How do you express those things in a systemd timer? E.g. run something 4x per day, */6 in cron.
In cron, you basically have to either use your configuration management to generate those times, or have a random delay script running before the command
In systemd timers, it's just
OnCalendar=0/6:00:00
RandomizedOffsetSec=60m
If you add
Persist=true
Nope. From crontab(5)
The RANDOM_DELAY variable allows delaying job startups by random amount
of minutes with upper limit specified by the variable. The random scal‐
ing factor is determined during the cron daemon startup so it remains
constant for the whole run time of the daemon.
I'll take payout from your lost bet now
https://dyn.manpages.debian.org/experimental/cron/crontab.5....?
OnCalendar=00/6
systemd-analyze calendar --iterations=6 '0/6:00:00'
https://www.freedesktop.org/software/systemd/man/latest/syst...
$ systemctl cat public-inbox-watch@.timer
# /etc/systemd/system/public-inbox-watch@.timer
[Unit]
Description=Periodic fetch of public mailing list
[Timer]
# twice a day
OnCalendar=*-*-* 5,17:35
RandomizedDelaySec=1h
Persistent=true
[Install]
WantedBy=multi-user.targetJust use Cron. It does one thing and does it well.
2) From the crontab(5) on my system:
The CRON_TZ variable specifies the time zone specific for the cron ta‐
ble. The user should enter a time according to the specified time zone
into the table. The time used for writing into a log file is taken from
the local time zone, where the daemon is running.
Mon,Fri *-01/2-01,03 *:30:45
[0] <https://www.freedesktop.org/software/systemd/man/latest/syst...>
[1] This question is sarcasm. SystemD is often like this... dead simple things look dead simple, but complex things are -if they're possible at all- at least as complex as they are everywhere else.
Looking at the other examples on that page, I'm gonna say that it's only arguably easier to read for basic stuff... especially if you're familiar with the syntax. The complex stuff is -at best- just as difficult.
Cron syntax is simpler for the easy cases because cron tries to do less. It ignores years and seconds entirely, and doesn't try to adhere roughly to ISO8601 ordering and field separators, instead using space universally for field separation and euro-style least-to-most significant field ordering. I like ISO8601, so I get along with systemd's style better, despite it introducing slightly more cognitive load.
The only thing that threw me for a loop and seems like "special magic" was
> "Mon *-05~07/1" means "the last Monday in May."
But good luck doing that in one line in cron.
Some cron-style libraries seem to support L/W/# for last / nearest-weekday / nth of month, but I don't know if any system crons do. (cronie? dcron? I don't think so. fcron? bcron? I don't see it there either.) '#' is syntactic sugar for DOW + 7-day range, while L is covered by the above quoted syntax.
If your cron has that kind of syntax, then for a case like "weekday closest to 1st of month", "W" is more convenient than writing 3 systemd timer rules to cover the three cases (weekday day 1, monday day 2, friday last day of month), but that's a big if. Generally you'd have to write 3 rules in cron anyway.
Two options:
0 0 25-31 5 1 0 0 L 5 1 # Run a command on the last day of the month. Only starts checking at midnight towards the end of the month. Assumes GNU date, which is fair if we're discussing a Linux-only cron-alike.
0 0 28-31 * * if [ $(date +\%d) -eq $(date --date="$(date +\%m)/1 + 1 month - 1 day" +\%d) ]; then /usr/local/bin/runCommand.sh; fi
If I were doing this for real, I'd either switch to a more capable cron, or take a serious try at the date math and then wrap it up as a standalone helper. Or I guess I'd look to see if someone already built that helper. ...I guess...
It ultimately doesn't matter "for real", because almost nobody without some horrific legacy system to integrate with would need to use anything more than lists, ranges, and increments. There's a reason nobody's added these features to system crons. Clever trigger times for events that don't really need to be triggered at clever times... sure, but lacking that capability wouldn't change anything, the trigger would just be a more boring one.
Events like "last Friday of the month" or "nearest weekday to the 1st of the month" or even "Friday the 13th" are more for business logic, not system crontabs.
