我使用follow命令添加水印图像并转换视频文件，然后检查输出文本文件。
avconv -i input.mp4 -vf "movie=watermark.png [watermark]; [in][watermark] overlay=10:10 [out]" -acodec copy -y output.swf 1> proccess_status.txt 2>&1
这是proccess_status.txt的一部分：
avconv version 11.9-6:11.9-1~deb8u1, Copyright (c) 2000-2017 the Libav developers
built on Apr 23 2017 21:47:31 with gcc 4.9.2 (Debian 4.9.2-10)
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/srv/site.com/public/htdocs/videos/8_e7d16e42ec66dc66fa.mp4':
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: mp41isom
...
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Stream #0:1 -> #0:1 (copy)
Press ctrl-c to stop encoding
frame= 46 fps= 0 q=0.0 size= 0kB time=1.58 bitrate= 0.2kbits/s
frame= 57 fps= 53 q=29.0 size= 27kB time=0.13 bitrate=1633.1kbits/s
frame= 69 fps= 44 q=29.0 size= 72kB time=0.53 bitrate=1108.8kbits/s
frame= 82 fps= 39 q=29.0 size= 110kB time=0.97 bitrate= 931.8kbits/s
...
[libx264 @ 0x15475e0] Weighted P-Frames: Y:10.1% UV:8.0%
[libx264 @ 0x15475e0] ref P L0: 72.3% 9.6% 13.3% 4.7% 0.0%
[libx264 @ 0x15475e0] ref B L0: 83.1% 14.7% 2.1%
[libx264 @ 0x15475e0] ref B L1: 93.3% 6.7%
[libx264 @ 0x15475e0] kb/s:861.12
我想知道如何在输出文本文件的开头添加其他文本，还是可以将ffmpeg命令行的输出附加到现有文件？