gradual dump of postgres data

#!/usr/bin/env bash

set -e

start=$(date -ud 20170413 "+%s") # start time in epoch time (seconds since Jan. 1st, 1970)
end=$(date -ud 20180831 "+%s") # end time

period=$((86400 * 7)) # 86400 is 24 hours

for ((i=start; i <= end; i+=$period)); do
    FROM="$(date -ud "@$i" +%Y-%m-%d)"
    TILL="$(date -ud "@$(($i + $period))" +%Y-%m-%d)"
    echo "dumping data for period from $FROM till $TILL"
    psql -c "COPY (select * from <TABLE> where <datefield> >= '${FROM}' and <datefield> < '${TILL}' ORDER BY <datefield>) TO STDOUT CSV DELIMITER E'\t' HEADER QUOTE E'\b' NULL AS '';" | gzip > /tmp/dump_${FROM}_${TILL}.tsv.gz
done