Cancel shell script on remote connect failure
I use a shell script to back up this site (and a variant of the same script to back up the Many Tricks site). I've been using these scripts for over a decade (wow), and though they've evolved, they're still fundamentally the same. (I did switch from cron to launchd for launching them, however.)
While the script typically runs very nicely, I recently noticed that my last backup was from a few weeks ago—uh oh. It didn't take long to figure out what had gone wrong: My ISP changed the hostname of the machine my site runs on, and my script uses ssh, scp and rsync, which connect via the hostname. Unfortunately, the failure mode is silence, because the script runs via a scheduled task. The only way I knew it failed was when I went to check the backup folder. Obviously, something more automatic than that would be desirable.
After much web searching, I couldn't find anything that seeemed like it'd do what I want: An email (and onscreen alert) if my backup failed. I found lots of Unix solutions to send mail using sendmail, but I didn't really want to enable that on my Mac. So I futzed around and built a simple checker that will mail me when it can't reach my web host.