity: Low to Medium
• Creates spurious files in version control
• Requires manual cleanup or .gitignore entries
• Confusing for users unfamiliar with the issue
• Happens frequently during normal workflow

Affected Scripts in Our Project

Multiple BAT scripts use >nul redirection:

• PEQs_FORECAST/Script/peqs-consolidator.bat
• PEQs_FORECAST/Script/shared/get_python.bat
• PEQs_FORECAST/Script/order-validator.bat

Example from get_python.bat:

for /f "tokens=*" %%i in ('call shared\get_python.bat 2^>nul') do set "PYTHON_CMD=%%i"

Root Cause Analysis

This is a known issue with the interaction between:

1. Windows-style null device redirection (NUL)
2. Unix-like shells (Git Bash, WSL, etc.)
3. How commands are executed through the shell

When Git Bash executes a Windows BAT file, it doesn't properly translate NUL to /dev/null or the Windows null device, causing it to be interpreted as a literal filename.

Workarounds Implemented

1. Added to .gitignore:

``gitignore
# Windows NUL file issue
NUL
**/NUL
``

1. Manual cleanup when noticed:

``bash
rm NUL
``

Suggested Solutions

Option 1: Translate NUL to proper null device

When executing BAT scripts, Claude Code's Bash tool could:

• Detect Windows environment
• Translate >nul to >$null (PowerShell) or handle it appropriately
• Or execute BAT scripts through cmd.exe instead of bash

Option 2: Pre-execution warning

• Detect BAT scripts containing >nul redirections
• Warn user about potential NUL file creation
• Suggest using cmd.exe execution mode

Option 3: Post-execution cleanup

• Automatically detect and remove spurious NUL files after BAT execution
• Only if NUL file is 0 bytes and recently created

Option 4: Execution mode flag

Add a flag to Bash tool for Windows BAT scripts:

{
  "command": "script.bat",
  "windows_native": true  // Execute through cmd.exe
}

Related Issues

This is a well-known issue in the Windows/Unix interoperability space:

• Git for Windows issue tracker has similar reports
• Common problem when running BAT scripts from WSL or Git Bash
• Affects any Unix-like shell environment on Windows

Additional Context

• Frequency: Happens every time affected BAT scripts are run
• User awareness: Users may not notice until git status shows untracked files
• Impact on workflow: Minor annoyance, but consistent

Recommendation

This issue should be addressed in the Bash tool's Windows execution logic. The most robust solution would be Option 1 (proper translation) or Option 4 (execution mode flag) to prevent the issue from occurring in the first place.

References

• Windows null device: https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file#file-and-directory-names
• Git Bash on Windows: https://gitforwindows.org/
• Similar issues in other tools dealing with Windows/Unix interoperability

---

Reported by: Claude Code user on Windows environment
Date: 2025-10-29
Project: alexion-expansion-dashboard
Component: PEQs_FORECAST automation scripts

What Should Happen?

Expected Behavior

Output should be redirected to the Windows null device (NUL) without creating a physical file. This is the standard behavior when executing BAT scripts from cmd.exe or PowerShell.

Error Messages/Logs

## Actual Behavior

A literal file named `NUL` is created in the current working directory. This file:
- Is 0 bytes
- Appears as an untracked file in git
- Gets recreated every time BAT scripts with `>nul` are executed
- Pollutes the repository

Steps to Reproduce

1. Create a BAT script with null redirection:

``batch
@echo off
call some_command.bat 2>nul
``

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.28

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_