利用h264_nvenc编码器再编码视频，有时可起到压缩的作用

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#!/bin/bash

# 此脚本暂没一般适用化，只做个记录

set -e

base_dir="$(dirname "$0")"
cd "$base_dir" || exit

list_file="$base_dir/list"

temp_file="$(mktemp --suffix=.mp4)"

function get_video_bitrate() {
    ffprobe -v error -select_streams v -show_entries stream=bit_rate \
        -of default=noprint_wrappers=1:nokey=1 \
        "$1"
}

# if file "./ok" exists, delete it
if [ -f "./ok" ]; then
    rm ok
fi
# same to "./err"
if [ -f "./err" ]; then
    rm err
fi
# same to "./ignored"
if [ -f "./ignored" ]; then
    rm ignored
fi

while read -r file; do
    if ffmpeg -y -nostdin -i "$file" -c:a copy -c:v h264_nvenc -b:v 0 "$temp_file"; then
        output_bitrate="$(get_video_bitrate "$temp_file")"
        input_bitrate="$(get_video_bitrate "$file")"

        if [[ output_bitrate -lt input_bitrate ]]; then
            mv -f "$temp_file" "$file"
            echo "$file" >>ok
        else
            echo "$file" >>ignored
        fi

    else
        echo "$file" >>err
    fi

done <"$list_file"

可以改编码器，比如改成hevc_nvenc，h265更省空间！