diff --git a/agenda b/agenda
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+++ b/agenda
@@ -0,0 +1,349 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# The home of all files read and written by agenda.
+#
+# If this is left empty, this will default to the xdg user DOCUMENTS directory (in a
+# child agenda directory), if `xdg-user-dir` is in PATH. Otherwise it will default
+# to `~/.agenda`.
+#
+# ```
+# $AGENDA_DIR
+# ├── {agenda-name}
+# │   ├── 0000-{backlog-name}.md
+# │   ├── 1970-01-01.md
+# │   ├── 1970-01-02.md
+# │   └── 1970-01-03.md
+# └── {agenda-name}
+#     ├── 1970-01-01.md
+#     ├── 1970-01-02.md
+#     └── 1970-01-03.md
+# ```
+#
+AGENDA_DIR=${AGENDA_DIR:-}
+
+# The default `-e` option.
+AGENDA_EXT=${AGENDA_EXT:-"md"}
+
+# The default name if the `-t` option is omitted.
+AGENDA_DEFAULT=${AGENDA_DEFAULT:-"agenda"}
+
+# A template that is automatically written to any created agenda file (excluding
+# backlogs). A few magic strings are replaced:
+#
+# - `{date}` The agenda date (`%Y-%m-%d`)
+# - `{name}` The agenda name
+# - `{slug}` The agenda name (slugged)
+AGENDA_TMPL="${AGENDA_TMPL:-"# {name} {date}\\n\\n- [ ] "}"
+
+__usage() {
+	fold -s <<-USAGE
+	Usage: ${0##*/} [-bcE] [-e <extension>] [-t <name>] [<date>]
+	       ${0##*/} <subcommand>
+
+	Organize and edit your agenda in any plain text format.
+
+	  -b             Backlog (use the date argument as backlog name)
+	  -c             Create the file if it does not exist
+	  -E             Instead of editing the file, print the path to STDOUT
+	  -e <extension> Specify the file extension
+	  -t <name>      Specify which agenda to edit
+
+	Subcommands:
+	  edit           The default, if subcommand is omitted.
+	  list           List a range of agenda files.
+	USAGE
+}
+
+__version() {
+	echo "${0##*/} version 0.0.1"
+}
+
+__main() {
+	case $* in
+		"--help") __usage && return ;;
+		"--version") __version && return ;;
+	esac
+
+	if [ -z "$AGENDA_DIR" ]; then
+		if type xdg-user-dir >/dev/null 2>&1; then
+			AGENDA_DIR="$(xdg-user-dir DOCUMENTS)/agenda"
+		else
+			AGENDA_DIR="$HOME/.agenda"
+		fi
+	fi
+
+	# Subcommand
+	case ${1:-} in
+		"list"|"ls") __list "${@:2}" ;;
+		"edit") __edit "${@:2}" ;;
+		*) __edit "$@" ;;
+	esac
+}
+
+__edit_usage() {
+	fold -s <<-USAGE
+	Usage: ${0##*/} edit [-bcE] [-e <extension>] [-t <name>] [<date>]
+
+	Edit an agenda file.
+
+	  -b             Backlog (use the date argument as backlog name)
+	  -c             Create the file if it does not exist
+	  -E             Instead of editing the file, print the path to STDOUT
+	  -e <extension> Specify the file extension
+	  -t <name>      Specify which agenda to edit
+	USAGE
+}
+
+__edit() {
+	case $* in
+		"--help") __edit_usage && return ;;
+	esac
+
+	local opt_b opt_c opt_E opt_e opt_t
+	opt_b="no"
+	opt_c="no"
+	opt_E="no"
+	opt_e="$AGENDA_EXT"
+	opt_t="$AGENDA_DEFAULT"
+	while getopts 'bcEe:t:' opt; do
+		case $opt in
+			b)
+				opt_b="yes"
+				;;
+			c)
+				opt_c="yes"
+				;;
+			E)
+				opt_E="yes"
+				;;
+			e)
+				opt_e=$OPTARG
+				;;
+			t)
+				opt_t=$OPTARG
+				;;
+			*)
+				__edit_usage && return 1
+				;;
+		esac
+	done
+	shift $(( OPTIND - 1 ))
+
+	if [ $# -eq 0 ]; then
+		if [ "$opt_b" = "yes" ]; then
+			set -- "backlog"
+		else
+			set -- "today"
+		fi
+	fi
+
+	local base ext slug file
+	if [ "$opt_b" = "yes" ]; then
+		base="0000-$(___slugify "$*")"
+	else
+		base=$(date -d "$*" +%F)
+	fi
+	ext=$opt_e
+	slug=$(___slugify "$opt_t")
+	file="$AGENDA_DIR/$slug/$base.$ext"
+
+	# Existing files may overwrite the `-e` option.
+	local exists
+	for exists in "${file%."$ext"}".*; do
+		if [ -e "$exists" ] && [ "$file" != "$exists" ]; then
+			echo "${0##*/}: ignoring '$ext' extension: '$exists' already exists" >&2
+			ext="${exists##*.}"
+			file="$exists"
+		fi
+	done
+
+	if ! [ -f "$file" ]; then
+		if [ "$opt_c" = "yes" ]; then
+			__agenda_create "$file" "$opt_b" "$opt_t" "$slug" "$base"
+		else
+			echo "${0##*/}: cannot access '$file': No such file" >&2
+			return 2
+		fi
+	fi
+
+	if [ "$opt_E" = "no" ]; then
+		export AGENDA_FILE="$file"
+		exec ${VISUAL:-${EDITOR:-vi}} "$file"
+	else
+		echo "$file"
+	fi
+}
+
+__list_usage() {
+	fold -s <<-USAGE
+	Usage: ${0##*/} list [-ac] [-u <date>] [-t <name>] [<date>]
+
+	List a range of agenda files.
