准备

首先介绍需要用到的几个接口：
1.av号获取cid

请求地址:http://www.bilibilijj.com/Api/AvToCid/{Av}/{P}

2.(历史)弹幕接口

http://comment.bilibili.com/rolldate,cid
http://comment.bilibili.com/dmroll,time,cid
http://comment.bilibili.com/cid.xml

然后需要用到的工具
curl iconv sed egrep

最后来理一下流程：
(输入1)网址——(输入2)av号——(输入3)cid——下载弹幕
最终目标：输入网址，av号或者cid号均可实现下载

代码区

是用最简单的bat，统一需要用到的几个变量：

input:%target%
av:%target_av%
cid:%target_cid%

P1

直接输入cid下载xml

set /p target=请粘贴cid号：

curl -k -L -# --compressed -O http://comment.bilibili.com/%target%.xml

P2

输入av号，解析出cid号并下载(可解析多P)，或者输入cid号直接下载

set /p target=请粘贴av/cid号：

echo %target%|findstr "^av" >nul
if %errorlevel%==0 echo %target% | sed "s/av//g" >target_av.temp

for /f %%i in (target_av.temp) do (
curl http://www.jijidown.com/Api/AvToCid/%%i/0 | sed "s/:/\n/g;s/,/\n/g" |egrep -A1 "CID" |egrep "[0-9]" >target_cid.temp
)

for /f %%i in (target_cid.temp) do (
curl -k -L -# --compressed -O http://comment.bilibili.com/%%i.xml
)

P3

接下来试试直接输入网址
https://www.bilibili.com/video/av2716342/

set /p target=请粘贴url/av/cid号：

echo %target%|findstr "https://www.bilibili.com/video/av" >nul
if %errorlevel%==0 echo %target% | sed "s#/#\n#g;s#av##g" | egrep "[0-9]" >target_av.temp
echo %target%|findstr "^av" >nul
if %errorlevel%==0 echo %target% | sed "s/av//g" >target_av.temp

for /f %%i in (target_av.temp) do (
curl http://www.jijidown.com/Api/AvToCid/%%i/0 | sed "s/:/\n/g;s/,/\n/g" |egrep -A1 "CID" |egrep "[0-9]" >target_cid.temp
)

for /f %%j in (target_cid.temp) do (
curl -k -L -# --compressed -O http://comment.bilibili.com/%%j.xml
)

P4

现在哔哩哔哩还有另一种网址
https://www.bilibili.com/bangumi/play/ep118250

set /p target=请粘贴url/av/cid号：

echo %target%|findstr "https://www.bilibili.com/bangumi/play/ep" >nul
if %errorlevel%==0 curl -k -L -o target_utf8.temp %target% && iconv -c -f UTF-8 -t GBK  target_utf8.temp > target_gbk.temp && sed "s#>#\n#g;s#<#\n#g"  target_gbk.temp | egrep "AV" | sed "s#AV##g" >target_av.temp && goto av2cid
echo %target%|findstr "https://www.bilibili.com/video/av" >nul
if %errorlevel%==0 echo %target% | sed "s#/#\n#g;s#av##g" | egrep "[0-9]" >target_av.temp && goto av2cid
echo %target%|findstr "^av" >nul
if %errorlevel%==0 echo %target% | sed "s/av//g" >target_av.temp && goto av2cid

:av2cid
for /f %%j in (target_av.temp) do (
curl http://www.jijidown.com/Api/AvToCid/%%j/0 | sed "s/:/\n/g;s/,/\n/g" |egrep -A1 "CID" |egrep "[0-9]" >target_cid.temp
)

:cid2xml
for /f %%i in (target_cid.temp) do (
curl -k -L -# --compressed -O http://comment.bilibili.com/%%i.xml
)

P5

P4已经可以把所有当前弹幕下载到当前文件夹了，不过还不是很方便，比如分P，还有视频标题，针对这几个问题做点调整

set downloads_folder=%cd%
set /p target=请粘贴url/av/cid号：

echo %target%|findstr "https://www.bilibili.com/bangumi/play/ep" >nul
if %errorlevel%==0 curl -k -L -o target_utf8.temp %target% && iconv -c -f UTF-8 -t GBK  target_utf8.temp > target_gbk.temp && sed "s#>#\n#g;s#<#\n#g"  target_gbk.temp | egrep "AV" | sed "s#AV##g" >target_av.temp && goto av2cid
echo %target%|findstr "https://www.bilibili.com/video/av" >nul
if %errorlevel%==0 echo %target% | sed "s#/#\n#g;s#av##g" | egrep "[0-9]" >target_av.temp && goto av2cid
echo %target%|findstr "^av" >nul
if %errorlevel%==0 echo %target% | sed "s/av//g" >target_av.temp && goto av2cid

