Add login shell environment checks with warnings for initialization failures and version mismatches

[Copilot]

Overview

This PR implements warning messages for shell initialization failures and version checks for Node.js and Python environments as requested in issue #[issue_number].

When shell initialization steps fail or environment versions don't match expectations, users now receive clear warnings on stderr to help diagnose configuration issues.

Changes

Warning System

Added a new _dots_warn() helper function that outputs warnings to stderr with a "WARNING:" prefix:

_dots_warn() {
    echo "WARNING: $*" >&2
}

Enhanced Initialization Functions

NVM (_dots_load_nvm())

• Warns when nvm.sh is missing or fails to load
• Warns when bash completion fails to load
• Only warns about missing Node.js when nvm is successfully loaded

Pyenv (_dots_load_pyenv())

• Warns when pyenv is not found in PATH
• Warns when pyenv initialization fails

Homebrew (_dots_load_brew())

• Warns when homebrew initialization fails

Version Checks

Node.js Version Check

• Compares current Node.js version against expected version from nvm alias file (lts/jod)
• Handles version strings with or without 'v' prefix
• Only checks when nvm is available (avoids false positives)

Python Version Check

• Compares current Python version against expected version (3.13.7)
• Checks both python and python3 commands
• Only checks when pyenv is available (avoids false positives)

Example Output

# System without nvm/pyenv installed:
WARNING: pyenv not found in PATH

# System with version mismatch:
WARNING: Node.js version mismatch: current=v18.0.0, expected=v20.11.0
WARNING: Python version mismatch: current=3.12.3, expected=3.13.7

# System with missing nvm.sh:
WARNING: nvm.sh not found at /home/user/.nvm/nvm.sh

Technical Details

• All warnings go to stderr - Allows proper separation from stdout and can be redirected independently
• POSIX-compatible syntax - Uses case statements and >/dev/null 2>&1 for maximum shell compatibility
• Smart detection - Contextual warnings only appear when tools should be present
• Proper cleanup - Variables are unset appropriately to avoid errors
• No breaking changes - Maintains full backward compatibility

Testing

• ✅ Verified warnings appear correctly on stderr
• ✅ Tested with bash and sh shells
• ✅ Confirmed environment setup continues to work despite warnings
• ✅ Tested version mismatch detection
• ✅ Verified POSIX compatibility

Benefits

Users can now:

• Quickly identify shell initialization failures during login
• Detect when Node.js or Python versions don't match expected versions
• Redirect warnings to a log file for debugging: bash -l 2>/tmp/shell-warnings.log
• Hide warnings when needed: bash -l 2>/dev/null
• See all warnings with: bash -l 2>&1 | grep WARNING

This helps maintain a healthy development environment and makes it easier to troubleshoot configuration issues.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Login shell environment checks</issue_title>
<issue_description>Print a warning message if any of the shell initialisation steps fail. Perform a quick check for the standard node/python environment versions and print a warning if they are not up-to-date</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #63

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