Schedule a cron task to be repeated every day?

Question

All the google results mention cron jobs. They don't work, I followed multiple guides exactly, and the script is never launched.

I just need to set the brightness to change at morning and dusk, it seems like a pretty simple task to me, but it's surprising that I can't figure it out. I use brightnessctl to control the brightness of my laptop, and it does not require sudo.

I'm running Arch Linux. Installed cronie, in terminal I crontab -e, and in the file I write 41 17 * * * dunstify "timer", which is the time a few minutes after I wrote that, as a test to see if it runs.

I save the file, I enable and start the cronie service in systemctl, then type crontab -l, and there's "no crontab for indeedwatson".

EDIT: Crontab error: no crontab For [user]

This post led me to this command sudo crontab -u indeedwatson -e from which I can create a crontab that does show up for crontab -l. I don't understand why. I'm using sudo to create it, but not to list them, and in any case, all the guides do not require the -u flag, to me it seems implied by running it without sudo, it's my own user.

Answer 1

Without knowing which Linux/Unix distribution you use, or what configuration changes you've made, or what command you've used in your cron job, it's tough to say for sure. I'll make some assumptions:

1. Assuming that your brightness command actually works from the command line, and that it requires su privileges:

Instead of using the normal user crontab, use the root crontab; i.e. open your crontab for editing with sudo. In this way, your cron job will run with the required privileges:

sudo crontab -e

2. Assuming you're not using a redirect to get the benefit of potentially helpful stderr & stdout, try that in your cron job; redirects are the >> to designate a file, and 2>&1 to include stderr and stdout :

...
0 7 * * * /full/path/to/bright-adj-pkg >> /home/user/mylog.txt 2>&1
0 19 * * * /full/path/to/bright-adj-pkg >> /home/user/mylog.txt 2>&1
...