The exciting list of Cron oddities

cron linux

Here follows a list of reasons why maybe you should think about using a proper job scheduler whenever you touch an existing cron job, or create a new one:

  • Cron doesn’t tell you if it fails to run.
  • Cron doesn’t tell you if the thing it ran, failed to run.
  • Cron likes to log to syslog, except nothing there is remotely interesting.
  • Cron instead will cry that you don’t have an MTA (email) configured on your server.
  • Cron will then just cry and cry and never show you any logs.
  • Cron will email you if you do have an MTA configured on your server.
  • Cron will probably be very chatty so you’ll likely configure your MTA to actually store emails locally.
  • Cron emails will therefore likely never be read, and accumulate in root’s inbox.
  • Why are we even talking about emails we just want to read jobs’ logs what the fuck cron.
  • Cron will ignore any PATH you have setup unless you specifically set it in a place it decides it will accept.
  • Cron will ignore and refuse to run all your properly configured jobs even if just one of them is misconfigured.
  • Cron has no native way to validate a cronfile without any side effects or actually trying to run it, because it wants to assert its dominance over human life forms.
  • Cron likes being told to run at times that make no sense to a human (and so these things exist)
  • Cron has no concept of anything under a minute.
  • Cron will helpfully replace your % (percent character) with newlines and send everything after it as STDIN because obviously that’s what you meant.
  • Cron will outright refuse to run if it doesn’t end in a newline probably because POSIX, like that is ever a good explanation for anything.
  • Cron will let you setup an /etc/crontab file and run it but also let you see per-user crontab’s with crontab -l. (check /var/spool/cron/crontabs for all of them).

Don’t assume Cron will run with:

  • The permissions you think it does.
  • At the times you think it will.
  • The user you think it does.
  • The shell you think it does.
  • The variables you think it has.
  • That it will run at all.