#scheduler #cron #anacron

app evry

A shell-script-centric task scheduler; uses exit codes to determine control flow

4 releases

new 0.1.6 Sep 9, 2020
0.1.5 Sep 7, 2020
0.1.4 Sep 7, 2020
0.1.3 Sep 7, 2020

#20 in Date and time

Apache-2.0

25KB
338 lines

evry

A shell-script-centric task scheduler; uses exit codes to determine control flow.

Install

Install rust/cargo, then:

cargo install evry

Rationale

Usage:
  evry [describe duration]... <-tagname>
  evry rollback <-tagname>
  evry help

Best explained with an example:

evry 2 weeks -scrapesite && wget "https://" -o ....

In other words, run the wget command every 2 weeks.

evry exits with an unsuccessful exit code if the command has been run in the last 2 weeks (see below for more duration examples), which means the wget command wouldn't run.

When evry exits with a successful exit code, it saves the current time to a metadata file for that tag (-scrapesite). That way, when evry is run again with that tag, it can compare the current time against that file.

This can sort of be thought of as cron alternative, but operations don't run in the background. It requires you to call the command yourself, but it won't run if its already run in the time frame you describe. (However, its not difficult to wrap tasks that run behind evry in an infinite loop that runs in the background, which is what bgproc does)

You could have an infinite loop running in the background like:

while true; do
  evry 1 month -runcommand && run command
  sleep 60
done

... and even though that tries to run the command every 60 seconds, evry exits with an unsuccessful exit code, so run command would only get run once per month.

The -runcommand is just an arbitrary tag name so that evry can save metadata about a command to run/job. Can be chosen arbitrarily, its only use is to uniquely identify runs of evry, and save a metadata file to your local data directory

Since this has no clue what the external command is, and whether it succeeds or not, this saves a history of one operation, so you can rollback when a tag was last run, in case of failure. An example:

evry 2 months -selenium && {
# evry succeeded, so the external command should be run
    python selenium.py || {
        # the python process exited with a non-zero exit code
        # we should rollback when the command was last run, so
        # we can re-try later
        evry rollback -selenium
    }
}

Duration

The duration (e.g. evry 2 months, 5 days) is parsed with a PEG, so its very flexible. All of these are valid duration input:

  • 2 months, 5 day
  • 2weeks 5hrs (commas are optional)
  • 60secs
  • 5wk, 5d
  • 5weeks, 2weeks (is additive, so this would result in 7 weeks)
  • 60sec 2weeks (order doesn't matter)

Debug

The EVRY_DEBUG environment variable can be set to provide information on what was parsed from user input, and how long till the next run succeeds.

EVRY_DEBUG=1 evry 2 months -pythonanywhere && pythonanywhere_3_months -Hc "$(which chromedriver)"

pythonanywhere:data directory: /home/sean/.local/share/evry/data
pythonanywhere:Parsed '2 months' into 5184000000ms
pythonanywhere:'60 days' (5184000000ms) hasn't elapsed since last run, exiting with code 1
pythonanywhere:Will next be able to run in '55 days, 13 hours, 56 minutes, 18 seconds' (4802178196ms)

How I use this

I have certain jobs (e.g. scraping websites for metadata, using selenium to login to some website and click a button, updating specific packages (e.g. running brew cask upgrade --greedy on mac)) that I want to run periodically.

Putting all my jobs I want to run periodically in one housekeeping script I run daily/weekly gives me the ability to monitor the output easily, but also allows me the flexibility of being able to schedule tasks to run at different rates. It also means that those scripts/commands can prompt me for input/confirmation, since this is run manually from a terminal, not in the background like cron.

This also means that all my 'cron-like' jobs are just bash scripts, and can be checked into version control easily.

I also have a background loop script that uses this to run tasks periodically, which I prefer to cron on my main machine.

Dependencies

~2MB
~45K SLoC