仅ubuntu上的ffmpeg python子进程错误

Question

ffmpeg -i {video_file} -vf fps={fps}

(standard_in) 1: syntax error
ffmpeg version 3.2.4-static http://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 5.4.1 (Debian 5.4.1-5) 20170205
  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc-5 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gray --enable-libass --enable-libfreetype --enable-libfribidi --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libzimg
  libavutil      55. 34.101 / 55. 34.101
  libavcodec     57. 64.101 / 57. 64.101
  libavformat    57. 56.101 / 57. 56.101
  libavdevice    57.  1.100 / 57.  1.100
  libavfilter     6. 65.100 /  6. 65.100
  libswscale      4.  2.100 /  4.  2.100
  libswresample   2.  3.100 /  2.  3.100
  libpostproc    54.  1.100 / 54.  1.100
Option accurate_seek (enable/disable accurate seeking with -ss) cannot be applied to output url test.mkv -- you are trying to apply an input option to an output file or vice versa. Move this option before the file it belongs to.
Error parsing options for output file test.mkv.
Error opening output files: Invalid argument

import subprocess
command = "for i in {0..3} ; do ffmpeg -accurate_seek -ss `echo $i*60.0 | bc` -i test.mkv -frames:v 1 images/test_img_$i.jpg ; done"
process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr = subprocess.STDOUT, shell=True)
output = process.communicate()
print output[0].replace('\\n' , '\n')

ffmpeg version 3.0.2 Copyright (c) 2000-2016 the FFmpeg developers
  built with Apple LLVM version 7.3.0 (clang-703.0.31)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/3.0.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libxvid --enable-libvpx --enable-vda
  libavutil      55. 17.103 / 55. 17.103
  libavcodec     57. 24.102 / 57. 24.102
  libavformat    57. 25.100 / 57. 25.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 31.100 /  6. 31.100
  libavresample   3.  0.  0 /  3.  0.  0
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
  Input #0, matroska,webm, from 'test.mkv':
  Metadata:
    COMPATIBLE_BRANDS: iso6avc1mp41
    MAJOR_BRAND     : dash
    MINOR_VERSION   : 0
    ENCODER         : Lavf57.25.100
  Duration: 00:02:10.20, start: 0.007000, bitrate: 2729 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Metadata:
      HANDLER_NAME    : VideoHandler
      DURATION        : 00:02:10.172000000
    Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:02:10.201000000
      [swscaler @ 0x7f9b4b008000] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to 'images/test_img_0.jpg':
  Metadata:
    COMPATIBLE_BRANDS: iso6avc1mp41
    MAJOR_BRAND     : dash
    MINOR_VERSION   : 0
    encoder         : Lavf57.25.100
    Stream #0:0: Video: mjpeg, yuvj420p(pc), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 23.98 fps, 23.98 tbn, 23.98 tbc (default)
    Metadata:
      HANDLER_NAME    : VideoHandler
      DURATION        : 00:02:10.172000000
      encoder         : Lavc57.24.102 mjpeg
    Side data:
      unknown side data type 10 (24 bytes)
      Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> mjpeg (native))
  Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=7.2 Lsize=N/A time=00:00:00.04 bitrate=N/A speed=   1x    
video:73kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
....

其他堆栈溢出引用：使用FFMPEG提取帧的最快方法？

Answer 1

问题是范围表示法{0.3}，它在bash中有效，但在其他shell中不有效。虽然我的Ubuntu系统上的正常登录shell是bash，但subprocess.popen()使用shell=true调用/bin/sh，在我的示例中不是bash，而是dash，它不支持大括号展开。

由于范围没有展开，所以循环只运行一次，变量$i包含字符串{0.3}。它传递给bc，导致第一个错误(standard_in)1:syntax error。因此，accurate_seek参数-ss到FFMPEG不会获得任何值。

在我的带有破折号shell的Ubuntu系统中，使用seq的循环工作：

$ sh
$ for i in $(seq 0 3); do echo $i; done    
0
1
2
3

$ sh
$ for i in {0..3}; do echo $i; done
{0..3}