Looking on the web for a quick and easy way to turn a video 90 degrees using the terminal I found HandBrake.
following this forum this is how I installed it:
there are two official HandBrake PPAs, ppa:stebbins/handbrake-releases and ppa:stebbins/handbrake-snapshots. The former contains stable releases, which are updated about once a year. These releases tend to be rather out-dated when their end-of-life is approaching. The current stable version (0.9.8) was released on 2012-07-18. The latter contains nightly builds, which are updated daily (or nightly, as it were). These are of course less stable, and undocumented to boot, but they are good software nonetheless. Additionally, as the stable release ages, the developers tend to start recommending users to try the nightly builds instead.
To add one of these to your sources, simply run:
depending on which you want.
Alternatively, if you would prefer the CLI over the GUI, replace the last line with:
Now, how to rotate a video 90 degress using HandBreak?