6 ms·

Show HN: Cronexpr, a Rust library to parse and iter crontab expression

160 points by tison 2y ago

cuu508 2y ago
Looks well done. I like how the docs describe every syntax feature, and every non-standard feature in detail. Hat tip to implementing Vixie's "*,10" quirk, and to handling DST.
- tison 2y ago
  You're welcome! As described in the "Why do you create this crate?" section, I actually started this domain only a few weeks ago. So I experienced how those tribal rules can be confusing and hard to search over the Internet the understand their semantic. And when I sorted out the parse structure [1] and finished the extensions [2][3], I believe I should write it down for others (and future me) :D [1] https://github.com/tisonkun/cronexpr/pull/4 https://github.com/tisonkun/cronexpr/pull/4 [2] https://github.com/tisonkun/cronexpr/pull/5 https://github.com/tisonkun/cronexpr/pull/5 [3] https://github.com/tisonkun/cronexpr/pull/6 https://github.com/tisonkun/cronexpr/pull/6
  - vlovich123 2y ago
    Is there a comparison to other cron parsing libraries like Saffron? https://github.com/cloudflare/saffron https://github.com/cloudflare/saffron
    - tison 2y ago
      Croner has a table for the syntax supported by it and saffron [1], you can compare it with the one cronexpr supported. [1] https://github.com/hexagon/croner-rust?tab=readme-ov-file#why-croner-instead-of-cron-or-saffron https://github.com/hexagon/croner-rust?tab=readme-ov-file#wh... As supported in [2], > There are several good candidates like croner and saffron, but they are not suitable for my use case. Both of them do not support defining timezone in the expression which is essential to my use case. Although croner support specific timezone later when matching, the user experience is quite different. Also, the syntax that croner or saffron supports is subtly different from my demand. > > Other libraries are unmaintained or immature to use. > > Last, most candidates using chrono to processing datetime, while I’d prefer to extend the jiff ecosystem. [2] https://docs.rs/cronexpr/latest/cronexpr/#why-do-you-create-this-crate https://docs.rs/cronexpr/latest/cronexpr/#why-do-you-create-...
Black616Angel 2y ago
Main drawback is the need for timestamps. Most crontab files or expressions I've seen didn't have them.
- assiniboine 2y ago
  It doesn't need timestamps. It parses crontab to/from timestamps where needed.
- tison 2y ago
  Could you elaborate a bit on the issue? I'm not sure you are commenting on cronexpr or other libraries. In cronexpr, there is no requirement for a timestamp until you'd like to find the next scheduled time, and thus you need to provide a related point. To decouple with certain datetime lib, I made a `MakeTimestamp` struct which provides multiple constructors. Later, I found it somehow like a function overload :D
  - deleted 2y ago
    [deleted]
- tricked 2y ago
  Do you mean timezones? if so it shouldn't be that hard to detect if there's no timezone and add it with your own code.
- tison 2y ago
  If you mean timezone, I wrote an FAQ [1]: "Why does the crate require the timezone to be specified in the crontab expression?" [1] https://docs.rs/cronexpr/latest/cronexpr/#why-does-the-crate-require-the-timezone-to-be-specified-in-the-crontab-expression https://docs.rs/cronexpr/latest/cronexpr/#why-does-the-crate...
  - aaomidi 2y ago
    So many of these restrictions feels arbitrary. Not supporting comments? Why? Assuming UTC for tz is not weird and cron users expect it, I guess why even support this specific syntax if the crons need to be edited to fit how this code expects them? There may have been better options if you were going full green field. Good work though!
Annatar 2y ago
[dead]
faangguyindia 2y ago
I've LLM based crontab bot. Where I simply write stuff like "run my backupBitBuckettoDropbox.sh from home directory at 3am everyday". I never really liked cronjob expressions.
- Jleagle 2y ago
  This sounds great, if i could trust it to get it right 100% of the time.
  - simonw 2y ago
    With the right harness around it you could trust it. Set it up so that it takes your input, generates the cron expression, then uses a deterministic cron library to spit out a summary of what that expression does plus a list of next upcoming instances. That should give you all the information you need to determine if it got the right expression.
    - encoderer 2y ago
      You think it would work to just pipe that description and next occurrences and ask an llm if it matches the intended prompt?
      - simonw 2y ago
        No, that still leaves a small probability of something weird happening - and since it's a small probability you might not ever spot it in your own testing. In this case I think it's better to run known, deterministic code that can turn a crontab into a clear explanation.
    - krupan 2y ago
      Wow. The things people will do to avoid learning cron syntax
qwertox 2y ago
If you're into Python, APScheduler is the way to go. It also has a cron scheduler [0] which includes scheduling down to seconds and perfectly integrates with asyncio [1]. async def myfunc(): print(f"\n{Fore.GREEN}Readout triggered at { datetime.now().strftime('%H:%M:%S') }{COLORS_RESET}\n") await disk_temperatures_handler.readout(storage) scheduler = AsyncIOScheduler() storage['scheduler'] = scheduler scheduler.add_job(myfunc, CronTrigger(second='*/15'), id='readout_job') scheduler.start() You can also modify the interval on-the-fly. [0] https://apscheduler.readthedocs.io/en/3.x/modules/triggers/cron.html#introduction https://apscheduler.readthedocs.io/en/3.x/modules/triggers/c... [1] https://apscheduler.readthedocs.io/en/3.x/modules/executors/asyncio.html#apscheduler.executors.asyncio.AsyncIOExecutor https://apscheduler.readthedocs.io/en/3.x/modules/executors/...
- deleted 2y ago
  [deleted]
- franga2000 2y ago
  +1 for APScheduler, we've been running it in production in multiple apps for going on 4 years now without any issue that I can remember.
epage 2y ago
After working with Jenkins which has "hashed value" support for automatic staggering of jobs, I think its essential in any cron syntax evaluater.
- tison 2y ago
  This can be optionally supported. To be clear, does it mean almost "RANDOM at construction" and the parser will assign a value by a hash/random function and then the crontab struct has an immutable value of that field?
  - epage 2y ago
    The way jenkins did it is they took the job name and hashed that. This gives you good staggering while still being consistent from run to run (compared to using a random number). This does mean it is immutable, not just at construction but ideally across separate constructions so long as the seed is the same. For an API, I could see either taking in something that is hashable or just taking in a number and letting the caller choose their hashing. From the person's perspective writing cron syntax, they shouldn't care so long as they get the intended affect.
the_duke 2y ago
There is also a cron parser library from Cloudflare which is used for their cron jobs: https://github.com/cloudflare/saffron https://github.com/cloudflare/saffron
- tison 2y ago
  I knew it. Please see also the comment at https://news.ycombinator.com/item?id=41666886 https://news.ycombinator.com/item?id=41666886 ... and this PR [1][2] [1] https://github.com/tisonkun/cronexpr/pull/12 https://github.com/tisonkun/cronexpr/pull/12 [2] https://docs.rs/cronexpr/latest/cronexpr/struct.Crontab.html#method.matches https://docs.rs/cronexpr/latest/cronexpr/struct.Crontab.html...
1oooqooq 2y ago
isn't cron removed from most distros? now you need 3 or 4 different files for systemd-cronie
- proaralyst 2y ago
  Systemd timer units are usually two: one for the service you want to run, one for the timer file. I think you're confusing systemd timer units with Cronie[0], a crond implementation that I think predates systemd? It's possible there's some systemd thing I don't know about though! I think most distros at least have an installable crond [0]: https://github.com/cronie-crond/cronie/ https://github.com/cronie-crond/cronie/
- encoderer 2y ago
  Cron is still part of posix standard
  - 1oooqooq 2y ago
    that ship sailed long ago.

