Cron Jobs on Linux – Comprehensive Guide with Examples
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The distinction between original Unix cron, anacron, and the at function.
What PATH applies.
Which POSIX are you on? Linux or BSD or SysV derived Unix differs in cron
Is cron even running by default? Containerised Unix may not do what you think
One syntax for /etc/cron.d (user to run as) and another for crontab -u <user> -e
The variation in which file it logs to in /var/log
Systemd and launchd and systemctl and launchctl and journalctl... (persisting daemons which run things according to the clock)
Daylight savings and ritualised habits about cronojbs at midnight
(Yes, I read the article. It's a great practical introduction to cron on most Linux situations people will find themselves in for the first time)
I have covered many of these topics here (see developer questions section)
* Do you know where it's going to send any output? If you're not certain then add a MAILTO entry at the start of the crontab to make it explicit
* Do your jobs require additional environment variables configuring?
* How noisy is your job's output? Any thing sent to STDOUT or STDERR will get emailed to either the recipients in the MAILTO option or the user that the crontab is for - initially you'll want a lot of output so you can check that it's working as expected, but once you're happy with it then you don't want all the information just any errors. There are many ways to achieve this like using --quiet options on commands to suppress output, or redirecting STDOUT to /dev/null or a file on disk (as errors should end up in STDERR).
Sep 6 02:20:01 ThinkPad-L560 CRON[3748665]: (root) CMD (/bin/bash /root/bin/backup.rsync /.git)
Sep 6 02:20:01 ThinkPad-L560 cron[3748668]: sendmail: fatal: open /etc/postfix/main.cf: No such file or directory
Sep 6 02:20:01 ThinkPad-L560 postfix/sendmail[3748668]: fatal: open /etc/postfix/main.cf: No such file or directory
Sep 6 02:20:01 ThinkPad-L560 CRON[3748664]: (root) MAIL (mailed 291 bytes of output but got status 0x004b from MTA#012)
...Yes, you need a systemd service file and a systemd timer file. Creating both is a matter of ~30m. Then, you do `systemctl start` and you're good... As a great benefit, you can see the status of each timer, which is great for debugging quick problems, like permission issues/runtime errors/... when setting the timer up.
I can write a cron entry in a minute or two, depending on the complexity.
In comparison, cron (and *nix tooling in general) asks you to learn a small bit of that tool’s syntax, and that’s it. You can add complexity if you’d like – read the man pages – but for the most part, you can be productive very quickly.
systemctl enable foo.timer --now
Simeone just needs to write some CLI for making really simple timers.
That's a post-it or a simple-systemd-timers.txt though. Don't bother Simeone with that :).
while true; do
date;
./doit.sh;
sleep 300;
done | tee doit.log
date "+%Y.%M.%dT%H:%m:%s-%Z`./doit.sh`"
1. Ensure commands are ran after you close the terminal you just opened.
2. Trigger actions when the process did fail (OnFailure=)
3. ensure the scheduling job survives system restarts
4. have proper scheduling, and handling of missed schedule doe to downtime (eg. for notebooks)
5. run the process in cgoups, under other user's context, or in a chroot, etc.
6. apply randomized jitter to the schedule to avoid crating overload and cascading failures resulting from that
7. Timing out tasks
8. Handle service dependencies...
just to name few from the top of my head...
cron is great, and systemd timers are also good, and even more refined solution to the task scheduling problem. Your propsal does not solve even the basics (configurable schedule). As other comment notes your script is more like the watch command.
There's really no reason to use cron today if you have systemd.
There’s a huge reason to use cron: it’s dead simple, doesn’t require a ton of complexity around it to function, and the syntax hasn’t changed in decades.
In a sense, you could say that Unix shell, or other tools like pipes or redirects constitute an interface that allows generic cooperation between tools written without knowledge of the other tools. So, you aren't too far off. However, the tradeoff here is that with very few very generic tools you must give up more complex things. Eg. you have to concede that everything is a string. When you can confine the interfaces to a smaller group of tools, you can take better advantage of the data they communicate (eg. compared to passing data through pipes, where you have to serialize to string and parse from string data that isn't inherently a string, you can work with structured data with many useful types).
> There’s a huge reason to use cron: it’s dead simple
systemd timers are still simpler. I.e. cron isn't complicated, but it has too many tools all over the place, many arbitrary conventions, its failures are difficult to diagnose. And it's OK. It was written at the time nobody even thought about systemd, so, they had to do some of that work themselves. Today, it's just an anachronism ;)
* I don't want to rewrite log or dependency management
* I *really* don't want to troubleshoot when it fails to function, presumably the work was important
* I barely want to set the job up at all! What service is deficient to beget this babysitting?
Go forth and recreate 'After=' and 'Requires=' for your cronjob and make it equally visible and robust, if you want, I guess. A command pipeline of all the right things and intimate knowledge/trust in their return codes will do. I'll go do something more productive or at least rewarding.
Embrace the two files, template them and never mind it again. I promise it can be worth it. I'd argue the Unix Philosophy is paradoxical, if not simply just argued in uncooperative/unproductive ways. Idealistic and impractical if lived truly.
Doing 'one thing' and doing it well actually requires doing the thing and telling others how it went... so really it's two things conflated, if not more! Enter 'systemd does too much': it's for our benefit, truly.
It's not a complete endorsement, of course. There are plenty of things I think are underdeveloped or out of bounds: spare the sniping
Pit me against some legacy holdout. Give us both a set of services and complex work to do in a real-world, representative, environment. I don't care what it is.
I guarantee my setup using "timers" will be delivered earlier and prove to be more robust; it's already been written and battle tested. It's ready as soon as I can SSH or clone my repository.
I just hear a bunch of self-selecting [read: made up] surface level complaints. Why is anyone editing these jobs so much to care if it's one file or two? Why not zero? Around two decades after 'cron' the concept of 'Configuration Management' was established.
What's more, interfacing to disable/stop on the regular isn't required with dependencies or relationships defined; the result is implicit. I didn't even mention 'PartOf=' or 'OnFailure=', more settings that allow for handling external influence.
The system will work for you, let it. I've soared through the ranks with this one simple trick: adaptation. The work isn't as unique as it is presented.
Everyone who does this enough knows it, everyone else [hopefully] rediscovers. Honest to goodness decades-long cargo culture.
I prefer not having this problem, using "systemd timers" instead. The job and any forked work have a reasonable default path, and observability is worlds better.
Did the last run succeed? When is the next? One command with timers, a guess with cron.
To where are you expecting to output to go? Does cron have permissions for that?
It also makes seeing the status of jobs and finding logs related to those jobs much easier.
Good overview is here: https://www.panozzaj.com/blog/2014/05/02/replace-local-cron-...
Sadly Jenkins is nowadays less and less popular and I'm concerned that it will become an abandon-ware at some point.
If anyone knows good open source alternative it would be great.
https://github.com/bdd/runitor
... which lets you quickly integrate any cron job with the https://healthchecks.io service. So darn handy. And you're supporting an indie startup that regularly shows up on HN.
Description (from its manpage): "Anacron can be used to execute commands periodically, with a frequency specified in days. Unlike cron(8), it does not assume that the machine is running continuously. Hence, it can be used on machines that aren't running 24 hours a day, to control daily, weekly, and monthly jobs that are usually controlled by cron."