Pragmatic Automation

Programs That Growl

Growl is a central notification system for Mac OS X. Applications and scripts can uniformly send notifications to Growl, which then displays the notification message on your screen in a stylish, modeless dialog. Growl works with a growing number of applications. More important, it’s easy to make your program growl using Growl’s command-line program, bindings for Python, Perl, and Tcl, or the Cocoa framework.

Now I find too many pop-up dialogs to be distracting, so I want to limit the notifications I receive to events that require my immediate attention. One example usage for Growl that comes to mind is monitoring a web site. If an important site goes down or gets sick, I want to be interrupted. Here’s a simple shell script that scrapes a web page using curl and grep, then uses the bare-bones growlnotify command-line program to send a notification if the web page isn’t available or if it’s displaying an error message:

  #!/bin/sh
  #
  # growl.sh - Makes a sick web site growl
  #

  url=$1
  outputfile="/tmp/growl-$$.html"
  message=""

  trap "rm -f $outputfile" 0

  if curl -so $outputfile $url
  then
    if grep -qiE "Error|Exception" $outputfile
    then
      message="Help! I'm sick!"
    else
      exit 0  # success
    fi
  else
    message="Sadly, the site crashed."
  fi

  echo $message | growlnotify "Web Site Status"

To test it on my DMS web application, I type:

  $ growl.sh http://localhost:8080/dms

When things have gone bad, here’s the notification I get, which fades nicely after a couple of seconds:

Scheduling the growl.sh script with cron is a cheap, but effective, way to periodically check the web site and rattle my cage if the site needs help. If you want more control, the Growl preferences pane in System Preferences lets you disable notifications that you don’t want to see.

One obvious limitation of Growl is that your have to watch your screen for the notification. Ideally, you’d also use cell phone notification for monitoring on the go, as described in the book.