avisynth (or a like tool) for linux

"Since no one mentioned it yet:

There's VapourSynth, a cross-platform toolkit very similar to AviSynth, which uses Python as its scripting language.

http://www.vapoursynth.com/

Example syntax:

import vapoursynth as vs
core = vs.get_core()
video = core.ffms2.Source(source='Rule6.mkv')
video = core.std.Transpose(video)
video.set_output()

To play a script file in a player (mpv):

vspipe --y4m script.vpy - | mpv -

To encode the script output using ffmpeg:

vspipe --y4m script.vpy - | ffmpeg -i pipe: encoded.mkv"

If you are willing to compile it, there is Avisynth 3.0 - source is available & there's a Gentoo binary distro.