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+#!/bin/sh
+# generate a Atom feed from a directory, recursively
+# using POSIX sh
+# AUTHOR: Case Duckworth <acdw@acdw.net>
+# LICENSE: MIT
+
+usage() {
+    cat <<END
+$0: generate an Atom feed from directories of files
+INVOCATION:
+        $0 [-h] [-c CONFIG] DIRECTORY...
+
+OPTIONS:
+        -h                      show this help
+        -c CONFIG       change the CONFIG file.
+                                Default: $PWD/$0.conf.sh
+END
+}
+
+# FEED VARIABLES
+FEED_TITLE=example
+FEED_SUBTITLE="an example"
+FEED_URL="https://example.com/atom.xml"
+SITE_URL="https://example.com"
+FEED_ID="${SITE_URL#*//}"
+FEED_AUTHOR="nobody"
+FEED_COPYRIGHT="(c) 2020 CC-BY-SA $FEED_AUTHOR"
+FEED_UPDATED=$(date -u +'%FT%TZ')
+
+# FUNCTIONS
+
+recent_files() { # recent_files DIRECTORY FIND_ARG...
+    # list files, in order of recency
+    # BOO: REQUIRES GNU FIND :( :( :( :(
+    # possibly look into https://unix.stackexchange.com/questions/9247/
+    # for more possibilities if you don't have GNU find.
+    dir="$1"
+    shift
+
+    find "$dir" -maxdepth 1 \
+        "$@" \
+        -not -name '.*' \
+        -printf '%T@\t%p\n' |
+        sort -nr |
+        cut -f2
+}
+
+# ENTRY FUNCTIONS
+
+skip_entry() {
+    return 1
+}
+
+entry_url() {
+    basename="${1#$DIRECTORY}"
+    echo "$FEED_URL/$basename"
+}
+
+entry_title() {
+    awk '/^#+[  ]\S/{
+        for(i=2;i<=NF;i++) {
+                printf $i;
+                if (i!=NF) printf " ";
+        }
+        printf "\n";exit}' "$1"
+}
+
+entry_summary() {
+    return 1
+}
+
+entry_author() {
+    echo "$FEED_AUTHOR"
+}
+
+entry_content() {
+    cat "$1"
+}
+
+entry_updated() {
+    # requires stat(1).
+    # probably another way.
+    # possibly using ls(1).
+    stat -c '%y' "$1" |
+        awk '{
+                sub(/\..*/,"",$2);
+                sub(/[0-9][0-9]/,"&:",$3);
+                print $1"T"$2$3;}'
+}
+
+# ATOM FUNCTIONS
+
+atom_header() {
+    cat <<END
+<?xml version="1.0" encoding="utf-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom">
+<title>$FEED_TITLE</title>
+<subtitle>$FEED_SUBTITLE</subtitle>
+<link href="$FEED_URL" rel="self" />
+<link href="$SITE_URL" />
+<id>$FEED_ID</id>
+<generator uri="https://git.sr.ht/~acdw/shatom" version="infinite">shatom</generator>
+<rights>$FEED_COPYRIGHT</rights>
+<updated>$FEED_UPDATED</updated>
+END
+}
+
+atom_footer() {
+    cat <<END
+</feed>
+END
+}
+
+atom_entry() { # atom_entry FILE
+    ENTRY_URL="$(entry_url "$1")"
+    ENTRY_TITLE="$(entry_title "$1")"
+    ENTRY_SUMMARY="$(entry_summary "$1")"
+    ENTRY_AUTHOR="$(entry_author "$1")"
+    ENTRY_CONTENT="$(entry_content "$1")"
+    ENTRY_UPDATED="$(entry_updated "$1")"
+    cat <<END
+<entry>
+<id>$ENTRY_URL</id>
+<link rel="alternate" href="$ENTRY_URL" />
+<title>$ENTRY_TITLE</title>
+<summary>$ENTRY_SUMMARY</summary>
+<updated>$ENTRY_UPDATED</updated>
+<author><name>$ENTRY_AUTHOR</name></author>
+<content type="text"><![CDATA[$ENTRY_CONTENT]]></content>
+</entry>
+END
+}
+
+main() {
+    CONFIGFILE="$PWD/$0.conf.sh"
+    case "$1" in
+        -h)
+            usage
+            exit 0
+            ;;
+        -c)
+            CONFIGFILE="$2"
+            shift 2
+            ;;
+    esac
+
+    if [ -f "CONFIGFILE" ]; then
+        . "$CONFIGFILE"
+    fi
+
+    atom_header
+    for DIR; do
+        for entry in $(recent_files "$DIR" -type f); do
+            if skip_entry "$entry"; then continue; fi
+
+            atom_entry "$entry"
+        done
+    done
+    atom_footer
+}
+
+if [ $DEBUG ]; then set -x; fi
+
+main "$@"