I am no cron scholar, nor am I young enough to have the energy required to do an exhaustive survey... so -sadly- I don't know of one. It seems like there'd be one being used internally in at least one business in the world, though, yanno? That just seems like the sort of thing that at least one bored programmer would take a few days to crank out.
> Events like "last Friday of the month" or "nearest weekday to the 1st of the month" or even "Friday the 13th" are more for business logic, not system crontabs.
Oh quite possibly, yeah. But, system crons are quite fine for one-shot things that are re-run on a regular schedule... as well as things that don't have complicated scheduling and/or retry requirements. If you're working on a Big Enterprise Project [0], then you're almost certainly going to have a scheduler inside of you, and -IME- you're very likely to use it to do a lot of that BEP's scheduled tasks. [1]
[0] Or a tiny one wearing the clothes of a BEP
[1] ...if for no other reason than it requires little effort to get the BEP's data and code into its internal scheduler, and can be a huge pain in the ass to get it into an external one.
At least busybox's cron implements it that way:
if (line->cl_Mins[ptm->tm_min]
&& line->cl_Hrs[ptm->tm_hour]
&& (line->cl_Days[ptm->tm_mday] || line->cl_Dow[ptm->tm_wday])
&& line->cl_Mons[ptm->tm_mon]
) {But first I‘d check the behavior of other crons ;-)
I found this amusing when in combination with
> The only thing that threw me for a loop and seems like "special magic" was
but -regardless- a careful reader notes that I never said that the Timer scheduling syntax was illogical or poorly thought out. It's at least as complicated as crontab-style time syntax, which was my entire point.
Related: Not that it's ether part of the core scheduler syntax or necessarily as nice as having it in the core syntax, but my crontab(5) suggests that one can use things like date(1) to get more fine-grained control over the time of execution:
As noted above, skip values only operate within the time period they´re attached to.
For example, specifying "0/35" for the minute field of a crontab entry won´t cause
that entry to be executed every 35 minutes; instead, it will be executed twice every
hour, at 0 and 35 minutes past.
For more fine-grained control you can do something like this:
* * * * * if [ $(expr \( $(date +%s) / 60 \) % 58) = 0 ]; then echo this runs every 58 minutes; fi
0 * * * * if [ $(expr \( $(date +%s) / 3600 \) % 23) = 0 ]; then echo this runs every 23 hours on the hour; fi
Adjust as needed if your date(1) command does not accept "+%s" as the format string
specifier to output the current UNIX timestamp.
Systemd enables cleaner, simpler syntax for common cases. Instead of "59 23 * * *", simply "23:59". Instead of "0 0 * * sun[day]", simply "sun[day]". 1st of the month and don't care exactly what time? Instead of "0 0 01 * *", simply "*-*-01".
Systemd started with the principle that they wanted to accept ISO8601 timestamps, then extended that with lists, increments, ranges. They developed that into a superset of basic cron capabilities, while maintaining similar syntax as best they could for increments and lists; they diverged for ranges and nth-from-last because those conflicted with the - date separator. They repurposed the little-used ~ cron randomization operator in the process. (systemd uses a separate RandomizedDelaySec line for that, which is also arguably superior because it randomizes per trigger, not on initial load of the timer)
The alternative is how AWS does it. They have separate cron() and at() syntax. at() takes timestamps. cron() takes cron+quartz syntax, mangled to remove seconds. They also have rate(value units), because apparently cron syntax is too complicated for most people. I'm sure in theory they did it to support "every 7 minutes" cases. I bet if you surveyed actual rate() AWS schedules, 99% would be cases that evenly divide a larger time unit, and could therefore be implemented with cron().
Cron syntax is more of a mess than I thought. While basic system crons don't support any advanced features, AWS and Cloudflare have adopted LW# capabilities, derived from quartz job scheduler syntax (Cloudflare references quartz explicitly). Quartz has extra fields for seconds and year (year optional, so it must go last, which breaks d m Y sequencing) so its syntax has 6 or 7 fields vs the standard 5. CF uses 5 fields, no seconds or years. AWS uses 6 fields, years but not seconds. Whenever you interact with cron-derived systems, you have to remember not just whether they support quartz-style syntax, but whether they support seconds and/or years. Is that really simpler?
