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I have about 70 mp4 files, and I want to extract the audio files directly without transcoding further. All the mp4 files have aac audio files.

I understand I can use the following command to extract the audio file from one of the mp4 files:

avconv -i "INPUT FILE" -map 0:1 -c:a copy "OUTPUT FILE"

How do I extract all the audio files from all the 70 mp4 files, so I end up with 70 aac files?

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3 Answers 3

up vote 11 down vote accepted

You can use a simple for loop:

for i in *.mp4; do
    avconv -i "${i}" -map 0:1 -c:a copy "${i%.mp4}.aac"
done

or on one line:

for i in *.mp4; do avconv -i "${i}" -map 0:1 -c:a copy "${i%.mp4}.aac"; done

What is does is run avconv once for every file named like *.mp4 where the filename is stored in the ${i} variable.

${i%.mp4} means ${i} (ie. the filename) with .mp4 stripped off from the end.

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IIRC, raw aac files can't contain metadata; if you want to keep metadata from the original files then use m4a (which is just another name for mp4, but is fairly widely recognised by audio players) instead of aac as a file extension. –  evilsoup Jan 4 '13 at 3:15
    
What is the -map option for? Why do you choose the 0:1 stream rather than other streams, such as 0:0, 0:2? –  qed Nov 28 '13 at 21:52
    
I dropped the -map option and it works just fine –  qed Nov 28 '13 at 22:00
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Xiao's answer is generally the most useful if you have all the files in one directory; but if there are MP4 files scattered in different directories, you can use this find command to convert them all.

find . -type f -name '*.mp4' -exec bash -c 'avconv -i "$0" -c:a copy "${0/%mp4/m4a}"' {} \;

This uses a slightly different form of bash string substitution at the end: "${0/%mp4/m4a}" tells bash to replace mp4 with m4a, but only if the mp4 is at the end of the string.

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I never knew that bash had that type of substitution. Awesome :) –  Xiao-Long Chen Jan 4 '13 at 4:40
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@xiao If you leave out the %,it'll do the substitution for the first instance of the string it finds; you can also use # to make it only substitute the string if it's at the start of a variable; so ${0/mp4/m4a} would replace the first instance of mp4 with m4a (which could be a problem if you have really weird filenames); and ${0/#mp4/m4a} will substitute mp4 for m4a, but only if the variable's string value begins with mp4 (no use here, obviously, but it might come in handy elsewhere). –  evilsoup Jan 4 '13 at 10:45
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sudo apt-get install parallel

This line will extract aac audio files from selected videos and put them into m4a containers.

parallel avconv -i '{}' -map 0:1 -c:a copy '{}.m4a' -- %F

Inspired by this answer, by a previous answer to this question, and by evilsoup's comment to the latter (saying: 'raw aac files can't contain metadata; if you want to keep metadata from the original files then use m4a (which is just another name for mp4, but is fairly widely recognised by audio players) instead of aac as a file extension').

I use such commands in Thunar custom actions,

enter image description here

limiting the application to videos that contain aac audio.

enter image description here

To have a terminal window open during processing, which closes at the end, the command can be changed like:

 gnome-terminal -e "parallel avconv -i '{}' -map 0:1 -c:a copy '{}.m4a' -- %F"
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