For every PERIOD lines, output a random line from that set of lines

sparse-random-lines.sh

#!/bin/bash

set -e -u

if [[ $# -ne 1 ]];then
    echo "Usage: $0 PERIOD"
    echo "For every PERIOD lines, output a random line from that set of lines"
    echo "E.g. printf '%s\n' {1..30} | $0 7"
    echo "might output 5, 12, 19, 23 and 30."
    exit 1
fi

gawk -vseed=$RANDOM -vperiod=$1 '
BEGIN{
    srand(seed)
}

NR%period == 1 {
    mod=int(period*rand())
}

NR%period == mod {
    print
}'