6 ms·

How do you express those things in a systemd timer? E.g. run something 4x per day, */6 in cron.

by eqvinox 5mo ago

iam-TJ 5mo ago
$ 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.target
- eqvinox 5mo ago
  I, er, what? It's... the same as cron? I'm confused now. It's not exactly the same, I guess?
- PenguinCoder 5mo ago
  So the `OnCalendar` stanza is the same as a Cron job; without the helpful comments of the ordering? And that is considered _easier_?? Just use Cron. It does one thing and does it well.
  - paradox460 5mo ago
    The systemd one supports timezones. You can say America/Denver and the like, or Etc/UTC
    - simoncion 5mo ago
      1) Your production equipment doesn't have its TZ set to UTC? Enjoy dealing with the intermittent and irregular hassle of DST changes, I guess. 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. If you have a job you need scheduled in a different timezone, dump a new file in /etc/cron.d, alter its CRON_TZ variable and go to town, as it were.
ecnahc515 5mo ago
Something like: OnCalendar=00/6 You can test it with: systemd-analyze calendar --iterations=6 '0/6:00:00' The format is `DayOfWeek Year-Month-Day Hour:Minute:Second` https://www.freedesktop.org/software/systemd/man/latest/systemd.time.html https://www.freedesktop.org/software/systemd/man/latest/syst...
PunchyHamster 5mo ago
That's simple but consider "run something 4x per day but randomize a delay by hour so all of the 200 servers doing that task won't run it all at once" 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 and the offset generated will be stable for the job on a given machine (i.e. always same on this machine but different on others) so you will get nice uniform distribution of load. If you add Persist=true the job will also be run once if there was one or more scheduled runs when the machine was down
- simoncion 5mo ago
  > 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 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. That's from my cronie install, but it looks like this has been a feature of some crons for at least a decade. (Notice that the post date of [0] is in 2016.) Given that cronie is based on vixie-cron, and I think I was was using vixie-cron in 2002, I bet it's been a thing for at least twenty years. [0] <https://stackoverflow.com/a/34815984 https://stackoverflow.com/a/34815984>
  - PunchyHamster 5mo ago
    still not a thing in vixie cron installed by default in Debian in 2026. I'll take payout from your lost bet now https://dyn.manpages.debian.org/experimental/cron/crontab.5.en.html https://dyn.manpages.debian.org/experimental/cron/crontab.5....?
- PenguinCoder 5mo ago
  `* 1 * * * sleep $(( $(od -N1 -tuC -An /dev/urandom) \% 45 ))m ; <your command here>`
simoncion 5mo ago
Sure, I'll pile on here. 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 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.
- p0358 5mo ago
  If you know the syntax, it's still actually rather trivial. Still easier to read than advanced cron magic.
  - simoncion 5mo ago
    > Still easier to read than advanced cron magic. 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.
- harshreality 5mo ago
  I found the systemd time spec syntax you referenced to be logical and well thought out. 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.
max-privatevoid 5mo ago
It's a mystery to me why everyone tries to use OnCalendar here, when "n amount of times within a certain timeframe" can be done much more easily with OnActiveSec, in this case that'd be OnActiveSec=6h.

How do you express those things in a systemd timer? E.g. run something 4x per day, */6 in cron.

by eqvinox 5mo ago

How do you express those things in a systemd timer? E.g. run something 4x per day, */6 in cron.

$ 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.target

eqvinox 5mo ago
I, er, what? It's... the same as cron? I'm confused now. It's not exactly the same, I guess?
PenguinCoder 5mo ago
So the `OnCalendar` stanza is the same as a Cron job; without the helpful comments of the ordering? And that is considered _easier_?? Just use Cron. It does one thing and does it well.
- paradox460 5mo ago
  The systemd one supports timezones. You can say America/Denver and the like, or Etc/UTC
  - simoncion 5mo ago
    1) Your production equipment doesn't have its TZ set to UTC? Enjoy dealing with the intermittent and irregular hassle of DST changes, I guess. 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. If you have a job you need scheduled in a different timezone, dump a new file in /etc/cron.d, alter its CRON_TZ variable and go to town, as it were.

ecnahc515 5mo ago

Something like: OnCalendar=00/6 You can test it with: systemd-analyze calendar --iterations=6 '0/6:00:00' The format is `DayOfWeek Year-Month-Day Hour:Minute:Second` https://www.freedesktop.org/software/systemd/man/latest/systemd.time.html https://www.freedesktop.org/software/systemd/man/latest/syst...

PunchyHamster 5mo ago

That's simple but consider "run something 4x per day but randomize a delay by hour so all of the 200 servers doing that task won't run it all at once" 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 and the offset generated will be stable for the job on a given machine (i.e. always same on this machine but different on others) so you will get nice uniform distribution of load. If you add Persist=true the job will also be run once if there was one or more scheduled runs when the machine was down

simoncion 5mo ago
> 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 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. That's from my cronie install, but it looks like this has been a feature of some crons for at least a decade. (Notice that the post date of [0] is in 2016.) Given that cronie is based on vixie-cron, and I think I was was using vixie-cron in 2002, I bet it's been a thing for at least twenty years. [0] <https://stackoverflow.com/a/34815984 https://stackoverflow.com/a/34815984>
- PunchyHamster 5mo ago
  still not a thing in vixie cron installed by default in Debian in 2026. I'll take payout from your lost bet now https://dyn.manpages.debian.org/experimental/cron/crontab.5.en.html https://dyn.manpages.debian.org/experimental/cron/crontab.5....?
PenguinCoder 5mo ago
`* 1 * * * sleep $(( $(od -N1 -tuC -An /dev/urandom) \% 45 ))m ; <your command here>`

simoncion 5mo ago

Sure, I'll pile on here. 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 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.

p0358 5mo ago
If you know the syntax, it's still actually rather trivial. Still easier to read than advanced cron magic.
- simoncion 5mo ago
  > Still easier to read than advanced cron magic. 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.
harshreality 5mo ago
I found the systemd time spec syntax you referenced to be logical and well thought out. 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.

max-privatevoid 5mo ago

It's a mystery to me why everyone tries to use OnCalendar here, when "n amount of times within a certain timeframe" can be done much more easily with OnActiveSec, in this case that'd be OnActiveSec=6h.