The other day I had one of those tasks that occasionally pop-up in the life of a developer where I had to perform a one-off action to one of my company’s applications. I say the other day but of course I mean the other night, because the action in question had to happen at 3am. Not for all the will in the world am I getting up at 3am to run a shell script. Thankfully I don’t have to 
Cron for MacOSX
So the solution I’m looking for is to schedule running a shell script using my development laptop (a Mac with OSX on). Obviously there is a strong argument for NOT launching these type of jobs on a development computer, there should be a more robust server bound solution for running scheduled tasks, and I completely agree. However the reality is sometimes it’s easier to do something ad-hoc, especially if there is no existing process for one-off scheduled tasks in your existing infrastructure.
In these situations Cron is the perfect solution. You can leave you development computer on and the task will be fired when you configure it to do so. So, how do configure a cron job to run on a Mac?
Configuring a Cron Job on Mac
To open your current scheduled task list, tap the following command into your terminal:
crontab -e
This with open a vim editor with the list of jobs (for the current user). This should be empty unless you’ve created any cron jobs recently.
(If you’re not familiar using vim to edit files before, check out a vim cheat sheet like this (for example).)
So let’s add a new cron job. But first, a gotcha…
I came across an issue whereby if I used any command line utilities in shell script, cron couldn’t find that utility, because it doesn’t have access to the $PATH env var. So before you add any cron jobs to you’re crontab file copy in your $PATH and $SHELL vars to the top of the file:
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
Once that is done, let’s add a job.
Cron works by assigning a time and and excution path in the following format (see here for extra details)
[minute] [hour] [day of month] [month] [day of week] /pathto/shellscript
Say we want to add cron job to execute a script on 2:14am August 29th
14 02 29 8 * /skynet/becomes-self-aware.sh
(* in this instance means any or ignore (sort of))
It’s best to add the full path to the script as the tilde ~ home shortcut can cause side effects.
Once you’ve saved the file and exited (:x in vim) cron will tell you it’s configuration has been updated.
crontab: installing new crontab
Tell me more…
If you want to pass in arguments to the shell script just add them after the path to the script.
14 02 29 8 * /skynet/become-self-aware.sh nukem --all
Logging
Logging out the stdout and stderr output from the file is a good idea, because cron logging is non-existent on mac. The following command will append both outputs to a judgment-day.log.
14 02 29 8 * /skynet/become-self-aware.sh nukem --all >> /skynet/judgment-day.log 2>&1
Debugging
Debugging using the logs you’ve just created is good start, additionally cron will send you mail!
Enter mail into a terminal after you’ve expected the cron job to have ran, and you should see a list of mail (in this instance I’ve told cron to schedule a echo 'hello from cron' command).
Mail version 8.1 6/6/93. Type ? for help.
"/var/mail/steve": 1 message 1 new
>N 1 steve@mac.l Thu Jun 6 15:06 19/729 "Cron <steve@mac> "
?
Hit <enter> to read the mail.
Message 1:
From steve@mac.local Thu Jun 6 15:06:01 2019
X-Original-To: steve
Delivered-To: steve@mac.local
From: steve@mac.local (Cron Daemon)
To: steve@mac.local
Subject: Cron <steve@mac> echo 'hello from cron'
X-Cron-Env: <SHELL=/bin/bash>
X-Cron-Env: <PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin>
X-Cron-Env: <LOGNAME=steve>
X-Cron-Env: <USER=steve>
X-Cron-Env: <HOME=/Users/steve>
Date: Thu, 6 Jun 2019 15:06:01 +0100 (BST)
hello from cron
As you can see there’s some basic details including when the cron job has been executed and superficial output.
(For more details on the mail utility enter man mail into a terminal.)
Conclusion
Cron is really useful utility for running scheduled tasks and it relatively simple to set up once you’ve navigated a few pitfuls (and manage to escape vim!)
One warning though, might be a good idea to delete one-off jobs once they are done, as they will happen exactly the same time next year if you’ve followed this guide!
Just crontab -e and delete the offending job then save the file.
Hope you have found this blog post useful!