Re-organize the shells' initialization logic

- unify ~/.config/shell/aliases.sh and ~/.config/shell/utils.sh
  into a common ~/.config/shell/init.sh
- split the contents of ~/.config/shell/aliases.sh
  into the folder ~/.config/shell/aliases.d
- split the contents of ~/.config/shell/utils.sh
  into the folder ~/.config/shell/utils.d

.config/shell/utils.d/python.sh

@@ -0,0 +1,101 @@
+# This file creates a function to install and update the Python develop environments
+
+
+_command_exists() {
+    command -v "$1" 1>/dev/null 2>&1
+}
+
+prepend-to-path () {  # if not already there
+    if [ -d "$1" ] ; then
+        case :$PATH: in
+            *:$1:*) ;;
+            *) PATH=$1:$PATH ;;
+        esac
+    fi
+}
+
+
+
+# The Python versions pyenv creates (in descending order)
+# Important: The first version also holds the "interactive" and "utils" environments
+_py3_versions=('3.10.6' '3.9.13' '3.8.13' '3.7.13')
+_py2_version='2.7.18'
+
+
+# Each Python version receives its own copy of black, pipenv, and poetry
+# (e.g., to avoid possible integration problems between pyenv and poetry
+#  Source: https://github.com/python-poetry/poetry/issues/5252#issuecomment-1055697424)
+_py3_site_packages=('black' 'pipenv' 'poetry')
+
+
+# The pyenv virtualenv "utils" contains some globally available tools (e.g., mackup)
+_py3_utils=('leglight' 'mackup' 'youtube-dl')
+
+
+# Important: this REMOVES the old ~/.pyenv installation
+_install_pyenv() {
+    echo "(Re-)Installing pyenv"
+
+    # Ensure that pyenv is on the $PATH
+    # (otherwise, the pyenv installer emits warnings)
+    mkdir -p "$PYENV_ROOT/bin"
+    prepend-to-path "$PYENV_ROOT/bin"
+
+    # Remove old pyenv for clean install
+    rm -rf "$PYENV_ROOT" >/dev/null
+
+    # Run the official pyenv installer
+    curl https://pyenv.run | bash
+
+    # Make pyenv usable after this installation in the same shell session
+    _init_pyenv  # defined in ~/.config/shell/utils.sh
+}
+
+
+create-or-update-python-envs() {
+    _command_exists pyenv || _install_pyenv
+
+    eval "$(pyenv init --path)"
+
+    # Keep a legacy Python 2.7, just in case
+    echo "Installing/updating Python $_py2_version"
+    pyenv install --skip-existing $_py2_version
+    pyenv rehash  # needed on a first install
+    PYENV_VERSION=$_py2_version pip install --upgrade pip setuptools
+    PYENV_VERSION=$_py2_version python -c "import sys; print sys.version"
+
+    for version in ${_py3_versions[@]}; do
+        echo "Installing/updating Python $version"
+        pyenv install --skip-existing $version
+        pyenv rehash  # needed on a first install
+
+        # Start the new environment with the latest pip and setuptools versions
+        PYENV_VERSION=$version pip install --upgrade pip setuptools
+        PYENV_VERSION=$version python -c "import sys; print(sys.version)"
+
+        # Put the specified utilities in the fresh environments or update them
+        for lib in ${_py3_site_packages[@]}; do
+            PYENV_VERSION=$version pip install --upgrade $lib
+        done
+    done
+
+    # Create a virtualenv based off the latest Python version to host global utilities
+    echo "Installing/updating the global Python utilities"
+    pyenv virtualenv $_py3_versions[1] 'utils'
+    pyenv rehash  # needed on a first install
+    PYENV_VERSION='utils' pip install --upgrade pip setuptools
+    for util in ${_py3_utils[@]}; do
+        PYENV_VERSION='utils' pip install --upgrade $util
+    done
+
+    # Create a virtualenv based off the latest Python version for interactive usage
+    echo "Installing/updating the default/interactive Python environment"
+    pyenv virtualenv $_py3_versions[1] 'interactive'
+    pyenv rehash  # needed on a first install
+    PYENV_VERSION='interactive' pip install --upgrade pip setuptools
+    # Install some tools to make interactive usage nicer
+    PYENV_VERSION='interactive' pip install --upgrade black bpython ipython
+
+    # Put all Python binaries/virtualenvs and the utilities on the $PATH
+    pyenv global 'interactive' $_py3_versions 'utils' $_py2_version
+}