pi-windows-nul-fix

A Pi extension that rewrites Windows-style > nul redirects to > /dev/null before the bash tool runs them, preventing Git Bash from creating literal files named nul on Windows.

The problem

On Windows, Pi runs shell commands through Git Bash (MSYS2). When an LLM emits a command like some-tool --quiet > nul or build.sh 2> nul, Git Bash does not treat nul as a null device — it creates a file literally named nul in the current directory.

Because nul is a reserved device name in Windows, these files are invisible to ls, cannot be opened in most editors, and cannot be deleted with standard tools — only with Remove-Item -LiteralPath '\\?\C:\path\nul' -Force in PowerShell.

The fix

This extension intercepts the bash tool's tool_call event and rewrites every bare > nul, 2> nul, &> nul, >> NUL, etc. to > /dev/null before execution. It tracks single quotes, double quotes, and backslash escapes and does not rewrite filenames like nul.txt or nul-backup.

On non-Windows platforms it is a no-op.

Install

pi install npm:pi-windows-nul-fix

Or install from the git source:

pi install git:github.com/i-snyder/pi-windows-nul-fix

To try without installing:

pi -e npm:pi-windows-nul-fix

What it covers

The extension rewrites redirects that target the bare nul token (case-insensitive) at a redirect boundary. Patterns rewritten:

Input	Rewritten to
> nul	>/dev/null
> NUL	>/dev/null
>> nul	>>/dev/null
2> nul	2>/dev/null
2>> nul	2>>/dev/null
&> nul	&>/dev/null
&>> nul	&>>/dev/null

Patterns left untouched:

• > nul.txt — filename, not a device reference
• "foo > nul" — inside a double-quoted string
• 'foo > nul' — inside a single-quoted string
• \> nul — escaped redirect operator

Scope and limits

This extension only affects commands invoked by the LLM via the built-in bash tool. It does not affect:

• Commands you run yourself in the Pi shell (! prefix)
• Extensions that bypass the built-in bash tool
• The pi CLI itself

Known limitations

• Heredoc bodies. The parser rewrites > nul anywhere it appears outside of quotes, including inside heredoc bodies. If the LLM writes a script file using a heredoc that intentionally contains > nul as text content, that text will be rewritten to > /dev/null. This is unlikely in practice but worth knowing.

• Quoted redirect targets. > "nul" and > 'nul' (where the target itself is quoted) are left untouched by this extension, but Git Bash still creates a literal nul file for them. These forms are uncommon in LLM-generated commands.

• >& nul. The csh-style combined redirect >& nul is not rewritten. Use &> nul (bash-style) which is what most LLMs emit and is covered.

How it works

Pi's tool_call event fires before each tool executes and exposes event.input as mutable. For bash tool calls, this extension rewrites event.input.command in place using a character-walking parser that tracks quote and escape state before applying a sticky regex against each redirect position.