Quick note on how to install ffmpeg (using Homebrew) and various mp4 conversion commands on Unix.

Full ffmpeg documentation available here.

Install ffmpeg for .webm/.mp4 conversion

# Uninstall any existing ffmpeg installation
brew uninstall ffmpeg

# Install (macOS using Homebrew)
brew install ffmpeg --with-fdk-aac --with-sdl2 --with-freetype --with-libass --with-libvorbis --with-libvpx --with-opus --with-x265

# Verify ffmpeg build configuration
ffmpeg -buildconf

# .webm -> .mp4
ffmpeg -i input.webm -codec copy output.mp4

# .mp4 -> .webm
ffmpeg -i input.mp4 -strict -2 output.webm

Convert all files in the same directory to .mp4

# Replace [wildcard] with e.g. *webm
# Replace [options] with your options...
for i in [wildcard]; do ffmpeg -i $i [options] $i.mp4; done

Merge .m4a with .mp4

ffmpeg -i video.mp4 -i audio.m4a -codec copy output.mp4

Extract still image at given time from video

# Enter time in hh:mm:ss
# Choose file format by changing extension of output to e.g. .png
ffmpeg -ss 00:00:46 -i input.mp4 -vframes 1 output.jpg