Every time someone asks "why should I use systemd instead of cron" they get these (technically correct -- the best kind of correct!) answers about how systemd timers allow you to attach jobs to cgroups, define dependencies, test unit files separately vs. changing crontabs for 1 minute ahead (which I don't think is a "cron idiom" -- I've certainly never done i -- but we all have our weird hacks), and how it has built-in jitter options and per-job environment settings and whatnot.
These are all very relevant for real-life workloads of super advanced cloud systems, I'm sure. (Which I'm not saying in a derogatory way. I haven't written back-end code in like 12 years now, the things that are happening there today blow my mind. I know these are complex systems so I'm pretty sure they have complex problems which have complex solutions.)
But guys, I just need to run a bash script every day at 2 AM. 99% of us just need to run a bash script every day at 2 AM. This has worked reliably in cron since practically forever. I have grumpily learned the new way of doing it but I can't really justify the time I've put into it, nor the (very extensive) troubleshooting that was involved in it. I just woke up one morning and sat down in front of a Linux machine and I found out that there's a new way to run things at 2 AM.
If you want to woo me with cool systemd-timers features, tell me that it finally has an equivalent to cron's MAILTO, so that I don't have to write yet another systemd unit file that wraps a script written by me if I want to get a notification when a job has failed, because somehow that hasn't made its way into the "everything but the kitchen sink" list which otherwise includes things like attaching jobs to cgroups.
Until such a feature is implemented, you could try:
ExecStart=/bin/bash -c 'shopt -o pipefail; mycommand | mail -s [email protected]'
Certainly not as convenient as MailTo=, but a lot less complicated than using OnFailure= with a separate service that tries to mail you the output.
But to be honest, not having to do <shell> -c <exec this or send me an email> ever again is one of the reasons why we're doing the whole systemd thing, and one of its big promises. If it comes to writing shell mantras again, I might as well use vixie cron (or one of its descendants) and benefit from 30+ years of bugfixes across more Unices than I can name, plus the acquired wisdom of the Interwebs.
Edit: plus... does this still allow you to get the other goodies, like being able to specify environment variables for a specific job?
As for 'exec this or send me an email' -- I would also much prefer to see this be done by systemd itself. Fortunately bash's 'pipefail' feature plus bsd-mailx's '-s' flag make this reasonably quick and sane; I wouldn't even bother to attempt it if I were stuck with POSIX sh or if the mail command lacked the logic to not send the mail if stdin is empty.
I thought that environment variable passed via the Environment= options might not be correctly set, but it seems they are :-).
Instead, try GNU mcron (guile-based).
"The mcron program represents a complete re-think of the cron concept originally found in the Berkeley and AT&T unices, and subsequently rationalized by Paul Vixie. The original idea was to have a daemon that wakes up every minute, scans a set of files under a special directory, and determines from those files if any shell commands should be executed in this minute.
The new idea is to read the required command instructions, work out which command needs to be executed next, and then sleep until the inferred time has arrived. On waking the commands are run, and the time of the next command is computed. Furthermore, the specifications are written in scheme, allowing at the same time simple command execution instructions and very much more flexible ones to be composed than the original Vixie format. This has several useful advantages over the original idea. (Changes to user crontabs are signalled directly to mcron by the crontab program; cron must still scan the /etc/crontab file once every minute, although use of this file is highly discouraged and this behaviour can be turned off). "
https://www.gnu.org/software/mcron/manual/mcron.html#Introdu...
Given the narrowness of the domain is there a good reason not to use English?
If you use *BSD, Jenkins, and everything else you use the standard cron syntax.
I'm so going to steal this
giving gold
(e.g. put all your logic in one script, test it with “env -“, lock exclusively with flock if you need it, or whatever else you have to hand in your language of choice.)
Package repo updates and Let's Encrypt renewals are a couple services I've seen use this. There isn't a great way to do this with cron that I know of, besides running some script that sleeps for a random duration.
That said, I still use cron for things that don't need this feature because it's portable and I'm just more familiar with it.
There are of course quite a few people who dislike systemd for justifiable reasons (but I doubt it receives the disdain that SELinux does), here's a starting point:
That said, I very much like systemd and I hope it isn't going anywhere. I think it's improved on a lot of things from the SysVinit days and I hope and believe that its shortcomings will be addressed and improved in the future.
So the super dismissive "systemd is better" response is kind of missing the point anyways, even with any type of legitimate justification.
For whatever reason it never works for me, including timers.
But I deploy a lot of services running as specific users and it works well. Good overview here:
https://wiki.archlinux.org/index.php/systemd/User
(You can install global services, and add `user=x`, etc, obviously. But I like to keep things self-contained.)
I hate this kind of syntax.