Special Cron Job

Today, I encountered a really nasty scripting “bug” that involves running a cron job. I have a simple script called run_monitor.sh that:

1. Curls a website to get its response status
2. Checks the response status
3. Updates the cron job depending on the status
   
   

#!/bin/sh

APPDIR=`pwd`
HTTP_RESPONSE=`curl -k -sL -w "%{http_code}" "http://www.google.com" -o /dev/null`

if [ ! $HTTP_RESPONSE -eq 200 ]; then
    # Replace this line to send a message to channel
    # Remove/add/update cron job to once every hour
    (crontab -l | grep -v "$APPDIR/run_monitor.sh"; echo "0 * * * * $APPDIR/run_monitor.sh") | sort - | uniq - | crontab -
else
    # Remove/add/update cron job to once every minute
    (crontab -l | grep -v "$APPDIR/run_monitor.sh"; echo "* * * * * $APPDIR/run_monitor.sh") | sort - | uniq - | crontab -

Things seemed fine when I ran the script locally and it would update the cron job as I had expected. So, I let it go on its merry way. A couple of weeks later and the site that I was monitoring goes down but I don’t get any notifications. Again, running the above code directly from the command line worked perfectly but the cron job was not doing anything. Upon further inspection, the cron job was somehow looking for the run_monitor.sh script in the home directory. Then it dawned on me that the pwd was the problem. Since the cron job is being executed relative to the home directory, the pwd command within the script actually returns the home directory and not the location of the run_monitor.sh script. So, instead, we need the $APPDIR to point to the location of the run_monitor.sh script like this:

#!/bin/sh

APPDIR="$( cd "$( dirname "$0" )" && pwd )"
HTTP_RESPONSE=`curl -k -sL -w "%{http_code}" "http://www.google.com" -o /dev/null`

if [ ! $HTTP_RESPONSE -eq 200 ]; then
    # Replace this line to send a message to channel
    # Remove/add/update cron job to once every hour
    (crontab -l | grep -v "$APPDIR/run_monitor.sh"; echo "0 * * * * $APPDIR/run_monitor.sh") | sort - | uniq - | crontab -
else
    # Remove/add/update cron job to once every minute
    (crontab -l | grep -v "$APPDIR/run_monitor.sh"; echo "* * * * * $APPDIR/run_monitor.sh") | sort - | uniq - | crontab -

Problem solved! It’s a subtle bug that was hard to detect/debug but became obvious only after I spent some time thinking about it. I hope to never encounter this again!