Bash script to convert Hugo markdown files to smartquotes, since Goldmark sucks at it

smartdown.sh

#!/usr/bin/env bash
#
# add smart quotes to Hugo Markdown source files, using the 
# reference implementation of CommonMark's CLI tool:
#     https://github.com/commonmark/commonmark-spec
# Notes:
#   - assumes TOML front matter
#   - converts footnote-style links to inline
#   - normalizes ordered/unordered list formatting
#
# WARNING: possible site-breaking changes:
#   ! rarely, cmark breaks *italic* and **bold** by backslashing
#     the asterisks
#   ! breaks description/definition-list formatting by reflowing it
#   ! cmark adds gratuitous backslashes before "[]*&!#_<>+"; stripping
#     them back out can break escapes in front matter
#   - adds blank line before shortcode that starts a line
#   - adds blank line after shortcode that ends a line
#   - converts &nbsp;, &rsquo;, &#8203;, etc into Unicode literals
#   - probably won't handle a "+++" line in body content

CMARK="cmark --to commonmark --width 70 --smart --unsafe"

for file in "$@"; do
    cat "$file" |
    # convert front matter to HTML comment, so it all gets ignored
    sed -e '1 s/^\+\+\+$/<!-- _FMPLUS_/' \
        -e 's/^\+\+\+$/_FMPLUS_ -->/' |
    # convert shortcodes to HTML comments, to keep it from
    # escaping their arguments
    sed -e 's/{{</<!-- _SC1OPEN_/g' \
        -e 's/>}}/_SC1CLOSE -->/g' \
        -e 's/{{%/<!-- _SC2OPEN_/g' \
        -e 's/%}}/_SC2CLOSE -->/g' |
    # pass through commonmark
    $CMARK |
    # restore shortcodes
    sed -e 's/<!-- _SC1OPEN_/{{</g' \
        -e 's/_SC1CLOSE -->/>}}/g' \
        -e 's/<!-- _SC2OPEN_/{{%/g' \
        -e 's/_SC2CLOSE -->/%}}/g' |
	# strip out mostly-gratuitous backslashes
	sed -e 's/\\\([][*&!#_<>+]\)/\1/g' |
    # restore front matter
    sed -e 's/^.*_FMPLUS_.*$/+++/' > "$file.new"
    # overwrite original (you have source control, right?)
    mv "$file.new" "$file"
done
exit 0