Check for expiring SSL certificates

From https://www.linuxjournal.com/content/bare-bones-monitoring-monit-and-rrdtool: "Here's an example of a script that alerts when an SSL certificate expiration date is within a specified number of days:"

#!/bin/bash

domainexpiredate() {
    openssl x509 -text -in <(echo -n | \
    openssl s_client -connect $1:$2 2>/dev/null | \
    sed -n '/-*BEGIN/,/-*END/p') 2>/dev/null | sed -n 's/
 ↪*Not After : *//p'
}

daysleft() {
    echo "((($(date -d "$(domainexpiredate $1 $2)" +%s)-$(date
 ↪+%s))/24)/60)/60" | bc
}

defaultport() {
    if [ -z "$1" ]; then
        echo "443"
    else
        echo "$1"
    fi
}

[[ $(daysleft $2 $(defaultport $3)) -le $1 ]] && exit 1 ||
 ↪exit 0