Systemd is a pretty neat little thing. It's what keeps me running Ubuntu and Debian servers instead of switching to Alpine, at least in those situations where I can't containerize what I want to do.
The thing that got me into it was systemd timers. I really like being able to split up what should run from when should it run. `cron` works best if you desire simplicity above all else, and Slack used `cron` for a very long time before moving to something more custom. But I ultimately don't feel like the learning curve for systemd timers is that bad, and the benefits it can bring (like writing all your echo statements to journalctl instead of some random hard coded logfile in a script) feel very worth it to me.
`cron` works best if you desire simplicity above all else
It may be simple, but it's config file is hard to comprehend compared to a systemd timer. Being able to just use "Friday 17:00" or "hourly" is so much more readable.
Debugging and tracability is just nuts too. With a timer and a run-once service it's easy to see what happens, happened, and will happen:
systemctl status foo.timer tells you when it has last triggered and when it will triggered next, if it is still enabled.
systemctl status foo.service tells you if it has been triggered from the timer and when, or ran manually with start
I get the conceptual utter simplicity of cron in place of timer + a script in place of service but in practice systemd is much simpler and more consistent to manage.
Exactly, and there are still alot of old school "cron" users who don't touch the better "anacron" processes. I hate managing that stuff.
Systemd > Anacron > Cron
Even worse, when I was finding "crons" run from Java apps with cron4j. I hated on SystemD a lot in the early days, and there are still many legit criticisms that don't have good answers, but I have come to appreciate it's power and usefulness very much. Writing your own units seems so much cleaner than the old way, and I say that as a everyday bash script writer/user. (example, systemd-nspawn for containerization)
PS: mcron is what I have been tinkering with and think will probably replace my systemd timers for most purposes.
I started writing a "oh, I never found it that difficult" comment. Then I thought to test my own belief and tried to type out a cron schedule for "run this every hour", and... Well...
As well as the already-mentioned `n * * * *` solution, there's also dropping a symlink in /etc/cron.hourly to the program to run. Or a 2-line shell script if you need to add command-line params. (The other line is the shebang.)
The cron.hourly directory is a non-standard Linux concept provided by run-parts. The FreeBSD equivalent is periodic, but that's really for system tasks and doesn't have anything more granular than daily.
Systemd itself, including timers, is deeply coupled with the Linux API, and cannot be (easily) ported on any other system. Cron, on the other hand, exists on almost every UNIX-like system.
systemd is is many things, but obvious and accessible is not one of them. It has a lot of power and flexibility, but the trade off is a lot of complexity. Just look at the unit configuration file:
I have no idea from that documentation how I'd run something every hour. I guess I have to create a new unit file, and use the OnCalendar stanza? But what do I set it to? I'm directed to this page:
I started my career as a Unix SA over 25 years ago and have worked with a lot of different Unix-flavored operating systems: SunOS/Solaris, HP/UX, FreeBSD, Linux (Slackware, RedHat, Debian), macOS/OS X, AIX. I'm familiar with all their different variations of init.
All of them are esoteric in one way or another. Some of them have their behavior right out in the open where it's easy to see (inittab and rc scripts). Others hide away massive complexity (launchd and systemd) and require extensive documentation to understand.
I appreciate all the power that systemd provides. It gets a lot of things right. But in terms of complexity, it's almost an operating system unto itself.
I'm not quite sure why you felt pressed to write this long-winded response to something I didn't write. I never said that systemd is easy. I said that if someone is running *BSD, today, in 2024, they'll cope with cron's syntax.
I'll still reply to one part:
Oh, I see I can use "hourly" but it turns out that's syntactic sugar for:
*-*-* *:00:00
Is that really any easier than the cron equivalent?
0 * * * *
...Yes? Ask someone who's not familiar either with cron or systemd to guess what each one mean, with the context that it's supposed to be something related to dates and times. If you're vaguely familiar with how dates are usually written down, and that a star is a wildcard, you can immediately guess that the first one means "any year, any month, any day, any hour, at the first minute, first second". The cron one? It's anyone's guess.
Hah, that's fair. I inferred from your comment the corollary that systemd is not arcane, and thought "oh, it's just arcane differently," hence my long-winded reply.
You can argue that no other system matters (which I strongly disagree with for reasons I don't want to get into right now), but that doesn't make systemd timers any more portable.
