general: linux audio extract now considers actual header len

The documentation on page /general/audio-file-format/ section 2. Audio file
format reports, that the header is of variable length. Thus, for precise
audio data extraction, the header's length must be parsed in order to
discard the correct number of bytes.

Previously, the code-snippet discarded always 4096 bytes, which would fail
when the header length should be ever different.
Also, i always like to have easy-to-copy&paste snippets in documentation.

Author: qyanu

content/docs/wiki/general/audio-file-format.md

@@ -76,9 +76,21 @@ But the in experiments used encoder did not have an obvious feature to do this.
 
 **We have developed a tool called [teddy](https://github.com/toniebox-reverse-engineering/teddy) to encode and decode these files.**
 
-# Audio file extraction with Linux OS
-- Remove Header of the file with ´dd bs=4096 skip=1 if=500304E0 of=trim.ogg´
-- then just use ffmpeg to convert it into mp3 ´ffmpeg -i trim.ogg done.mp3´
+# Audio file extraction with Linux OS and bash shell
+
+1. read the *header_len* (in hex): `dd status=none if=50012345 bs=1 count=4 | xxd -plain`
+2. convert to decimal and add 4 in order to get the length (in decimal) of the whole header
+3. read the audio_data into file: `dd if=50012345 bs=1 skip="${header_len}" of=50012345.audio_data.ogg`
+4. (optional) convert audio_data from ogg to mp3: `ffmpeg -i 50012345.audio_data.ogg 50012345.audio_data.mp3`
+
+The same procedure as one block of spagetthi code for easier copy&paste:
+
+```bash
+inputfile=50012345
+header_len="$((4+0x"$(dd status=none if="${inputfile}" bs=1 count=4 | xxd -plain)"))"
+dd if="${inputfile}" bs=1 skip="${header_len}" of=${inputfile}.audio_data.ogg
+ffmpeg -i "${inputfile}.audio_data.ogg" "${inputfile}.audio_data.mp3"
+```
 
 # Audio file extraction with Windows
 - Remove Header of the file with: powershell -Command "$in = [System.IO.File]::OpenRead('00000000'); $out = [System.IO.File]::Create('trim.ogg'); $in.Seek(4096, 'Begin') | Out-Null; $in.CopyTo($out); $in.Close(); $out.Close()"