160 points by tison 2y ago

Looks well done. I like how the docs describe every syntax feature, and every non-standard feature in detail. Hat tip to implementing Vixie's "*,10" quirk, and to handling DST.

tison 2y ago
You're welcome! As described in the "Why do you create this crate?" section, I actually started this domain only a few weeks ago. So I experienced how those tribal rules can be confusing and hard to search over the Internet the understand their semantic. And when I sorted out the parse structure [1] and finished the extensions [2][3], I believe I should write it down for others (and future me) :D [1] https://github.com/tisonkun/cronexpr/pull/4 https://github.com/tisonkun/cronexpr/pull/4 [2] https://github.com/tisonkun/cronexpr/pull/5 https://github.com/tisonkun/cronexpr/pull/5 [3] https://github.com/tisonkun/cronexpr/pull/6 https://github.com/tisonkun/cronexpr/pull/6
- vlovich123 2y ago
  Is there a comparison to other cron parsing libraries like Saffron? https://github.com/cloudflare/saffron https://github.com/cloudflare/saffron
  - tison 2y ago
    Croner has a table for the syntax supported by it and saffron [1], you can compare it with the one cronexpr supported. [1] https://github.com/hexagon/croner-rust?tab=readme-ov-file#why-croner-instead-of-cron-or-saffron https://github.com/hexagon/croner-rust?tab=readme-ov-file#wh... As supported in [2], > There are several good candidates like croner and saffron, but they are not suitable for my use case. Both of them do not support defining timezone in the expression which is essential to my use case. Although croner support specific timezone later when matching, the user experience is quite different. Also, the syntax that croner or saffron supports is subtly different from my demand. > > Other libraries are unmaintained or immature to use. > > Last, most candidates using chrono to processing datetime, while I’d prefer to extend the jiff ecosystem. [2] https://docs.rs/cronexpr/latest/cronexpr/#why-do-you-create-this-crate https://docs.rs/cronexpr/latest/cronexpr/#why-do-you-create-...

