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., `install-dotfiles` and `update-dotfiles`,
   and a `ensure-locales` helper)
- Add README.md with info on the installation and general notes

.config/shell/README.md

@@ -0,0 +1,11 @@
+# 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.
+
+[`welcome`](./welcome) and [`logout`](./logout)
+    depend on helper functions defined in [`~/.profile`](../../.profile),
+    so they cannot work standalone.

.config/shell/aliases

@@ -0,0 +1,10 @@
+#!/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 grep='grep --exclude-dir=.git'

.config/shell/env

@@ -0,0 +1,38 @@
+#!/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="$HOME/.local/share"  # also set in ~/.local/bin/dotfiles-{install,update}
+export XDG_STATE_HOME="$HOME/.local/state"
+
+# Make up a XDG directory for binaries (that does not exist in the standard)
+export XDG_BIN_HOME="$HOME/.local/bin"
+
+
+# Convenient names for various places in the system
+
+export DOTFILES_DIR="$XDG_DATA_HOME/dotfiles"  # also set in ~/.local/bin/dotfiles-{install,update}
+
+
+# Generic shell configs
+
+
+[ -t 0 ] && export GPG_TTY=$(tty)
+
+
+export PAGER=less
+export LESS="--chop-long-lines --ignore-case --LONG-PROMPT --no-init --raw-control-chars --status-column --quit-if-one-screen"
+
+
+[ -f "$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

@@ -0,0 +1,24 @@
+#!/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