To prevent cron emails do I need to redirect output to /dev/null?
What I've done to fix this is create a bash script that executes the civicrm api command, captures the output to a variable, tests for the string [is_error] => 0, if there's no match, then it echos the output of the command.

Here's the script:

 #!/bin/bash

output=$(/usr/local/bin/wp --user=cronuser --url=https://mysite.org --path=/srv/www/mysite.org/public_html civicrm api job.execute auth=0)

### only print if error is not 0

if [[ $output != *"[is_error] =&gt; 0"* ]]
then
  echo "$output";
fi