+
+	  -a        Print the files' absolute path
+	  -c        Print the files' content
+	  -u <date> Stop listing at <date>
+	  -t <name> Specify which agenda to list
+	USAGE
+}
+
+__list() {
+	case $* in
+		"--help") __list_usage && return ;;
+	esac
+
+	local opt opt_a opt_c opt_u opt_t
+	opt_a="no"
+	opt_c="no"
+	opt_u="next week" # FIXME: Not portable.
+	opt_t=""
+	while getopts 'abcu:t:' opt; do
+		case $opt in
+			a)
+				opt_a="yes"
+				;;
+			c)
+				opt_c="yes"
+				;;
+			u)
+				opt_u="$OPTARG"
+				;;
+			t)
+				opt_t="$OPTARG"
+				;;
+			*)
+				list_usage
+				return 1
+				;;
+		esac
+	done
+	shift $(( OPTIND - 1 ))
+
+	# TODO: Check if stdout is a file that is a terminal or a pipe that is
+	#       connected to a terminal.
+	local padlength
+	padlength=80
+
+	if [ $# -eq 0 ]; then
+		# FIXME: Not portable.
+		set -- "last week"
+	fi
+
+	local date until dir
+	date=$(date -d "$*" +%F)
+	until=$(date -d "$opt_u" +%F)
+	if [ -n "$opt_t" ]; then
+		slug=$(slugify "$opt_t")
+		dir="$AGENDA_DIR/$slug"
+	else
+		dir="$AGENDA_DIR"
+	fi
+
+	if ! [ -d "$dir" ]; then
+		echo "${0##*/}: cannot access '$dir': Directory does not exist" >&2
+		return 1
+	fi
+
+	local date_cmd until_cmd
+	date_cmp="${date%%-*}${date#*-}"
+	date_cmp="${date_cmp%%-*}${date_cmp#*-}"
+	until_cmp="${until%%-*}${until#*-}"
+	until_cmp="${until_cmp%%-*}${until_cmp#*-}"
+
+	while read -r REPLY; do
+		if ! [ -f "$REPLY" ]; then
+			echo "${0##*/}: cannot access '$REPLY': File does not exist" >&2
+		fi
+
+		local file_date
+		file_date="${REPLY##*/}"
+		file_date="${file_date%.*}"
+
+		local file_date_cmp
+		file_date_cmp="${file_date%%-*}${file_date#*-}"
+		file_date_cmp="${file_date_cmp%%-*}${file_date_cmp#*-}"
+
+		# Agenda may allow special files that are not named by date in the
+		# future.
+		# https://stackoverflow.com/a/18620446
+		case $file_date_cmp in
+			*[!0-9]* | '') continue ;;
+			*) ;;
+		esac
+
+		# This was originally written in POSIX shell. Let's keep it that way for
+		# now.
+		# https://unix.stackexchange.com/a/84472
+		if [ "$file_date_cmp" -ge "$date_cmp" ] && [ "$file_date_cmp" -le "$until_cmp" ]; then
+			local file_name
+			file_name="$REPLY"
+			if [ "$opt_a" = "no" ]; then
+				file_name="${file_name#"$AGENDA_DIR/"}"
+				file_name="${file_name%.*}"
+			fi
+
+			if [ "$opt_c" = "yes" ]; then
+				printf "%*s\n" "$padlength" "[$file_name]" | tr " " "-"
+				cat "$REPLY"
+				echo
+			else
+				echo "$file_name"
+			fi
+		fi
+	done < <(
+		# Sort the paths by basename.
+		find "$dir" -type f \
+			| __agenda_sort_prefix \
+			| sort \
+			| __agenda_strip_sort_prefix
+	)
+}
+
+__agenda_create() {
+	local file=$1 backlog=$2 name=$3 slug=$4 date=$5
+
+	if [ -e "$file" ]; then
+		echo "${0##*/}: cannot create '$file': File already exists" >&2
+		return 1
+	fi
+
+	local content
+	if [ "$backlog" = "no" ]; then
+		content=$(echo "$AGENDA_TMPL" \
+			| sed "s/{date}/$date/g" \
+			| sed "s/{name}/$name/g" \
+			| sed "s/{slug}/$slug/g")
+	fi
+
+	mkdir -p "${file%/*}"
+
+	# shellcheck disable=SC2059
+	printf "${content:-}" > "$file"
+}
+
+# https://stackoverflow.com/a/49035906
+___slugify() {
+    echo "$1" \
+        | iconv -c -t ascii//TRANSLIT \
+        | sed -E "s/[~^]+//g" \
+        | sed -E "s/[^a-zA-Z0-9]+/-/g" \
+        | sed -E "s/^-+|-+$//g" \
+        | tr "[:upper:]" "[:lower:]"
+}
+
+__agenda_sort_prefix() {
+	while read -r REPLY; do
+		echo "${REPLY##*/}---$REPLY"
+	done
+}
+
+__agenda_strip_sort_prefix() {
+	while read -r REPLY; do
+		echo "${REPLY#*---}"
+	done
+}
+
+__main "$@"
+
+# vi: noet sts=4 sw=4 tw=84