I’d argue that the degree of portability implied by the term is very much context dependent. “Portable between Linux systems” is a perfectly valid use of the term, imo.
Do you take portable to mean a program that can run on every processor arch and OS available?
Do you take portable to mean a program that can run on every processor arch and OS available?
The definition of the word "portable" is not the point of this comment thread (although if you want to hear my definition, see the reply to a sibling comment).
Let's take the context into account:
> hashworks:
While I support systemd timers over cron, AFAIK cron has stuff like @hourly.
> caiusdurling:
Some cron implementations do, it's not portable.
> bheadmaster:
Neither are systemd timers. I don't think there's a single system out there implementing the systemd timer interface.
User caiusdurling said @hourly is not portable between cron implementations as a way to discredit hashworks' argument about cron having @hourly.
However, that's a disingenious argument, because systemd timers aren't any more portable than cron implementations that use @hourly - in fact, there isn't a single system out there implementing the systemd timer interface except systemd.
Portable means easy to make it work while changing the underlying system or its implementation details - where the underlying system refers to architecture, OS, API or any other dependencies needed to run the program.
In context of systemd, systemd isn't portable because it highly depends on Linux API and thus cannot be ported on any other UNIX-like (or unlike) OS.
In context of systemd timers, systemd timers aren't portable because there isn't as single alternative implementation that can interpret systemd timer unit files.
Comments
Systemd is a pretty neat little thing. It's what keeps me running Ubuntu and Debian servers instead of switching to Alpine, at least in those situations where I can't containerize what I want to do.
The thing that got me into it was systemd timers. I really like being able to split up what should run from when should it run. `cron` works best if you desire simplicity above all else, and Slack used `cron` for a very long time before moving to something more custom. But I ultimately don't feel like the learning curve for systemd timers is that bad, and the benefits it can bring (like writing all your echo statements to journalctl instead of some random hard coded logfile in a script) feel very worth it to me.
It may be simple, but it's config file is hard to comprehend compared to a systemd timer. Being able to just use "Friday 17:00" or "hourly" is so much more readable.
Debugging and tracability is just nuts too. With a timer and a run-once service it's easy to see what happens, happened, and will happen:
systemctl status foo.timer tells you when it has last triggered and when it will triggered next, if it is still enabled.
systemctl status foo.service tells you if it has been triggered from the timer and when, or ran manually with start
I get the conceptual utter simplicity of cron in place of timer + a script in place of service but in practice systemd is much simpler and more consistent to manage.
Exactly, and there are still alot of old school "cron" users who don't touch the better "anacron" processes. I hate managing that stuff. Systemd > Anacron > Cron
Even worse, when I was finding "crons" run from Java apps with cron4j. I hated on SystemD a lot in the early days, and there are still many legit criticisms that don't have good answers, but I have come to appreciate it's power and usefulness very much. Writing your own units seems so much cleaner than the old way, and I say that as a everyday bash script writer/user. (example, systemd-nspawn for containerization)
PS: mcron is what I have been tinkering with and think will probably replace my systemd timers for most purposes.
https://www.gnu.org/software/mcron/manual/mcron.html#Introdu...
You're touching on something subtle where, like markdown, all cron look the same until they differ.
I started writing a "oh, I never found it that difficult" comment. Then I thought to test my own belief and tried to type out a cron schedule for "run this every hour", and... Well...
https://crontab.guru/#*_0/1_*_*_*
Oops. Point taken :)
As well as the already-mentioned `n * * * *` solution, there's also dropping a symlink in /etc/cron.hourly to the program to run. Or a 2-line shell script if you need to add command-line params. (The other line is the shebang.)
The cron.hourly directory is a non-standard Linux concept provided by run-parts. The FreeBSD equivalent is periodic, but that's really for system tasks and doesn't have anything more granular than daily.
Yeah, but the comparison was specifically comparing to systemd timers, so I'm pretty sure the GP wasn't considering non-Linux compatibility anyway.
Run this every hour is:
Adjust the leading digit to whichever minute of the hour you wish to run this.While I support systemd timers over cron, AFAIK cron has stuff like @hourly.
Some cron implementations do, it's not portable.
Neither are systemd timers. I don't think there's a single system out there implementing the systemd timer interface.
What do you mean? There's a single implementation as far as I know, deployed on many distributions and working identically.
