Allow overriding of templates; document in usage()

fedi2html

@@ -8,7 +8,8 @@
 # Date: 2024-03
 #―――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――
 
-POST_TEMPLATE='
+if test -z "$POST_TEMPLATE"; then
+	POST_TEMPLATE='
 <article class="comment">
 <a class="user" href="$ACCOUNT_URL">
     <img class="avatar" src="$ACCOUNT_AVATAR">
@@ -29,17 +30,21 @@ POST_TEMPLATE='
 </div>
 </article>
 '
+fi
 
-
-ATTACH_TEMPLATE='
+if test -z "$ATTACH_TEMPLATE"; then
+	ATTACH_TEMPLATE='
 <a href="$ATTACH_URL">
     <div class="attachment">
         <strong>$ATTACH_NAME</strong>
         $ATTACH_DESC
     </div>
 </a>'
+fi
 
-ATTACH_IMAGE_TEMPLATE='
+
+if test -z "$ATTACH_IMAGE_TEMPLATE"; then
+	ATTACH_IMAGE_TEMPLATE='
 <figure>
     <a href="$ATTACH_URL">
        <img class="attachment" src="$ATTACH_URL" alt="$ATTACH_DESC" title="$ATTACH_DESC">
@@ -55,8 +60,12 @@ ATTACH_IMAGE_TEMPLATE='
     </figcaption>
 </figure>
 '
+fi
 
-EMOJI_TEMPLATE='<img class="emoji" src="$EMOJI_URL" alt="$EMOJI_SHORTCODE" title="$EMOJI_SHORTCODE">'
+
+if test -z "$EMOJI_TEMPLATE"; then
+	EMOJI_TEMPLATE='<img class="emoji" src="$EMOJI_URL" alt="$EMOJI_SHORTCODE" title="$EMOJI_SHORTCODE">'
+fi
 
 
 # Given a note’s JSON, render it as HTML.
@@ -345,7 +354,18 @@ usage() {
 	echo "       $(basename "$0") [-h] [-IRc] [-bBt] USER_URL"
 	echo
 	echo "$(basename "$0") does exactly what it says on the tin: It formats"
-	echo 'a fediverse post (and its replies) into simple-and-embeddable HTML.'
+	echo 'a fediverse post (and its replies) into text. By default, into'
+	echo 'simple-and-embeddable HTML.'
+	echo
+	echo '  -c             only print the responses (children) of a post'
+	echo '  -h             print this message and exit'
+	echo '  -I             display posts in reverse-chronological order'
+	echo '  -l             output at maximum the specified amount of posts'
+	echo '  -R             do not recursively display posts’ responses'
+	echo
+	echo '  -b             exclude reblogs/repeats from user feed'
+	echo '  -B             exclude top-level replies from user feed'
+	echo '  -t             filter posts from user feed by tag'
 	echo
 	echo 'It works with posts from any server that supports Mastodon’s API,'
 	echo 'including Pleroma, Akkoma, Glitch, etc.'
@@ -355,15 +375,26 @@ usage() {
 	echo 'if privacy or total archival, is a concern, to use wget(1)’s --mirror'
 	echo '(or something like it) to fetch even these foreign files.'
 	echo
-	echo '  -c             only print the responses (children) of a post'
-	echo '  -h             print this message and exit'
-	echo '  -I             display posts in reverse-chronological order'
-	echo '  -l             output at maximum the specified amount of posts'
-	echo '  -R             do not recursively display posts’ responses'
-    echo
-    echo '  -b             exclude reblogs/repeats from user feed'
-    echo '  -B             exclude top-level replies from user feed'
-    echo '  -t             filter posts from user feed by tag'
+	echo '“Template” environment variables are used to generate the text'
+	echo 'output. A template is a string that contains shell-style variable'
+	echo "names (\$NAME) that $(basename "$0") will replace."
+	echo 'They are $POST_TEMPLATE, $ATTACH_TEMPLATE, and $EMOJI_TEMPLATE.'
+	echo
+	echo 'The variables that can be used in $POST_TEMPLATE are:'
+	echo '  $ACCOUNT_ID, $ACCOUNT_URL, $ACCOUNT_NAME, $ACCOUNT_AVATAR,'
+	echo '  $POST_URL, $POST_DATE, $POST_CONTENT, $POST_ATTACHMENTS,'
+	echo '  and $POST_RESPONSES.'
+	echo
+	echo 'The variables that can be used in $ATTACH_TEMPLATE are:'
+	echo '  $ATTACH_URL, $ATTACH_NAME, $ATTACH_TYPE, $ATTACH_DESC,'
+	echo '  and $ATTACH_PREVIEW.'
+	echo
+	echo 'The variables that can be used in $EMOJI_TEMPLATE are:'
+	echo '  $EMOJI_SHORTCODE and $EMOJI_URL.'
+	echo
+	echo 'See the first few lines of fedi2html for the default (example)'
+	echo 'template values; see the README for a more detailed description'
+	echo 'of these variables’ meanings.'
 }