# usage:
# split.sh log.txt 20000
set -e
LANG=C LC_ALL=C  # 1 char = 1 byte
PART=1
BYTES=0
exec 3<>"$1.part${PART}"  # open file
while IFS='' read -r line ; do
    echo "$line" >&3
    BYTES=$(( BYTES + ${#line} ))
    if (( BYTES > $2 )) ; then
        exec 3>&-
        (( PART++ ))
        exec 3<>"$1.part${PART}"
        BYTES=0
    fi
done < "$1"
exec 3>&-  # close file