How long will it take for a file to reach a target size?

howlong.sh

#!/bin/bash

FILE=$1
TARGET_SIZE=$2
SIZES=()
INTERVAL=5

CURRENT_SIZE=($(wc -c "$FILE"))
while [[ $CURRENT_SIZE -lt $TARGET_SIZE ]]; do
	SIZES=("${SIZES[@]}" $CURRENT_SIZE)
	OLDEST_SIZE=${SIZES[0]}
	COMPLETE_PERCENT=$((100 * $CURRENT_SIZE / $TARGET_SIZE))
	if [[ $CURRENT_SIZE -le $OLDEST_SIZE ]]; then
		BYTES_SEC=0
		ETA="Unknown"
	else
		BYTES_SEC=$((($CURRENT_SIZE - $OLDEST_SIZE) / ((${#SIZES[@]} - 1) * $INTERVAL)))
		REMAINING_TIME=$((($TARGET_SIZE - $CURRENT_SIZE) / $BYTES_SEC))
		ETA_SECONDS=$(($(date +%s) + $REMAINING_TIME))
		ETA=$(date -d @$ETA_SECONDS 2>/dev/null || date -r $ETA_SECONDS)
	fi
	echo -ne "\r\e[K$COMPLETE_PERCENT% : $BYTES_SEC bytes/sec : ETA $ETA"
	if [[ ${#SIZES[@]} -ge 6 ]]; then
	        SIZES=("${SIZES[@]:1}")
	fi

        sleep $INTERVAL
		CURRENT_SIZE=($(wc -c "$FILE"))
done

echo -e "\r\e[KFinished"