Terminally Geeky: How to tell if a script is being called from launchd

Warning: command line geekiness ahead.

When writing shell scripts, I often send interactive output to the user via echo to give feedback or information. But if the shell script is called on a schedule via OS X's behind-the-scenes process launcher launchd, rather than from a Terminal session, chances are that I won't ever see that message.

Fortunately, it's relatively easy to figure out if a shell script has been called from launchd or not, simply by checking the $PPID variable. (Note: this works in zsh and may work in bash as well. If it doesn't work in your shell, this is a good time to upgrade to zsh.)

But how can I make sure my messages are seen if a shell script has been called via launchd? For that I use the Swiss Army Knife of notification tools, Growl. Specifically, I use the growlnotify optional package, which allows me to send Growl notifications from shell scripts.

For example, imagine that I wrote a script where I wanted to tell the user that a certain process had succeeded or failed. Normally I might just use

echo 'SUCCESS!'

or

echo 'FAILED!'

But now, instead of 'echo' I use a function called msg (short for 'message'). If the script was called from launchd then msg will use growlnotify, but if the script was called from the command line, it will just use echo. Here's how that works:

This method is not foolproof. For example, if you call a shell script from launchd and that shell script calls another shell script, it might not realize that it was originally executed from launchd. In practice, I have not run into that problem, but it did seem worth mentioning.