[Bug] Claude Code overrides user default SHELL environment variable

Bug Description
Claude Code overrides user default SHELL

Environment Info

• Platform: darwin
• Terminal: iTerm.app
• Version: 2.1.37
• Feedback ID: 09397305-34ef-462a-9b70-204f85b7b9bd

Claude Code Shell Investigation

Problem

Claude Code sessions were using /bin/zsh despite the developer machine being configured entirely for bash.

Environment

• Login shell (dscl): /opt/homebrew/bin/bash (GNU Bash 5.3.9)
• /etc/shells: /opt/homebrew/bin/bash is listed
• User's actual shell usage: Always bash, never zsh
• Claude Code session shell: /bin/zsh (5.9)

Root Cause

Claude Code has its own shell detection logic that does not simply read the user's login shell from dscl or honor $SHELL as set by the login process. Its automatic detection was resolving to /bin/zsh — likely because macOS defaults to zsh since Catalina, and Claude Code's detection logic may prioritize the system default over the user's configured login shell.

Fix

Added CLAUDE_CODE_SHELL to ~/.claude/settings.json:

{
  "env": {
    "CLAUDE_CODE_SHELL": "/opt/homebrew/bin/bash"
  }
}

This environment variable explicitly tells Claude Code which shell to use for all Bash tool invocations, overriding the automatic detection.

Implications

• The current session (at time of fix) continues using zsh — restart required
• All future sessions will use /opt/homebrew/bin/bash
• Shell-specific syntax differences (e.g., shopt in bash vs setopt in zsh, array indexing, BASH_SOURCE vs ZSH_EVAL_CONTEXT) may have caused subtle issues in prior sessions
• The BASH_ENV setting in settings.json (pointing to ~/.config/bash/functions.sh) was likely not being sourced correctly under zsh, since BASH_ENV is a bash-specific variable

Related Settings

| Variable | Purpose |
|----------|---------|
| CLAUDE_CODE_SHELL | Override shell binary for Bash tool |
| CLAUDE_CODE_SHELL_PREFIX | Wrap all bash commands with a prefix |
| CLAUDE_BASH_MAINTAIN_PROJECT_WORKING_DIR | Return to project dir after each command |