#!/bin/sh

injection="$1"
tmp="$(mktemp)"

awk '
BEGIN {
    s="<!-- start injection -->"
    e="<!-- end injection -->"
}

FILENAME != "index.html" {
    d=d $0 "\n"
    next
}
$0 == s || $0 == e { print }
$0 == s,$0 == e {
    if(d) {
        printf d;
        d = ""
    }
    next
}
{ print }
' "$injection" index.html > "$tmp" \
    && mv "$tmp" index.html