In my job script, I have the following statements (this job only runs on the Jenkins master):
# This is the condition test I use to set the build status as UNSTABLE
if [ ${PERCENTAGE} -gt 80 -a ${PERCENTAGE} -lt 90 ]; then
echo WARNING: disc usage percentage above 80%
# Download the Jenkins CLI JAR:
curl -o jenkins-cli.jar ${JENKINS_URL}/jnlpJars/jenkins-cli.jar
# Set build status to unstable
java -jar jenkins-cli.jar -s ${JENKINS_URL}/ set-build-result unstable
fi
You can see this and a lot more information about setting build statuses on the Jenkins wiki: https://wiki.jenkins-ci.org/display/JENKINS/Jenkins+CLI