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

.config/shell/README.md

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+# Shell-related Configurations
+
+This folder contains files that are sourced by `bash` and `zsh`.
+
+They are not executable.
+The `#!/bin/sh` line documents the dialect for readers and editors;
+    it has no effect, since the files are sourced and never run directly.
+
+[`aliases`](./aliases),
+[`env`](./env),
+[`logout`](./logout), and
+[`welcome`](./welcome)
+    depend on helper functions defined in [`~/.profile`](../../.profile),
+    so they cannot work standalone.

.config/shell/aliases

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+#!/bin/sh
+
+# Aliases used in all kinds of shells
+
+
+# Manage the bare `git` repository in ~/ holding the dotfiles
+alias dotfiles='git --git-dir=$DOTFILES_DIR --work-tree=$HOME'
+
+
+
+alias path='echo $PATH | tr ":" "\n"'
+
+
+# Easier `cd` movements
+alias ..='cd ..'
+alias ...='cd ../..'
+alias ....='cd ../../..'
+alias .....='cd ../../../..'
+
+
+# Use human-readable sizes
+alias df='df --human-readable'
+alias du='du --human-readable'
+alias free='free --human'
+
+
+# Make the terminal more colorful
+
+alias diff='diff --color=auto'
+alias grep='grep --color=auto --exclude-dir=.git'
+alias ip='ip -color=auto'
+
+if _command_exists dircolors; then
+
+    # Compute `$LS_COLORS` only once because this file is also sourced
+    # by every non-interactive `bash` via `$BASH_ENV` (e.g., ":!cmd" in `vi`)
+    if [ -z "${LS_COLORS:-}" ]; then
+        if [ -r "$XDG_CONFIG_HOME/dircolors" ]; then
+            eval "$(dircolors -b "$XDG_CONFIG_HOME/dircolors")"
+        else
+            eval "$(dircolors -b)"
+        fi
+    fi
+
+    alias ls='ls --color=auto'
+
+fi
+
+
+# Add common `ls` utilities
+alias l='ls --classify --format=vertical'
+alias la='ls --classify --format=vertical --almost-all'
+alias lal='ls --classify --format=long --almost-all'
+alias ll='ls --classify --format=long'
+alias lla='lal'

.config/shell/env

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+#!/bin/sh
+
+# Environment variables for all kinds of shells
+
+
+# Standard XDG base directories
+# See: https://wiki.archlinux.org/title/XDG_Base_Directory
+
+export XDG_CACHE_HOME="$HOME/.cache"
+export XDG_CONFIG_HOME="$HOME/.config"
+export XDG_DATA_HOME="$HOME/.local/share"
+export XDG_STATE_HOME="$HOME/.local/state"
+
+# Make up a XDG directory for binaries (that does not exist in the standard)
+export XDG_BIN_HOME="$HOME/.local/bin"
+
+
+# Convenient names for various places in the system,
+# shown here not only to be used but also document things
+
+export DOTFILES_DIR="$XDG_DATA_HOME/dotfiles"
+export DOTFILES_HTTPS="https://code.webartifex.de/alexander/dotfiles"
+export DOTFILES_SSH="git@git.webartifex.de:alexander/dotfiles.git"
+
+
+# Generic shell configs
+
+
+# Fall back to a widely known terminal type
+# if this host lacks the terminfo entry for `$TERM`
+# (e.g., `alacritty` over `ssh` on a fresh server)
+if _command_exists infocmp && ! infocmp "$TERM" >/dev/null 2>&1; then
+    export TERM=xterm-256color
+fi
+
+
+[ -t 0 ] && export GPG_TTY=$(tty)
+
+
+export EDITOR=nano
+
+
+export PAGER=less
+export LESS="--chop-long-lines --ignore-case --LONG-PROMPT --no-init --quit-if-one-screen --RAW-CONTROL-CHARS --status-column"
+
+
+[ -f "$XDG_CONFIG_HOME/shell/locale" ] && . "$XDG_CONFIG_HOME/shell/locale"
+
+
+# Move common tools' config and cache files into XDG directories
+
+export LESSHISTFILE="$XDG_STATE_HOME/less/history"

.config/shell/locale

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+#!/bin/sh
+
+# Locale settings for headless machines
+
+
+# Drop what a client may forward over `ssh`,
+# because `AcceptEnv LANG LC_*` may be set in /etc/ssh/sshd_config
+unset \
+    LC_ALL \
+    LC_ADDRESS \
+    LC_COLLATE \
+    LC_CTYPE LC_IDENTIFICATION \
+    LC_MEASUREMENT \
+    LC_MESSAGES \
+    LC_MONETARY \
+    LC_NAME \
+    LC_NUMERIC \
+    LC_PAPER \
+    LC_TELEPHONE \
+    LC_TIME
+
+
+# Set every LC_* category implicitly with the `$LANG` fallback
+export LANG=C.UTF-8