Check if a Let's Encrypt certificate needs renewal, and renew if possible. This script is an adapted version of the one made by Erika Heidi for Digital Ocean. The only difference is this works on nginx.

le-renew

#!/bin/bash
#================================================================
# Let's Encrypt renewal script for Nginx on Ubuntu/Debian
# @author Erika Heidi<[email protected]>
# Edited by Sam Bellen <@sambego>
# Usage: ./le-renew.sh [base-domain-name]
# More info: http://do.co/1mbVihI
#================================================================
domain=$1
le_path='/opt/letsencrypt'
le_conf='/etc/letsencrypt'
exp_limit=30;

get_domain_list(){
        certdomain=$1
        config_file="$le_conf/renewal/$certdomain.conf"

        if [ ! -f $config_file ] ; then
                echo "[ERROR] The config file for the certificate $certdomain was not found."
                exit 1;
        fi

        domains=$(grep --only-matching --perl-regex "(?<=domains \= ).*" "${config_file}")
        last_char=$(echo "${domains}" | awk '{print substr($0,length,1)}')

        if [ "${last_char}" = "," ]; then
                domains=$(echo "${domains}" |awk '{print substr($0, 1, length-1)}')
        fi

        echo $domains;
}

if [ -z "$domain" ] ; then
        echo "[ERROR] you must provide the domain name for the certificate renewal."
        exit 1;
fi

cert_file="/etc/letsencrypt/live/$domain/fullchain.pem"

if [ ! -f $cert_file ]; then
        echo "[ERROR] certificate file not found for domain $domain."
        exit 1;
fi

exp=$(date -d "`openssl x509 -in $cert_file -text -noout|grep "Not After"|cut -c 25-`" +%s)
datenow=$(date -d "now" +%s)
days_exp=$(echo \( $exp - $datenow \) / 86400 |bc)

echo "Checking expiration date for $domain..."

if [ "$days_exp" -gt "$exp_limit" ] ; then
        echo "The certificate is up to date, no need for renewal ($days_exp days left)."
        exit 0;
else
        echo "The certificate for $domain is about to expire soon. Starting renewal request..."
        domain_list=$( get_domain_list $domain )
        # In order to successfully run the Let's Encrypt renewak script, nginx needs to be stopped.
        # This means your site will be offline for a few seconds, 
        # but don't wory, we'll restart nginx as soon as the script finishes.
        echo "Stopping nginx..."
        /usr/sbin/service nginx stop
        "$le_path"/letsencrypt-auto certonly --renew-by-default --domains "${domain_list}"
        echo "Starting nginx..."
        /usr/sbin/service nginx start
        echo "Renewal process finished for domain $domain"
        exit 0;
fi