On 12 September 2016 at 21:22, Mike Wright <nobody@xxxxxxxxxxxxxxxxxxxx> wrote:
> Hi all,
>
> This one's not in my unix nutshell book :/
>
> Extracting the sound from some music videos (mp4's) into mp3's:
>
> for f in "$(ls *.mp4)"; do
> ffmpeg -i "$f" -c:a libmp3lame "${f%.mp4}.mp3";
> done
>
> I'm not familiar with this notation: ${f%.mp4}, where $f is the name of an
> mp4, e.g. "my favorite video.mp4". Can somebody explain this to me?
>
This is bash parameter expansion, from the manual page of bash:
<quote>
${parameter%word}
${parameter%%word}
Remove matching suffix pattern. The word is expanded to produce a
pattern just as in pathname expansion. If the pattern matches a
trailing portion of the expanded value of parameter, then the result
of the expansion is the expanded value of parameter with the shortest
matching pattern (the ``%'' case) or the longest matching pattern (the
``%%'' case) deleted. If parameter is @ or *, the pattern removal
operation is applied to each positional parameter in turn, and the
expansion is the resultant list. If parameter is an array variable
subscripted with @ or *, the pattern removal operation is applied to
each member of the array in turn, and the expansion is the resultant
list.
</quote>
so IIUC, it would change "my favorite video.mp4" to "my favorite video.mp3".
--
Ahmad Samir
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