The one feature I found interesting, "W", may not work the way I anticipated according to quartz's documentation. I thought "nearest weekday to the 1st of the month" might be useful because it maps to some billing cycles, but quartz (at least through 2.3) couldn't do that. The 2.3 docs say that 1W, if the 1st falls on a Saturday, triggers on the 3rd because it won't jump backwards over a month boundary. That was my only interesting use case for "W". Systemd does everything else, though it requires more verbosity in some cases, it's simpler in many others. 2.4/2.5 are from the last couple of years, and their docs are restructured and don't mention that, so maybe it was finally fixed? I'm not about to install java crud to test.
I didn't find the *-*~01 syntax that strange (last day of every month). The "fri *-*~07/1" syntax for "last friday of every month" is what I found magical, but it makes sense now that I've thought about it more.
What would you prefer for 3rd friday of the month? Cron (5-field, quartz syntax)
0 0 ? * fri#3 (special syntax is needed because cron uses logical-or to combine day and weekday fields)
fri *-*-15..21 (works because systemd day and weekday are joined with logical-and)
[aws] https://docs.aws.amazon.com/scheduler/latest/UserGuide/sched...
I disagree. It's all a matter what you're accustomed to. On top of that, what's at least as important as clear syntax for simple cases is the establishing of a handful of rules that well serve both the simple and complex cases. Consistency that leads to complicated cases being made easy should not be avoided just to make the simple cases extremely welcoming to the newcomer. Note that I'm not saying that 'systemd-crond' falls into this trap, but I am saying that oh so many tools that aim to be "approachable by newcomers" enthusiastically throw themselves into it.
> Cron syntax is more of a mess than I thought.
Nah. It's just there are a very large number of cron implementations. If The Systemd Project were actually a thing that people could make an independent reimplementation of, we'd see at least as much inconsistency in 'systemd-cron' scheduling syntax as people extended it in their reimplementations to meet their needs.
> What would you prefer for 3rd friday of the month? Cron (5-field, quartz syntax)
I don't understand why the "day of month" column has a "?" instead of a "*", and I don't understand why "fri#3" isn't rendered as "Fri/3", but whatevz. I prefer that to the systemd syntax.
FWIW, I think you can directly translate your systemd scheduler line into this for most crons:
* * 15-21 * Fri
Note: The day of a command’s execution can be specified in the following two fields — ’day of month’, and ’day of week’. If both
fields are restricted (i.e., do not contain the "*" character), the command will be run when either field matches the current time.
For example,
"30 4 1,15 * 5" would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.
Everything else is simpler. You don't have to count fields to figure out whether you're dealing with a day vs a month, or an hour vs minute, no matter how the fields are aligned or how long or complex they are. You can scan left and right and look for space and dash and colon to figure out where you are. The formats for time and date are distinctive and ubiquitous. You don't have to guess what you're looking at when you just see one of them in isolation.
Very little follows from cron's format. You have to learn which fields are which for your particular implementation: does it include seconds? You have to learn the bizarre interaction between day and weekday, to do something non-trivial with them. You have to carefully keep track of which field you're in because you can't determine that from neighboring syntax. The visual clutter, if you want to call it that, of systemd's iso8601-based syntax only applies if you're doing complicated rules. In the vast majority of cases it would simply be an ISO8601 timestamp with various values replaced with *'s, and at most one /.
That first impression you had is what I thought too, before diving into it for my previous replies. But no, cron logic is that day and weekday are ORed. It lets you do clever things like run a script on the 1st and 15th of every month, AND on mondays, all in one line. I don't think I've ever seen such a thing in the wild. I'd go so far as to say it's stupid, because it's likely to be misinterpreted if anyone did use it that way. All the other fields are ANDed. I would say that systemd's AND logic is better... and more consistent, which is your own acceptance criterion.
Nope. I don't give a shit how "clean" something looks, [0] I just care how difficult it is to learn and either retain or remind yourself of the syntax when you need to do something complex with it. In that regard, [1] systemd-crond's scheduler syntax is at least as complicated as that of most crons'.
> You don't have to count fields to figure out whether you're dealing with a day vs a month, or an hour vs minute, no matter how the fields are aligned or how long or complex they are. You can scan left and right and look for space and dash and colon to figure out where you are.
