`launchd` notes to self (mostly)

Since Apple has deprecated cron on macOS, I've decided to get my feet wet with its replacement, launchd. I used to used cron to automate post publishing. One downside of static site generators is that you can't SELECT * FROM posts where pub_date < NOW(). You have to find another solution. For me, that was cron, until it wasn't.

launchd — specifically, the .plist file syntax — is easy to learn. There's a whole man page for launchd.plist that will help you write your own agents. I also found launchd.info helpful.

Status codes, on the other hand, can be opaque and hard to search. Running launchctl list gives you a list of running agents. Its output looks a bit like the example below.

693     0       com.apple.CoreAuthentication.agent
632     0       com.apple.syncdefaultsd
-       0       com.apple.sidecar-display-agent
652     0       com.apple.accessibility.heard
653     0       com.apple.corespeechd
-       0       com.apple.AMPSystemPlayerAgent
777     0       com.apple.itunescloudd
-       0       com.apple.scrod
-       0       com.myusername.publishblog

The first column indicates the process identifier (PID). Not every agent will have one. The second column is the status code, if any, that occurs when this agent runs. The last column reflects the agent label, as specified by the <Label> field in the corresponding .plist file.

A 0 in the second column indicates that launchd was able to run that file without any issues. Other values may indicate a problem.

This is my cheat sheet for those values. I may be wrong about the details, but they should help you (and future me) figure out where to begin. I'm sure there's a list of these values somewhere.

Error code	Possible cause
0	Everything is okay. There are no issues with your `.plist` file, or any commands it calls.
29	Your command wrote something to stdout. The command ran just fine, but you may wish to stop writing to stdout. Alternatively, add a `StandardOutPath` property key to your `.plist` and write to a file instead.
78	Function not implemented. Check that your `.plist` file is calling the correct command or referencing the correct script.
255	Your script has a runtime error and can't be executed.

Possibly helpful things written by other people

MacOS launchd plist StartInterval and StartCalendarInterval examples: A quick introduction to launchd.plist with code examples that you can adapt to your needs.
Mac crontab: Creating macOS startup jobs with crontab, er, launchd: A post detailing how to write your own launchd agent.
Creating Launch Daemons and Agents: How to use launchd, straight from Apple's developer documentation.