How to extract audio from video with ffmpeg, into separate files

extract-audio.bat

REM This script will check all MP4 files in the current folder
REM and process them to extract audio into separate files (M4A).
REM The audio files use the same file name as the original video file, with a number, and the M4A extension
REM This is a batch file, to be run on Windows, in Command Line. 
REM For Linux, see the "extract-audio.sh" gist below
REM In this example, there are 2 audio tracks, which get split out to 2 separate audio files.
REM The original video file remains unchanged.
REM If you want have a video file with more than 2 audio tracks, 
REM just add more "-map 0:a:0 -c copy "%~na audio track 1.m4a"" sections:
REM -map 0:a:0 -c copy "%~na audio track 1.m4a" 
REM -map 0:a:1 -c copy "%~na audio track 2.m4a" 
REM -map 0:a:2 -c copy "%~na audio track 3.m4a" 
REM -map 0:a:3 -c copy "%~na audio track 4.m4a" 
REM etc ...

REM TO RUN THIS SCRIPT:
REM You MUST HAVE ffmpeg INSTALLED!
REM How to install ffmpeg is beyond the scope of this script. Google it.
REM Either copy all this text into a text file and save it under the name "extract-audio.bat", on your Desktop for example.
REM Or, download this gist and save it under the name "extract-audio.bat" on your Desktop, for example.
REM If you save this file into your C:\Windows folder, 
REM you will be able to run it just by using the script name, "extract-audio.bat"
REM Otherwise, you will have to provide the full path (or location) of the file when you run it: 
REM "C:\Users\Username\Desktop\extract-audio.bat"
REM To use the script, open a Command Prompt window, navigate to the folder that has the MP4 files you want to process, and run it:
REM "cd C:\Users\Username\Videos"
REM "C:\Users\Username\Desktop\extract-audio.bat"
REM
REM If you just want to run the command directly, and not the batch file, 
REM please REMEMBER to change the variable name from %%a to a single %a, in all instances where the variable is used!

for /f "delims=*" %%a in ('dir /b .\*.mp4'); do ffmpeg -hide_banner -n -threads 8 -i "%%a" -vn -map 0:a:0 -c copy "%%~na audio track 1.m4a" -map 0:a:1? -c copy "%%~na audio track 2.m4a"

REM for /f %%a in = this is a the loop section that will look for files to process, 
REM %%a = this is the variable that will be subsctituted with the actual file name when the for loop executes
REM ('dir .\*.mp4') = this command looks for MP4 files in the current folder
REM do ffmpeg = this part actually executes ffmpeg and tells it to extract the audio fro the MP4 files it finds.
REM -hide_banner = hides the info banner when the command is executed.
REM -n = option to not overwrite existing files
REM -threads 8 = option to use 8 separate CPU threads for the work. You can increase the number depending on your CPU.
REM -i "%%a" = this option defines the input file
REM -vn = this option means no video to be copied
REM -map 0:a:0 = this options maps the first available audio track to be extracted
REM -map 0:a:1 = this options maps the second available audio track to be extracted
REM  ? = this option will avoid an error if there are no multiple audio tracks in a file.
REM -c copy = this option means please copy the audio without altering it
REM %~na = this expression is used to get file name only, no extension
REM "%~na audio track 1.m4a" = the filename of the extracted audio track

extract-audio.sh

# This script will check all MP4 files in the current folder,
# and process them to extract audio into separate files (M4A).
# The audio files use the same file name as the original video file, with a number, and the M4A extension
# This is a bash file, to be in on Linux, in Command Line. It will not run on WINDOWS!
# For Windows, see the "extract-audio.bat" gist.
# In this example, there are 2 audio tracks, which get split out to 2 separate audio files.
# The original video file remains unchanged.
# If you want have a video file with more than 2 audio tracks, 
# just add more "-map 0:a:0 -c copy ""${a%.*} audio track X.m4a"" sections:
# -map 0:a:0 -c copy "${a%.*} audio track 1.m4a" 
# -map 0:a:1 -c copy "${a%.*} audio track 2.m4a" 
# -map 0:a:2 -c copy "${a%.*} audio track 3.m4a" 
# -map 0:a:3 -c copy "${a%.*} audio track 4.m4a" 
# etc ...

# TO RUN THIS SCRIPT:
# You MUST HAVE ffmpeg INSTALLED!
# How to install ffmpeg is beyond the scope of this script. Google it.

for a in *.mp4; do ffmpeg -hide_banner -n -threads 8 -i "$a" -vn -map 0:a:0? -c copy "${a%.*} audio track 1.m4a" -map 0:a:1? -c copy "${a%.*} audio track 2.m4a" ; done

# for a in = this is a the loop section that will look for files to process, 
# $a = this is the variable that will be subsctituted with the actual file name when the for loop executes
# do ffmpeg = this part actually executes ffmpeg and tells it to extract the audio fro the MP4 files it finds.
# -hide_banner = hides the info banner when the command is executed.
# -n = option to not overwrite existing files
# -threads 8 = option to use 8 separate CPU threads for the work. You can increase the number based on the available cores in your CPU
# -i "$a" = this option defines the input file
# -vn = this option means no video to be copied
# -map 0:a:0 = this options maps the first available audio track to be extracted
# ? = this option will avoid an error if there are no multiple audio tracks in a file.
# -c copy = this option means please copy the audio without altering it
# ${a%.*} = this expression is used to get file name only, no extension
# "${a%.*} audio track 1.m4a" = the filename of the extracted audio track