ffmpeg

reduce filesize

ffmpeg -i input.avi -vcodec libx264 -crf 24 output.avi

trim

trim start :

ffmpeg
-i 1.mp4
-ss start_time
-c copy
retult.mp4

trim end :

ffmpeg
-i 1.mp4
-to end_time
-c copy
retult.mp4

audio or video only

-vn -acodec copy -> only audio
-an -vcodec copy -> only video

joinfiles

Ehhh...
ls * | perl -ne 'print "file $_"' | ffmpeg -f concat -i - -c copy merged.mp4

video <--> images

to images : ffmpeg -i video.flv image%d.jpg
from images : ffmpeg -f image2 -i image%d.jpg imagestovideo.mpg

video -> gif

set palt=palette.png
set filters=scale=800:-1:flags=lanczos
rem set filters=fps=10,scale=500:-1:flags=lanczos

rem make palette.png
set a=ffmpeg -y -i %1 -filter_complex "%filters%,palettegen" %1-%palt%
rem set a=ffmpeg -y -i %1 -filter_complex "%filters%,palettegen" %1-%palt%

rem make gif: palette.png + video.mp4
set b=ffmpeg -y -i %1 -i %1-%palt% -filter_complex "%filters%[x];[x][1:v]paletteuse" %2
rem set b=ffmpeg -y -i %1 -i %1-%palt% -filter_complex "%filters%[x];[x][1:v]paletteuse" %2

%a%
%b%