Black616Angel 2y ago

Main drawback is the need for timestamps. Most crontab files or expressions I've seen didn't have them.

assiniboine 2y ago
It doesn't need timestamps. It parses crontab to/from timestamps where needed.
tison 2y ago
Could you elaborate a bit on the issue? I'm not sure you are commenting on cronexpr or other libraries. In cronexpr, there is no requirement for a timestamp until you'd like to find the next scheduled time, and thus you need to provide a related point. To decouple with certain datetime lib, I made a `MakeTimestamp` struct which provides multiple constructors. Later, I found it somehow like a function overload :D
- deleted 2y ago
  [deleted]
tricked 2y ago
Do you mean timezones? if so it shouldn't be that hard to detect if there's no timezone and add it with your own code.
tison 2y ago
If you mean timezone, I wrote an FAQ [1]: "Why does the crate require the timezone to be specified in the crontab expression?" [1] https://docs.rs/cronexpr/latest/cronexpr/#why-does-the-crate-require-the-timezone-to-be-specified-in-the-crontab-expression https://docs.rs/cronexpr/latest/cronexpr/#why-does-the-crate...
- aaomidi 2y ago
  So many of these restrictions feels arbitrary. Not supporting comments? Why? Assuming UTC for tz is not weird and cron users expect it, I guess why even support this specific syntax if the crons need to be edited to fit how this code expects them? There may have been better options if you were going full green field. Good work though!

Annatar 2y ago

[dead]

faangguyindia 2y ago

I've LLM based crontab bot. Where I simply write stuff like "run my backupBitBuckettoDropbox.sh from home directory at 3am everyday". I never really liked cronjob expressions.

Jleagle 2y ago
This sounds great, if i could trust it to get it right 100% of the time.
- simonw 2y ago
  With the right harness around it you could trust it. Set it up so that it takes your input, generates the cron expression, then uses a deterministic cron library to spit out a summary of what that expression does plus a list of next upcoming instances. That should give you all the information you need to determine if it got the right expression.
  - encoderer 2y ago
    You think it would work to just pipe that description and next occurrences and ask an llm if it matches the intended prompt?
    - simonw 2y ago
      No, that still leaves a small probability of something weird happening - and since it's a small probability you might not ever spot it in your own testing. In this case I think it's better to run known, deterministic code that can turn a crontab into a clear explanation.
  - krupan 2y ago
    Wow. The things people will do to avoid learning cron syntax

qwertox 2y ago

If you're into Python, APScheduler is the way to go. It also has a cron scheduler [0] which includes scheduling down to seconds and perfectly integrates with asyncio [1]. async def myfunc(): print(f"\n{Fore.GREEN}Readout triggered at { datetime.now().strftime('%H:%M:%S') }{COLORS_RESET}\n") await disk_temperatures_handler.readout(storage) scheduler = AsyncIOScheduler() storage['scheduler'] = scheduler scheduler.add_job(myfunc, CronTrigger(second='*/15'), id='readout_job') scheduler.start() You can also modify the interval on-the-fly. [0] https://apscheduler.readthedocs.io/en/3.x/modules/triggers/cron.html#introduction https://apscheduler.readthedocs.io/en/3.x/modules/triggers/c... [1] https://apscheduler.readthedocs.io/en/3.x/modules/executors/asyncio.html#apscheduler.executors.asyncio.AsyncIOExecutor https://apscheduler.readthedocs.io/en/3.x/modules/executors/...

deleted 2y ago
[deleted]
franga2000 2y ago
+1 for APScheduler, we've been running it in production in multiple apps for going on 4 years now without any issue that I can remember.

epage 2y ago

After working with Jenkins which has "hashed value" support for automatic staggering of jobs, I think its essential in any cron syntax evaluater.

tison 2y ago
This can be optionally supported. To be clear, does it mean almost "RANDOM at construction" and the parser will assign a value by a hash/random function and then the crontab struct has an immutable value of that field?
- epage 2y ago
  The way jenkins did it is they took the job name and hashed that. This gives you good staggering while still being consistent from run to run (compared to using a random number). This does mean it is immutable, not just at construction but ideally across separate constructions so long as the seed is the same. For an API, I could see either taking in something that is hashable or just taking in a number and letting the caller choose their hashing. From the person's perspective writing cron syntax, they shouldn't care so long as they get the intended affect.

the_duke 2y ago

There is also a cron parser library from Cloudflare which is used for their cron jobs: https://github.com/cloudflare/saffron https://github.com/cloudflare/saffron

tison 2y ago
I knew it. Please see also the comment at https://news.ycombinator.com/item?id=41666886 https://news.ycombinator.com/item?id=41666886 ... and this PR [1][2] [1] https://github.com/tisonkun/cronexpr/pull/12 https://github.com/tisonkun/cronexpr/pull/12 [2] https://docs.rs/cronexpr/latest/cronexpr/struct.Crontab.html#method.matches https://docs.rs/cronexpr/latest/cronexpr/struct.Crontab.html...

1oooqooq 2y ago

isn't cron removed from most distros? now you need 3 or 4 different files for systemd-cronie

proaralyst 2y ago
Systemd timer units are usually two: one for the service you want to run, one for the timer file. I think you're confusing systemd timer units with Cronie[0], a crond implementation that I think predates systemd? It's possible there's some systemd thing I don't know about though! I think most distros at least have an installable crond [0]: https://github.com/cronie-crond/cronie/ https://github.com/cronie-crond/cronie/
encoderer 2y ago
Cron is still part of posix standard
- 1oooqooq 2y ago
  that ship sailed long ago.