So, you have to learn a tool-specific format. Just like you do for crontab. I've read both man pages, and I'm 100% unconvinced that systemd-crond's scheduler syntax is actually simpler. I do agree that it can express specific schedules that one cannot in the syntaxes used by most cron implementations.
> Very little follows from cron's format. You have to learn which fields are which for your particular implementation:
Sure. And I expect that exactly nothing other than systemd-crond uses its weirdo syntax. And -as I mentioned in the comment you're replying to-
Nah. It's just there are a very large number of cron implementations. If The Systemd Project were actually a thing that people could make an independent reimplementation of, we'd see at least as much inconsistency in 'systemd-cron' scheduling syntax as people extended it in their reimplementations to meet their needs.
On this I agree with you. Changing the rules for the Day Of The Week column (but only when it's used in combination with the day of the month column) is catastrophically idiotic. The only time I've ever had need to use the DoW column was to schedule something to run weekly... so I've been blissfully unaware of how incredibly idiotic that behavior is.
[0] If anything, I've noticed a strong inverse correlation between how "clean" a syntax is regarded to be and how easy it is to both do complicated things with it, and come back to those complicated pieces of work six+ months later and be able to understand what the hell you did.
[1] With the notable exception of cron's DoW column.... read on for further discussion.
Nobody is all that happy with cron's limitations. That's why there are so many variants, with varying features. When a variant has more than 5 fields, the syntax becomes ambiguous... not to a parser, but to whoever's reading and writing the lines, unless they already know the exact cron variant in use. Google, the one large cloud provider that seems to have stuck with basic cron syntax without quartz-scheduler extras, even invented their own pseudo-natural-language scheduling syntax to fill in for features cron lacks, or for people who don't like cron syntax.
Systemd's is definitely novel, but as I keep repeating, it's ISO8601 + cron. An alternative would be some extension to ISO8601 interval syntax, but that looks clunky and has probably never been used outside of niche applications for data interchange, probably inside xml files; it's used in https://docs.cibseven.org/manual/2.0/reference/bpmn20/events... (syntax at https://en.wikipedia.org/wiki/ISO_8601#Durations ). I think systemd's syntax is better than that.
The direct benefits, even if cron syntax suffices: Calendar timers are integrated with systemd, with all its process-management capabilities[1]. Unifying to systemd timers also avoids duplication of functionality ("is this scheduled process managed by systemd or cron?"). Cron doesn't let you `systemctl list-timers --all`. You'd have to grep through cron logs, possibly (and ironically) using journalctl, or have those logs already in a management dashboard, and that still doesn't let you see the next time the timer would fire. Cron also doesn't let you beta-test cron lines like you can with `systemd-analyze calendar --iterations 10 "Fri *~8..14/1"`
There's even a systemd-cron translator/generator that's in the official debian/ubuntu repos, and in Arch's AUR of course, which avoids the need for system cron but allows you to retain existing crontabs by importing them as systemd timers. They won't be quite as well integrated because it uses defaults for task names and other parameters, but the crontabs get run... without cron.
I'll repeat my opinion, as expressed in my first post:
To do nontrivial scheduling you'd use the entirely-obvious-and-intuitive syntax described at [0]. For example:
Mon,Fri *-01/2-01,03 *:30:45
Who'd ever want to go back crontab format for nontrivial scheduling? [1]
[0] <https://www.freedesktop.org/software/systemd/man/latest/systemd.time.html#Calendar%20Events>
[1] This question is sarcasm. SystemD is often like this... dead simple things look dead simple, but complex things are -if they're possible at all- at least as complex as they are everywhere else.You keep saying systemd calendar format is "at least as complicated", but it seems to me the converse is true. Simple cases are simpler to represent in systemd because fields are optional in various circumstances; complex cases, where they can be implemented in system cron at all, look equivalently complex. Removing the weekday constraint, rounding to minutes, and removing the star for year since it's optional:
31 * 01,03 01/2 *
01/2-01,03 *:31
For cron, despite years of looking at it, I can never remember what those numbers mean and I need to Google almost every time.