The goal of these function are to automatically add and remove your Rails ./bin/ directory to your and remove it when it doesn't exist. I am sure there are edge cases I missed or maybe there is a better way to do this. :shurg:

add_to_path.sh

# NOTE: 
# Make sure you add this function to your zshrc file or link to the file.
# it could look like this line in your `~/.zshrc`
# . PATH_TO_FILE/add_to_path.sh

#!/bin/sh

# Function to add a directory to PATH
#
# Usage:
#   add_to_path "/path/to/add"
#
# Arguments:
#   $1 - The directory path to add to PATH
#
# Description:
#   Adds the specified directory to the beginning of PATH if it's not already present.
#
# Edge cases handled:
#   - Checks if the directory exists and is readable
#   - Avoids adding duplicate entries
#   - Handles directories with spaces in the name
#   - Maintains : at the start/end of PATH if originally present
add_to_path() {
    if [ -z "$1" ]; then
        echo "Usage: add_to_path <directory>" >&2
        return 1
    fi

    if [ ! -d "$1" ]; then
        echo "Error: '$1' is not a directory" >&2
        return 1
    fi

    if [ ! -r "$1" ]; then
        echo "Error: '$1' is not readable" >&2
        return 1
    fi

    case ":$PATH:" in
        *":$1:"*) :;; # Already in PATH
        *)
            if [ "${PATH#:}" != "$PATH" ]; then
                PATH=":$1$PATH" # Maintain leading :
            elif [ "${PATH%:}" != "$PATH" ]; then
                PATH="$1:$PATH:" # Maintain trailing :
            else
                PATH="$1:$PATH"
            fi
            ;;
    esac
}

rails-cd.sh

# NOTE: 
# Make sure you add this function to your zshrc file or link to the file.
# it could look like this line in your `~/.zshrc`
# . PATH_TO_FILE/rails-cd.sh

#!/bin/sh

# Function to override the cd command
#
# Usage:
#   cd [directory]
#
# Arguments:
#   [directory] - The directory to change to. If omitted, changes to the home directory.
#
# Description:
#   This function overrides the built-in cd command. It changes the current
#   directory and manages the PATH based on the presence of a ./bin/rails file.
#
# Edge cases handled:
#   - Fails gracefully if the target directory doesn't exist
#   - Checks if the old directory still exists before modifying PATH
#   - Verifies both existence and readability of bin/rails files
#   - Works with relative and absolute paths
#   - Handles 'cd' with no arguments (change to home directory)
#   - Handles spaces in directory names
cd() {
  old_dir="$PWD"

  # Check if another cd function is defined
  if (( $+functions[cd_func] )); then
    cd_func "$@"
  else
    # Use builtin cd for zsh, fallback to command cd for other shells
    if [ -n "$ZSH_VERSION" ]; then
      builtin cd "$@"
    else
      command cd "$@"
    fi
  fi

  if [ $? -ne 0 ]; then
    echo "cd: no such file or directory: $*" >&2
    return 1
  fi

  if [ "$PWD" != "$old_dir" ]; then
    # Check if old directory still exists and had bin/rails
    if [ -d "$old_dir" ] && [ -f "$old_dir/bin/rails" ]; then
      remove_from_path "$old_dir/bin"
      echo "Removed $old_dir/bin from PATH"
    fi

    # Check if new directory has bin/rails
    if [ -f "./bin/rails" ] && [ -r "./bin/rails" ]; then
      add_to_path "$PWD/bin"
      echo "Added $PWD/bin to PATH"
    fi
  fi
}

remove_from_path.sh

# NOTE: 
# Make sure you add this function to your zshrc file or link to the file.
# it could look like this line in your `~/.zshrc`
# . PATH_TO_FILE/remove_from_path.sh

#!/bin/sh

# Function to remove a directory from PATH
#
# Usage:
#   remove_from_path "/path/to/remove"
#
# Arguments:
#   $1 - The directory path to remove from PATH
#
# Description:
#   Removes all occurrences of the specified directory from PATH.
#
# Edge cases handled:
#   - Handles directories with spaces in the name
#   - Maintains : at the start/end of PATH if originally present
#   - Handles cases where the directory appears multiple times in PATH
#   - Avoids creating empty entries or consecutive colons
remove_from_path() {
    if [ -z "$1" ]; then
        echo "Usage: remove_from_path <directory>" >&2
        return 1
    fi

    # Store the original first and last characters of PATH
    first_char="${PATH%"${PATH#?}"}"
    last_char="${PATH#"${PATH%?}"}"

    # Remove all occurrences of the directory, handle spaces in path names
    new_path=$(echo "$PATH" | sed -e "s#^$1:##" -e "s#:$1:##g" -e "s#:$1\$##")

    # If PATH was changed, remove any resulting empty entries and duplicate colons
    if [ "$new_path" != "$PATH" ]; then
        new_path=$(echo "$new_path" | sed -e 's#^:##' -e 's#:$##' -e 's#::+#:#g')

        # Restore leading/trailing colon if they were present originally
        [ "$first_char" = ":" ] && new_path=":$new_path"
        [ "$last_char" = ":" ] && new_path="$new_path:"

        PATH="$new_path"
    fi
}

Update for POISX and fixed some edge cases.

handle when a cd function already exists

Split it up into a few files to make it easier to understand.