avprobe <file>

ffmpeg -i input.video -threads 4 -b:v 1M -crf 10 output.webm

mencoder -idx input.video -ovc copy -oac copy -o output.video

# 5 images per seconds
ffmpeg -y -framerate 5 -pattern_type glob -i '*.jpg' -c:v libx264 -vf "fps=25,scale=720:-1" out.mp4

convert to mkv

mkvmerge -o output.mkv <inputfile>

convert to mkv and downgrade quality

avconv -i <inputfile> -map 0 -c:v libx264 -crf 20 -c:a copy -c:s copy <outfile>.mkv

concatenate all *.mp4 files to output.mkv

mkvmerge -o output.mkv $(echo *.mp4 | sed "s| | +|g")

convert files to mkv and downgrade them if greater than specific value

tomkv (chmod +x)

#!/bin/bash

OUTDIR=out
LIMIT_MBYTE=1500
QUALITY=20 # lower values are better

mkdir -p $OUTDIR
set -x
for f in "$@"
do
    [ -f "$f" ] || continue

    SIZE=$(stat -c%s "$f")
    OUTNAME=$OUTDIR/${f%.*}.mkv
    [ -f "$OUTNAME" ] && ( echo "Skip $OUTNAME"; continue )

    if [ $SIZE -gt $(($LIMIT_MBYTE*1000000)) ]; then
        echo avconv $f
        avconv -i "$f" -map 0 -c:v libx264 -crf $QUALITY -c:a copy -c:s copy "$OUTNAME"
    else 
        echo mkvmerge $f
        mkvmerge "$f" -o "$OUTNAME"
    fi
done

Usage to convert into ./out folder

tomkv file1 file2 ...