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Using FFmpeg to Get an MP3's Duration and 4 Ways to Get the File Size


Here's a Shell script I created to automate a task that used to take a bunch of steps to do manually.

Quick Jump: Demo Video

After 80+ episodes of my Running in Production podcast I finally automated the process of getting an MP3’s duration and file size in bytes so I can add it to the meta data of each episode (the site is built with Jekyll and is open source).

This video goes over what I used to do manually and how I automated it with a Shell script, FFmpeg and a few Unix tools. The takeaway here is you might be able to extract some of the patterns into your own scripts. For example, we’ll go over a while loop that removes leading zeros and colons from a string using a bit of Shell scripting and more.

Demo Video


Here’s 4 ways to get a file’s size in bytes (replace $file with your file):

# POSIX compliant.
wc -c < "${file}"

# Likely only works on Linux but is the fastest.
stat c %s "${file}"

# Likely only works on Linux.
du -sb "${file}"

# Hey it's possible!
python3 -c "import os; print(os.path.getsize('$file'))"

Here’s 2 ways to get the duration of an MP3 using ffmpeg.

You’ll need to install ffmpeg with sudo apt-get install ffmpeg, brew install ffmpeg or use whatever package manager your OS comes with.

# This is fast but it's only an estimate based on the bitrate of the file.
ffmpeg -i "${file}" 2>&1 | grep -oE "[0-9]{1}:[0-9]{2}:[0-9]{2}"

# More accurate but much slower. The first method hasn't failed me yet, it's
# usually only a difference of a few milliseconds.
ffmpeg -i "${file}" -f null /dev/null 2>&1 | grep -oE "[0-9]{1}:[0-9]{2}:[0-9]{2}" | tail -n 1

Recently I found you can do this with ffprobe too which comes with ffmpeg. Here’s that post.


  • 0:14 – Focusing on the why and then the how
  • 0:41 – Using this script to get meta data about each podcast episode
  • 2:01 – The manual process before I made this script
  • 3:45 – The script fixed a bunch of human errors I made in the past
  • 4:19 – Running the script to see how it works
  • 5:43 – Skimming the script
  • 6:27 – Set -e and preparing the file path to modify
  • 7:55 – Handling no episode or invalid episode names
  • 9:35 – Using find, sort and tail to get the latest episode
  • 12:51 – 4 different ways to get the file size in bytes on Linux and Unix
  • 17:40 – Using FFmpeg and a regular expression to get the duration of an MP3
  • 22:00 – Using a Bash while loop to strip leading 0s and leading colons
  • 25:58 – Figuring out if an episode is published or a draft
  • 30:43 – Outputting the details if we’re doing a dry run
  • 31:54 – Using Perl to do in place edits on the file in a cross platform way
  • 33:23 – A Bash 1 liner to run the script on every episode
  • 34:53 – Getting a more accurate duration with FFmpeg

What are you going to do with this script? Let me know below.

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