Cron jobs have a funny way of being ignored. Either no one knows the job is failing because the job doesn’t tell anyone. Or, the job is spamming your e-mail inbox many times a day, regardless of success or failure, which means you just ignore the e-mails.

I’ve seen the “Monitor an external job” option for new Jenkins jobs before, and never paid much attention. Turns out it’s a great bucket for storing logs and results of cron jobs.

The external-monitor-job plugin seems to ship with the Jenkins war file. So, your Jenkins should have it out of the box.

Creating a job is pretty simple. It’s just a name and description. Click “New Item” in Jenkins and select the “Monitor an external job” option. This creates a job of type hudson.model.ExternalJob.

The wiki describes a fairly complicated method to download the Jenkins jar files onto the server running your cron jobs, and then use the Java runtime to run a jar with your cron script as an argument. The jar presumably forks your a new shell to run your desired cron command and sends the output/result to Jenkins.

There’s a much easier way to do this. Redirect or tee your job’s stdout/stderr output to a temp file. Then post the result code and log file via curl to Jenkins. No need to download jar files. No need to even have Java runtime on the server.

Just POST a small XML document with the log contents (binary encoded) and the exit code to Jenkins @ /job/:jobName/postBuildResult where :jobName is the URL encoded name of your monitoring job in Jenkins.

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#!/bin/sh
# example cron script to post logs to Jenkins

# exit on error
set -e

log=`mktemp -t tmp`
timer=`date +"%s"`
jenkins_job=my_monitoring_job
jenkins_server=http://jenkins.example.com:8080/jenkins/job/$jenkins_job/postBuildResult
# see http://jenkins.example.com:8080/me/configure to get your username and API token
jenkins_username=myusername
jenkins_token=abcdef0123456789fedcba9876543210

function banner() {
  echo $(printf '#%.0s' {1..80}) >> "$log"
}

function report() {
  result=$?
  timer=$((`date +"%s"` - $timer))

  banner
  echo "`whoami`@`hostname -f` `date`: elapsed $timer second(s)" >> "$log"
  echo "exit code $result" >> "$log"

  # binary encode the log file for Jenkins
  msg=`cat "$log" | hexdump -v -e '1/1 "%02x"'`

  # post the log to jenkins
  echo curl -X POST \
       -u "$jenkins_username:$jenkins_token" \
       -d "<run><log encoding=\"hexBinary\">$msg</log><result>$result</result><duration>$timer</duration></run>" \
        $jenkins_server/job/$jenkins_job/postBuildResult
}

trap report EXIT;

banner
echo "hello, world @ `date`!" | tee "$log"

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MAILTO=""
0 * * * * /bin/sh /your/directory/myjob.sh

A sample of the build log on Jenkins with a green/red build status:

Credit to Taytay on Stackoverflow.com for figuring out how to use hexdump to properly encode the XML for Jenkins.