Convert mp4 to mp3

[bsd-keith]

Convert mp4 to mp3

I have a load of mp4 files with spaces in their names, the first line replaces the spaces with _ so that the second line can convert them to mp3.

#!/bin/sh

for f in *; do mv "$f" `echo $f | tr ' ' '_'`; done

for X in *.mp4;do ffmpeg -i $X $X.mp3;done

[J65nko]

Instead of using back ticks ```` (the key under the ~) you can use the POSIX command substitution"$( command )" construct. It is easier on the eyes, and does not have some quirks that plagues the older ` command ` format.

for f in *; do mv "$f" $( echo $f | tr ' ' '_'); done

[bsd-keith]

Thanks, it's the first time I've used tr, so copied what I'd found, (after trying it out in safety), & it worked OK. But I will alter my personal script to use $() in future.

[victorvas]

I think you need to use -vn flag to extract just the audio stream out of .mp4

ffmpeg -i video.mp4 -vn audio.mp3

[IdOp]

Also, if $X is "video.mp4" then isn't $X.mp3 going to be "video.mp4.mp3" ? Probably you want to get rid of the ".mp4" part of the final result? One way might be using

$( dirname $X )/$(basename $X .mp4).mp3

as the output file.

[bsd-keith]

Yes, I had to use sed to swap '.mp4.' to just a single '.' - messy, but worked.

##To remove .mp4 from *.mp4.mp3 files
for file in *; do mv "$file" $(echo "$file" | sed -e 's/.mp4././g'); done

I also had to remove excess punctuation from filenames, this is what I used, (I expect there are better solutions, but again, it worked reasonably well).

##To keep A-Za-z0-9.- & remove all other
#!/bin/bash
for file in *; do mv "$file" $(echo "$file" | sed -e 's/[^A-Za-z0-9.-]//g'); done

As you can probably see, I'm no expert at scripting, but I noticed others were trying to do what I wanted to do, so posted what worked in my case.

[victorvas]

for X in *.mp4; do ffmpeg -i $X ${X%.mp4}.mp3; done

This will get rid of .mp4 extension.

[Head_on_a_Stick]

And if the variables are properly quoted then the spaces don't have to be replaced with underscores:

for X in *.mp4; do ffmpeg -i "$X" "${X%.mp4}.mp3"; done

[IdOp]

Originally Posted by victorvas

${X%.mp4}

Good find, thanks!

[victorvas]

Originally Posted by IdOp

Good find, thanks!

You're welcome! There are others:

$ foo=some.thing.mp4

$ echo ${foo%.*}       
some.thing

$ echo ${foo%%.*}
some

$ echo ${foo#*m}
e.thing.mp4

$ echo ${foo##*m}
p4

[J65nko]

These are documented in ksh(1):

The following forms of parameter substitution can also be used:

     [snip]

     ${name#pattern}
     ${name##pattern}
             If pattern matches the beginning of the value of parameter name,
             the matched text is deleted from the result of substitution.  A
             single `#' results in the shortest match, and two of them result
             in the longest match.

     ${name%pattern}
     ${name%%pattern}
             Like ${..#..} substitution, but it deletes from the end of the
             value.

From sh(1):

     ${parameter%[word]}
             Substitute parameter, deleting the smallest possible suffix
             matching word.

     ${parameter%%[word]}
             Substitute parameter, deleting the largest possible suffix
             matching word.

     ${parameter#[word]}
             Substitute parameter, deleting the smallest possible prefix
             matching word.

     ${parameter##[word]}
             Substitute parameter, deleting the largest possible prefix
             matching word.

[Head_on_a_Stick]

Keith's using a /bin/sh shebang so the POSIX reference is probably better:

https://pubs.opengroup.org/onlinepub...V3_chap02.html

(Section 2.6.2 — Parameter Expansion)