:av2cid
for /f %%a in (target_av.temp) do (
curl -o target_utf8.temp http://www.jijidown.com/Api/AvToCid/%%a/0
iconv -c -f UTF-8 -t GBK  target_utf8.temp > target_gbk.temp
sed -i "s#\"#\n#g" target_gbk.temp
sed -i "s/://g;s/,//g" target_gbk.temp
egrep -A1 "CID" target_gbk.temp |egrep "[0-9]" | uniq >target_cid.temp
sed -n "12p" target_gbk.temp |sed "s#\/##g;s#[[:blank:]]#_#g" >target_title.temp
sed -n "56~10p" target_gbk.temp |sed "s#\/##g;s#[[:blank:]]#_#g" >target_ptitle.temp

:cid2xml
set /a n=0 && setlocal enabledelayedexpansion
for /f %%t in (target_title.temp) do (
if not exist %downloads_folder%\%%t_av%%a\ md %downloads_folder%\%%t_av%%a\
for /f %%c in (target_cid.temp) do (
set /a n=!n!+1
for /f "delims=" %%p in ('sed -n "!n!p" target_ptitle.temp') do set target_ptitle=%%p
curl -k -L -# --compressed -o %downloads_folder%\%%t_av%%a\!target_ptitle!_cid%%c.xml http://comment.bilibili.com/%%c.xml
)
)
)
del *.

P6

现在可以增加YouTube-dl 下载视频功能

set downloads_folder=%cd%
set /p target=请粘贴url/av/cid号：

echo %target%|findstr "https://www.bilibili.com/bangumi/play/ep" >nul
if %errorlevel%==0 curl -k -L -o target_utf8.temp %target% && iconv -c -f UTF-8 -t GBK  target_utf8.temp > target_gbk.temp && sed "s#>#\n#g;s#<#\n#g"  target_gbk.temp | egrep "AV" | sed "s#AV##g" >target_av.temp && goto av2cid
echo %target%|findstr "https://www.bilibili.com/video/av" >nul
if %errorlevel%==0 echo %target% | sed "s#/#\n#g;s#av##g" | egrep "[0-9]" >target_av.temp && goto av2cid
echo %target%|findstr "^av" >nul
if %errorlevel%==0 echo %target% | sed "s/av//g" >target_av.temp && goto av2cid

:av2cid
for /f %%a in (target_av.temp) do (
curl -o target_utf8.temp http://www.jijidown.com/Api/AvToCid/%%a/0
iconv -c -f UTF-8 -t GBK  target_utf8.temp > target_gbk.temp
sed -i "s#\"#\n#g" target_gbk.temp
sed -i "s/://g;s/,//g" target_gbk.temp
egrep -A1 "CID" target_gbk.temp |egrep "[0-9]" | uniq >target_cid.temp
sed -n "12p" target_gbk.temp |sed "s#\/##g;s#[[:blank:]]#_#g" >target_title.temp
sed -n "56~10p" target_gbk.temp |sed "s#\/##g;s#[[:blank:]]#_#g" >target_ptitle.temp

:cid2xml
set /a n=0 && setlocal enabledelayedexpansion
for /f %%t in (target_title.temp) do (
if not exist %downloads_folder%\%%t_av%%a\ md %downloads_folder%\%%t_av%%a\
for /f %%c in (target_cid.temp) do (
set /a n=!n!+1
for /f "delims=" %%p in ('sed -n "!n!p" target_ptitle.temp') do set target_ptitle=%%p
curl -k -L -# --compressed -o %downloads_folder%\%%t_av%%a\!target_ptitle!_cid%%c.xml http://comment.bilibili.com/%%c.xml

:av2video
echo https://www.bilibili.com/video/av%%a/#page=!n! >>target_video.temp
)
youtube-dl -a target_video.temp --external-downloader aria2c --external-downloader-args " --split=16 --min-split-size=3M --max-connection-per-server=16 "
)
)
del *.
del *.temp

P7

不过一般不需要下载视频，倒是历史弹幕可以下载试试