That's exactly what I mean.
Systemd itself, including timers, is deeply coupled with the Linux API, and cannot be (easily) ported on any other system. Cron, on the other hand, exists on almost every UNIX-like system.
I'm sure the few people who run BSD systems will be fine with cron's arcane syntax.
systemd is is many things, but obvious and accessible is not one of them. It has a lot of power and flexibility, but the trade off is a lot of complexity. Just look at the unit configuration file:
https://www.freedesktop.org/software/systemd/man/latest/syst...
I've never heard of a systemd timer before, but I guessed it was a type of unit file, so I just read the documentation:
https://www.freedesktop.org/software/systemd/man/latest/syst...
I have no idea from that documentation how I'd run something every hour. I guess I have to create a new unit file, and use the OnCalendar stanza? But what do I set it to? I'm directed to this page:
https://www.freedesktop.org/software/systemd/man/latest/syst...
Oh, I see I can use "hourly" but it turns out that's syntactic sugar for:
Is that really any easier than the cron equivalent? Which can also be written (on FreeBSD) as: https://man.freebsd.org/cgi/man.cgi?query=crontab&sektion=5&...I started my career as a Unix SA over 25 years ago and have worked with a lot of different Unix-flavored operating systems: SunOS/Solaris, HP/UX, FreeBSD, Linux (Slackware, RedHat, Debian), macOS/OS X, AIX. I'm familiar with all their different variations of init.
All of them are esoteric in one way or another. Some of them have their behavior right out in the open where it's easy to see (inittab and rc scripts). Others hide away massive complexity (launchd and systemd) and require extensive documentation to understand.
I appreciate all the power that systemd provides. It gets a lot of things right. But in terms of complexity, it's almost an operating system unto itself.
I'm not quite sure why you felt pressed to write this long-winded response to something I didn't write. I never said that systemd is easy. I said that if someone is running *BSD, today, in 2024, they'll cope with cron's syntax.
I'll still reply to one part:
...Yes? Ask someone who's not familiar either with cron or systemd to guess what each one mean, with the context that it's supposed to be something related to dates and times. If you're vaguely familiar with how dates are usually written down, and that a star is a wildcard, you can immediately guess that the first one means "any year, any month, any day, any hour, at the first minute, first second". The cron one? It's anyone's guess.
Hah, that's fair. I inferred from your comment the corollary that systemd is not arcane, and thought "oh, it's just arcane differently," hence my long-winded reply.
Portable has a well-defined meaning.
You can argue that no other system matters (which I strongly disagree with for reasons I don't want to get into right now), but that doesn't make systemd timers any more portable.
I’d argue that the degree of portability implied by the term is very much context dependent. “Portable between Linux systems” is a perfectly valid use of the term, imo.
Do you take portable to mean a program that can run on every processor arch and OS available?
The definition of the word "portable" is not the point of this comment thread (although if you want to hear my definition, see the reply to a sibling comment).
Let's take the context into account:
User caiusdurling said @hourly is not portable between cron implementations as a way to discredit hashworks' argument about cron having @hourly.However, that's a disingenious argument, because systemd timers aren't any more portable than cron implementations that use @hourly - in fact, there isn't a single system out there implementing the systemd timer interface except systemd.
Okay, can you define precisely what portable means?
Portable means easy to make it work while changing the underlying system or its implementation details - where the underlying system refers to architecture, OS, API or any other dependencies needed to run the program.
In context of systemd, systemd isn't portable because it highly depends on Linux API and thus cannot be ported on any other UNIX-like (or unlike) OS.
In context of systemd timers, systemd timers aren't portable because there isn't as single alternative implementation that can interpret systemd timer unit files.
systemd also has shorthands like "hourly": https://www.man7.org/linux/man-pages/man7/systemd.time.7.htm...
The man page lists: minutely, hourly, daily, monthly, weekly, yearly, quarterly, semiannually (and "annually" further down in examples).
It doesn't have a "hourly" directory where you can drop in scripts though, AFAIK.
And SLURM's scrontab has @fika, @teatime, and - as of yesterday - @elevenses.
I also enjoy that you can ask systemd to validate your timer/cron, in case you struggle with the syntax:
As a sysadmin, sandboxing and the general ease of use is what really sold systemd to me.
journalctl alone makes it so good IMO