find bitcoin block number with a provided timestamp

fjb.sh

#!/bin/bash

# USAGE:
# fjb.sh <timestamp>
# returns: <blocknum>

TIMESTAMP=$1
LOWER=0
UPPER=$(bitcoin-cli getblockcount)

while (( LOWER <= UPPER )); do
    MID=$(( (LOWER + UPPER) / 2 ))
    BLOCKHASH=$(bitcoin-cli getblockhash $MID)
    BLOCKTIME=$(bitcoin-cli getblockheader $BLOCKHASH | jq .time)

    if (( BLOCKTIME < TIMESTAMP )); then
        LOWER=$(( MID + 1 ))
    elif (( BLOCKTIME > TIMESTAMP )); then
        UPPER=$(( MID - 1 ))
    else
        echo "$BLOCKTIME"
        exit 0
    fi
done

echo "$UPPER"