Converting MP4 To Webm Using FFMPEG and Bash

We just recently added [video headers]({% post_url 2016-11-10-adding-video-backgrounds-with-jekyll-front-matter %}) to our blog because videos are awesome. We wrote a bash script to convert MP4 videos to Webm videos to make the experience of adding a new video header easy for our writers. We are also taking the first frame of the video for a poster image. A pre-requisite is that you will need ffmpeg installed. For macOS users, you can use the following Homebrew command.

> brew install ffmpeg --with-libvpx --with-libvorbis --with-fdk-aacc

Note the dependencies required to convert a video to Webm. Now for the fun part, the bash script.

#!/bin/bash
# macOS Users : Install ffmpeg via homebrew
#  > brew install ffmpeg --with-libvpx --with-libvorbis --with-fdk-aacc

# stop script on errors
set -e

function convert_mp4_to_webm
{
    if [ ! -f "${1%%.mp4}.webm" ]; then
       echo "Converting $1"
       ffmpeg -y -i "$1" -vframes 1 -f image2 "${1%%.mp4}.jpg"; i=$((i+1))
       ffmpeg -y -i "$1" -c:v libvpx -crf 30 -b:v 1M -an "${1%%.mp4}.webm"       
    else
        echo "Skipping $1"
    fi
}

VIDEOS=./video
cd "$VIDEOS"
for item in *.mp4; do
    convert_mp4_to_webm "$item"
done

exit

This script will loop through a directory and only convert videos to webm that have not already been converted previously.

There are a few things this script is doing that you should know:

• Audio is removed from the Webm version (not needed for header)
• The poster image is a jpg
• Conversion speeds depend on your GPU / CPU speed

I would like to thank Thomas Harold for lending me his bash expertise.