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Every day's a school day - learned about arrays in bash.

#!/bin/bash
start=$SECONDS
inputfile=$1
mapfile -t < $inputfile

# Part 1 - 2 numbers
for i in "${MAPFILE[@]}"; do
    mycount=$(( mycount + 1 ))
    if [[ " ${MAPFILE[@]} " =~ " $(( 2020 - $i )) " ]] ; then
        echo "$i, $(( 2020 -$i)), $(( $i * (2020 - $i)))"
        duration=$(( SECONDS - start ))
        echo "$duration seconds $mycount records"
        break
    fi
done


# Part 2 - 3 numbers
mycount=0
start=$seconds
for i in "${MAPFILE[@]}"; do
    for j in "${MAPFILE[@]}"; do
        mycount=$(( mycount + 1 ))
        if [[ " ${MAPFILE[@]} " =~ " $(( 2020 - $i -$j )) " ]] ; then
            echo "$i, $j, $(( 2020 -$i - $j )), $(( $i * $j * (2020 - $i - $j) ))"
            duration=$(( SECONDS - start ))
            echo "$duration seconds $mycount records"
            exit
        fi
    done
done

 $ ./array_test.sh input1.txt 
1632, 388, 633216
0 seconds 82 records
1607, 196, 217, 68348924
3 seconds 14573 records