Bash tool: Environment variables stripped when command contains pipes

Bug Description

When using the Bash tool with a command that contains a pipe (|), environment variables (e.g., $HOME, $PATH, $USER) are replaced with empty strings before execution.

Steps to Reproduce

Copy and paste this prompt into any Claude Code session:

I need you to help me test a potential bug in Claude Code's Bash tool. Please run these commands exactly as written and report the results.

Test 1: Without pipe
echo "HOME=$HOME"

Test 2: With pipe
echo "HOME=$HOME" | cat

Test 3: Workaround
bash -c 'echo "HOME=$HOME"' | cat

The bug hypothesis is: When a Bash command contains a pipe (|), environment variables like $HOME are replaced with empty strings before execution.

Expected results if the bug exists:
- Test 1: Shows the full path (e.g., HOME=/Users/yourname)
- Test 2: Shows just HOME= with no value
- Test 3: Shows the full path again (workaround works)

Please run all three tests and tell me what you observe.

Expected Behavior

All three tests should output the full path:

HOME=/Users/username

Actual Behavior

• Test 1: HOME=/Users/jtran ✓ (works correctly)
• Test 2: HOME= ✗ (variable stripped to empty string)
• Test 3: HOME=/Users/jtran ✓ (workaround succeeds)

Impact

This bug affects any command that needs to:

1. Use environment variables (like $HOME, $PATH, $USER)
2. Pipe output to another command

Common examples that would fail:

echo $PATH | tr ':' '\n'
ls $HOME/Documents | grep pattern
cat $HOME/.bashrc | head -10

Workaround

Wrap the command in bash -c '...' with single quotes:

bash -c 'echo $PATH | tr ":" "\n"'
bash -c 'ls $HOME/Documents | grep pattern'

Environment

• OS: macOS (Darwin 24.6.0)
• Claude Code version: Latest (as of 2025-12-17)
• Model: Claude Sonnet 4.5