Remove audio from a video on Linux

If you are working on a computer vision project, there is a great chance that you have been recording videos with sound, which you have no use of. To save space and processing power, you can strip the audio from the video just like this:

avconv -i input_video.mp4 -an output_video.mp4

(Replace avconv with ffmpeg if necessary.)

Just so you know, there is something dishonest with avconv as they make you believe ffmpeg is dead or obsolete. In fact it's nothing like that. avconv is a fork of ffmpeg and has become the default on Ubuntu. So if you are reading to use a filter that isn't present in avconv, then you need to download the latest version of ffmpeg from source or from a PPA. Read more on this subject here.

Rotate a video on Linux

When you record videos on your smartphone, the video itself is not rotated. The video recorder simply adds a tag telling the player how to rotate the video.
That's why mediainfo reports "Rotation: 180°" or other angles on videos recorded from my Samsung Galaxy S4.

Unfortunately unlike the video player on your phone, the vast majority of video players on desktop computers and TVs don't support this tag.

If the video is upside down:

avconv -i original_video.mp4 -vf "vflip,hflip" output_video.mp4
or
avconv -i original_video.mp4 -vf "transpose=2,transpose=2" output_video.mp4

(You can replace avconv with ffmpeg if avconv is not found on your system.)

Note that the first version is slightly faster (2-5 %).

If the video was recorded in portrait mode:

if the top of the image is on the left of the screen, it needs a +90° rotation:

avconv -i original_video.mp4 -vf "transpose=1" output_video.mp4

if the top of the image is on the right of the screen, it needs a -90° or +270° rotation:

avconv -i original_video.mp4 -vf "transpose=2" output_video.mp4

Ubuntu: run a script when a USB device is plugged in

There's a lot of stuff you can do with udev. One of them is running a script when a USB device is plugged in.

It's as simple as:

1. figure out the vendor id and product id with lsusb
2. edit /etc/udev/rules.d/85-something.rules:

ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="04c5", ATTR{idProduct}=="11a2", RUN+="/absolute/path/to/your/script/that/will/be/executed/as/root"

3. Write the script mentioned above and make sure it's executable.
4. Restart udev: service udev restart.

You can do other things with udev such as managing /dev/ and the permissions on the device.
However you should not use udev to mount USB disks, there are other tools for that.

To run a script when detaching a USB device, replace ACTION=="add" with ACTION=="remove". You can put several lines in the same ".rules" file.

Downloading subtitles

Subliminal is a command-line utility written in Python that downloads subtitles for TV episodes or movies.

While media players such as Kodi (formerly known as XBMC) offer this feature, it's a painful process to pause the video, open the subtitles menu, download the subtitles file, wait until the download completes and so on.
I stream videos directly from my NAS, so that wasn't even an option because the smart TV is unable to store the subtitles file anywhere, it has to be at the same location as the video.

With one little command, Subliminal fetches subtitles from various sources and finds what it thinks is the best subtitles track for the media files you throw at it.

Here's how to install and use it:

1. Try to find if you can install Subliminal directly from your package manager. If so, skip to step 4.
2. Install Python PIP.
3a. Recommended method: using virtualenv(wrapper): mkvirtualenv subtitles; pip install subliminal colorlog
3b. Alternate method: directly on the system: sudo pip install subliminal colorlog
4. Download the subtitles: subliminal -l en -s --color videoOrDirectory anotherVideoOrDirectory...
You can use other flags. These will download a subtitle track in English. The "-s" flag means the file will be named something.srt instead of something.en.srt. I use this option because some players are not able to associate the ".en.srt" subtitles file with the video. "--color" is just for eye candy. You can use "-v" for more messages.

To my knowledge subliminal won't browse directories recursively. You can achieve that effect with:
find . -type d -exec sh -c 'subliminal -l en -s  "{}"' \;

Subliminal is smart enough to detect if the file is a video, and if it needs subtitles.
I was confused with the message "No video to download subtitles for". It will be printed if there was no video in the path(s) you provided and if the video(s) already contain(s) a subtitles track. You can check that with mediainfo.

As with Kodi, you should follow certain naming conventions. It is best to include the full name of the show or movie in the title, and describe the episode as "S01E02" (Season 1 Episode 2) or "1x02". For best compatibility avoid spaces and non-ASCII characters. If you downloaded the video from the Internet, you should however try to keep the filename as it was, including the uploader's nickname and the video quality (HDTV, 720p, 1080p...) There might be a subtitles track for the exact video file you have.

Parallel Gzip with a progress bar

Everybody knows how to create a "tar.gz", probably using something like this:

tar zcvf foo.tar.gz bar-directory/

This is fine to compress something relatively small (a few megs).
However, if you have to archive hundreds of gigabytes, you probably want features such as:

• Speeding things up
• Displaying a progress bar
• Displaying an ETA

This command* offers all of these:

tar cf - bar-directory/ -P | pv -s $(du -sb bar-directory/ | awk '{print $1}') | pigz > foo.tar.gz

You'll probably have to install pigz, which is a parallelized version of Gzip. You can substitute it with pbzip2 if you want a "tar.bz2" archive.

Supposing the CPU was the bottleneck and not the I/O, which is probably the case if you are working on the local filesystem, this will speed things up by a factor of "number of CPU cores". For instance, instead of 1 hour with gzip, it took only 15 minutes with pigz on my machine with 4 CPU cores.
In fact it was almost as fast as copying the directory without archiving / compression.

As a bonus you get a nice progress bar in your terminal:

*On OS X and *BSD, du uses another unit, use awk to convert the size into bytes.

ownCloud Client asking for password when starting a KDE session

UPDATE: On another machine I have, the fix described below is not enough. You have to make sure that the KWallet daemon is enabled for the current user. Enable with: System Settings => Account details => KWallet => Enable

The ownCloud client is integrated with KDE's password manager (KWallet).
When it starts, it checks if the kwallet is unlocked and looks for the password.

Yeah, that's how it should be and that is a very software design. Unfortunately it has never worked and the ownCloud client asks for the password every single time!

In the latest version the problem is actually explained in the dialog ("No keychain available"):

In my case, which seems to be the default, the problem is that kwalletd is not running when the ownCloud client starts. It's a timing issue.

My solution :

1. Open KDE's "Autostart" control module (a quick search from the start menu will get you there)
2. Change the ownCloud entry to point to a script of your own.
3. Click OK.
4. Write the following shell script:



5. Make it executable.
6. Reboot (or kill kwalletd and try your script. Note that logging out doesn't kill the daemon.)

Of course if your KWallet is protected by a password, then you will be asked to provide it.

VBoxManage made simple

As you might know, VirtualBox machines can be managed from the command-line. However, I find the syntax of the VBoxManage command-line utility cumbersome and hard to remember.

Copy this in .bash_aliases:



Now you can do the following:

vm-start starts a VM in headless mode
vm-savestate suspends a VM to disk
vm-powerbutton simulates a press on the power button
vm-poweroff simulates unplugging the power cord (some OS don't shutdown completely)
vm-running lists the running VMs

Autocompletion is enabled and dangerous actions will ask for confirmation.

(You will need to re-login or "source" .bash_aliases whenever you add / remove a VM.)
(The script doesn't support VM names with spaces.)