Add base configuration

- Follow XDG standard: ~/.config and ~/.local folders
- Set environment variables and define aliases within ~/.config/shell
- Add installation script for easy setup and maintenance
  (i.e., `dotfiles-install` and `dotfiles-update`)
- Add README.md with info on the installation and general notes

.local/bin/dotfiles-update

@@ -0,0 +1,116 @@
+#!/bin/sh
+
+# Replace the local `dotfiles` repository with a fresh install
+# by re-running the latest `dotfiles-install` taken from the `origin`
+#
+# Refuses to run if there are uncommitted or unpushed changes,
+# unless called with `--force`
+
+set -eu
+
+
+XDG_DATA_HOME="${XDG_DATA_HOME:-$HOME/.local/share}"
+
+DOTFILES_DIR="${DOTFILES_DIR:-$XDG_DATA_HOME/dotfiles}"
+
+
+
+_force=0
+
+if [ "$#" -gt 1 ]; then
+    echo "Usage: $(basename "$0") [--force]" >&2
+    exit 1
+fi
+
+case "${1:-}" in
+    "")      ;;
+    --force) _force=1 ;;
+    *)
+        echo "Usage: $(basename "$0") [--force]" >&2
+        exit 1
+        ;;
+esac
+
+
+
+if [ ! -d "$DOTFILES_DIR" ]; then
+    echo "" >&2
+    echo "The dotfiles are not installed at: $DOTFILES_DIR" >&2
+    echo "" >&2
+    echo "=> See instructions at https://code.webartifex.de/alexander/dotfiles#installation" >&2
+    echo "" >&2
+
+    exit 1
+fi
+
+
+
+_git() {
+    git --git-dir="$DOTFILES_DIR" --work-tree="$HOME" "$@"
+}
+
+
+
+# Keep the machine on its current branch (e.g., "desktop")
+branch=$(_git symbolic-ref --short HEAD) || {
+    echo "" >&2
+    echo "Detached HEAD => Re-run after 'dotfiles checkout <branch>'" >&2
+    echo "" >&2
+
+    exit 1
+}
+
+
+
+if [ "$_force" -ne 1 ]; then
+
+    if [ -n "$(_git status --porcelain)" ]; then
+        echo "" >&2
+        echo "There are uncommitted changes!" >&2
+        echo "" >&2
+        _git --no-pager diff HEAD --stat >&2
+        echo "" >&2
+        echo "=> Commit and push them, or re-run with '--force'" >&2
+        echo "" >&2
+
+        exit 1
+    fi
+
+    if _git rev-parse --verify --quiet "origin/$branch" >/dev/null \
+        && [ -n "$(_git log --oneline "origin/$branch..$branch")" ]; then
+        echo "" >&2
+        echo "There are unpushed commits!" >&2
+        echo "" >&2
+        _git --no-pager log --oneline "origin/$branch..$branch" >&2
+        echo "" >&2
+        echo "=> Push them, or re-run with '--force'" >&2
+        echo "" >&2
+
+        exit 1
+    fi
+
+fi
+
+
+
+# Take the latest installer out of the local repository,
+# after fetching from the `origin`
+
+_git fetch --prune origin
+
+if ! _git rev-parse --verify --quiet "origin/$branch" >/dev/null; then
+    echo "" >&2
+    echo "Branch '$branch' does not exist on origin!" >&2
+    echo "" >&2
+    echo "=> Push it first, or 'dotfiles checkout' one that does" >&2
+    echo "" >&2
+
+    exit 1
+fi
+
+installer_path=$(mktemp)
+trap 'rm -f "$installer_path"' EXIT
+
+_git show "origin/$branch:.local/bin/dotfiles-install" > "$installer_path"
+
+DOTFILES_BRANCH="$branch" sh "$installer_path" --force