bat for 迴圈 if set 無效

AudioToAAC.bat
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for /r "%dir%" %%i in (*.mkv) do (
ffmpeg -i "%%~i" -y -vn -async 1 -ac 2 -f wav - | qaac --tvbr 100 --quality 2 --rate keep --ignorelength -o "%outdir%\%%~ni.m4a" -
ffmpeg -i "%%~i" 2>map.txt
set sub=
for /f "delims=" %%a in ('type map.txt^|findstr /i "Stream"') do (
for /f "tokens=3 delims= " %%b in ("%%a") do (
if "%%~b"=="Subtitle:" set sub=-map 0:s -c:s copy
)
)
ffmpeg -i "%%~i" -i "%outdir%\%%~ni.m4a" -map 0:v -c:v copy -map 1:a -c:a copy %sub% -y "%outdir%\%%~ni.mkv"
)
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map.txt
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Input #0, matroska,webm, from 'D:\Samples\Sample.H264.720p.DTS.TEXT.ASS.mkv':
Metadata:
creation_time : 2014-01-01 13:43:52
Duration: 00:01:00.85, start: 0.043000, bitrate: 3774 kb/s
Stream #0:0(eng): Video: h264 (High), yuv420p, 1280x720, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
Stream #0:1(jpn): Audio: dts (DTS), 48000 Hz, 5.1(side), fltp, 1536 kb/s (default)
Metadata:
title : Japanese DTS 5.1-Ch
LANGUAGE : jpn
Stream #0:2(chi): Subtitle: subrip (default)
Metadata:
title : Traditional Chinese TEXT/UTF8
Stream #0:3(chi): Subtitle: ssa
Metadata:
title : Traditional Chinese SSA/ASS
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為什麼 Subtitle Stream 存在，但 變數 sub 還是空值
請各位前輩給指教