Analysis of a letsencrypt renewal cron script from: https://blog.benroux.me/running-multiple-https-domains-from-the-same-server/#autorenewourcerts

letsencrypt-autorenew-cron.sh

#!/bin/sh
# The line above this one basically just indicates that the file is a shell script.

# This script renews all the Let's Encrypt certificates with a validity < 30 days

# This first line is a conditional block it tries to run the program at
# `/opt/letsencrypt/letsencrypt-auto` with the command `renew`
# Earlier in the guide the letsencrypt binaries were installed to opt
# `sudo git clone https://github.com/letsencrypt/letsencrypt /opt/letsencrypt`
# The `!` at the beginning is a boolean conditional negation operator and in this
# case the complete expression will return true if the program doesn't succeed.
# Also in the conditional it uses the redirection operator `>` to write the output
# of the command to the file, I recommend using `>>` instead as it will append
# `>` just overwrites the contents of the file.
# `>` or `>>` alone will just redirect the contents of the Standard Output Stream, or
# stdout, `2>&1` placed at the end will cause the Standard Error Stream, or stderr to
# be redirected into the `renew.log` file as well.
if ! /opt/letsencrypt/letsencrypt-auto renew > /var/log/letsencrypt/renew.log 2>&1 ; then
# After the command fails to renew the certs it will say so
echo Automated renewal failed:
# And it will also try to print out the log immediately, I don't recommend this.
cat /var/log/letsencrypt/renew.log
# Finally this block exits the script with a failure status (anything other than 0)
exit 1
fi

# This runs some nginx commands `nginx -t` tests the config file,
# `nginx -s` sends a signal to the running process, in this case
# the reload signal. So it reloads nginx with the new valid config.
nginx -t && nginx -s reload

As for the logic of

if ! /opt/letsencrypt/letsencrypt-auto renew > /var/log/letsencrypt/renew.log 2>&1 ; then

This is how I understand the logic if is testing

1. Run auto renew and output to renew.log whether the command ran successfully or not (using > and 2>&1).
2. If there was an error (! false) then print output that no one will see echo Automated renewal failed: and cat /var/log/letsencrypt/renew.log (this is for a cron job... so who'll see the output? 😛) and then terminate the script exit 1 (thus not running the last line of code).

Is this correct?

Yep, that's correct, I thought it was weird too... I suppose the cronjob facility might be receiving all command output and injecting it into it's own logs but in general I thought it was silly. Perhaps it was for testing the script before making it a cronjob.