#!/bin/bash
source_dir='.'
file_ptrn='*.webm'
dest_dir='truncated'
for file in $(find -L $source_dir -name $file_ptrn); do
  s=$(avconv -i $file 2>&1 | grep -o -E '|Duration: [^.]+|')
  sec=${s##*:}
  s=${s%:*}
  min=${s##*:}
  duration=$(( ${sec#0} + ${min#0} * 60 ))
  max_start=$(( $duration - 300 ))

  start=0
  while (( $start < $max_start )); do
    dest=$dest_dir/$(</dev/urandom tr -dc A-Za-z0-9 | head -c 6).webm
    start=$(( $start + $max_start / 5 ))
    trunc_m=$( (( $start > 60 )) && printf %02d "$(( $start /  60 ))" || echo '00' )
    trunc_s=$( printf %02d $(( $start - $trunc_m * 60 )) )
    avconv -i $file -ss 00:$trunc_m:$trunc_s -t 00:05:00 -codec copy $dest
  done
done