Can a crontab job run concurrently with itself?

Question

If I have a crontab job that runs, for instance, every hour, but that job may take more than an hour to complete, will the next job fail to run?

Answer 1

No, cron scripts run parallelly, if you do not implement some locking mechanism.

See Quick-and-dirty way to ensure only one instance of a shell script is running at a time and Correct locking in shell scripts? for possible solutions.

A simpler way is to use lockfile, like in this answer or the run-one package (see this answer) - thanks to gertvdijk for suggesting it.

Answer 2

There's also whenjobs which aims to be "a powerful but simple cron replacement". From its manpage:

PRE FUNCTIONS

Before a job runs, you can arrange that a "pre" function is called. This function may decide not to run the job (by returning "false").

One use for this is to prevent a particular job from running if there is already an instance of the same job running:

job "only one"
pre (Whentools.one ())
every 10 seconds :
<<
  # Takes longer than 10 seconds to run, but 'Whentools.one ()'
  # will ensure only one is ever running.
  sleep 11
>>

(It also provides Whentools.max n, allowing n instances of the job to run. By default, it works like Cron.)