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benjamin10
Asked: 2020-03-04 03:26:28 +0800 CST

Desktop entry to convert audio/video files (naming question)

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My question: I'd like to create a desktop entry in (kde dolphins servicemenu) to convert audio/video files.

I have tried it with the follwing:

[Desktop Entry]
Actions=Convertwav2Mp3
Icon=audio-x-flac
MimeType=audio/*
ServiceTypes=KonqPopupMenu/Plugin
Type=Service
X-KDE-Priority=TopLevel

[Desktop Action Convertwav2Mp3]
Exec=ffmpeg -i %f -codec:a libmp3lame -b:a 320k .out.mp3 && mv .out.mp3 %f
Icon=audio-x-flac
Name=Convertwav2Mp3

Works like a charm when put in /usr/share/kservices5/ServiceMenus/.

BUT: The mv command renames the output file to the original filename with its file extension (which is wav but should be mp3).

How can i change the command as to rename it from file.wav to file.mp3 within the Exec field?

ffmpeg .desktop

1 Answers

  1. Option #1: append the .mp3

    Exec=ffmpeg -i %f -codec:a libmp3lame -b:a 320k %f.mp3

    This will result in e.g. converting

    my_file.wav

    to

    my_file.wav.mp3

    which may not be desirable.

    Fun fact: there was used to be the %n key that gave the base name like this:

    Exec=ffmpeg -i %f -codec:a libmp3lame -b:a 320k %n.mp3

    The FreeDesktop spec has listed this as deprecated from version 1.0:

    Deprecated Exec field codes: ... %n (the base name of a file) ...

    https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-1.0.html

    This may have worked in older versions of Dolphin, but in the version I used (17.12.3), the %n key does the same as the %f key, so this will not work as desired.

    Option #2: use bash parameter expansion

    To get this instead:

    my_file.mp3

    we will need to remove the file extension. This can be accomplished using bash parameter expansion:

    Exec=bash -c 'wavfile='\''%f'\''; mp3file="${wavfile%.wav}.mp3"; ffmpeg -i '\''%f'\'' -codec:a libmp3lame -b:a 320k "$mp3file"'

    We have to invoke bash explicitly because the Exec key is passed through /bin/sh, which does not support this syntax.

    Caveats

    Both of these options will work with filenames that have spaces. However, they will not work as expected in other cases, such as:

    • If the MP3 filename already exists, it will silently fail.

    • If the WAV filename contains a parameter expansion string such as $0 or $USER it will silently fail.

    • If the WAV filename contains a command substitution string such as `date` or $(date) it will silently fail.

    • If the WAV filename is e.g. example.WAV instead of example.wav, it will result in example.WAV.mp3.

    To make these problems more tractable and easier to debug, I would recommend writing a separate shell script and invoking it directly; there are many examples:

    Further comments

    As an aside, you may already know this, but the desktop file can be copied to

    ~/.local/share/kservices5/ServiceMenus/

    instead of

    /usr/share/kservices5/ServiceMenus/

    which is useful if you want to install it for a single user or don't have root privileges.

    Finally, I would recommend using

    MimeType=audio/x-wav

    instead of

    MimeType=audio/*

    since this only works on WAV files.

    Related questions:

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