Best practice to make sure cron doesn't launch a task already launched by anacron

l0f4r0 · 09-05-2019, 08:46 AM

Hello,

I want to use both anacron and cron but with priority given to cron because I would prefer that my specific task is triggered at a specific time.

So let's assume my script to be triggered is /root/myTask.sh.
I want it to run at 3.00 am daily.
As a backup, if my laptop was off or something, I want anacron to launch it ASAP.

Here is my implementation:

Quote:

Originally Posted by /etc/crontab

0 3 * * * /root/myTask.sh; anacron -u myTask

Quote:

Originally Posted by /etc/anacrontab

@daily 20 myTask /root/myTask.sh

/root/myTask is executable

Everything should work as expected EXCEPT if my laptop reboots between 0.00 and 3.00 because anacron will launch myTask AND then cron as well at 3.00.

What is the best practice to make sure cron doesn't launch a task already launched by anacron please? Is it by creating a kind of flag/file and checking its value by cron?

Thank you in advance

pan64 · 09-06-2019, 01:01 AM

I don't think there is a "best practice" for it.
Most probably I would try to check if it was already running to avoid second execution.

michaelk · 09-06-2019, 04:52 AM

A flag would work or have the cron job check the anacron timestamp /var/spool/anacron.

Lsatenstein · 09-06-2019, 09:40 AM

Your best option is to set a flag when one of the two methods, cron or anacron launches the application. The flag should include a timestamp and a sequence number and an identification as to which of cron/anacron did the set.

If your laptop was off for a weekend, and you happen to start the laptop coincident with the crontab entry timestamp, you will encounter a problem. One way to circumvent such situations is to not get into them.

Using the who, and the sequence number, and date, you should be able to resolve problems due to long passages of time.

It might help to add a sleep command to the anacron launched application the "job" will have to wait for the sleep command to expire. In that time interval, it could be that the crontab entry will launch and complete. The sleep command is to prevent race conditions.

michaelk · 09-06-2019, 10:29 AM

The OP's anacron entry has a 20 minute delay which means the only time there could be a problem is if the laptop starts at ~0240. The cron and anacron job could also check in addition to a flag/timestamp if there is an existing mytask running process.

I assume that it is working but system cron jobs i.e. /etc/crontab should have a user name, never tried without so I guess it defaults to root.

Quote:

0 3 * * * root /root/myTask.sh; anacron -u myTask

TenTenths · 09-06-2019, 10:45 AM

You could make use of a lock file, as has been said, have either of your reboot processes create a file /var/tmp/BOOT_LOCK and then check if the file is more than a period of time old and only reboot if it's older than expected. The code example below is modified from a production script snippet that will delete a lock file that's more than 4 hours old but will exit if there's a lock file that's under 4 hours old.

Code:

# We just process the lock file right at the start, because if it's here then we really don't
# care about anything else, we're just going to exit quietly

# This is a bit funky, we see if the lock file has been modified in the last 4 hours.  If the
# mtime is more than 240 then it's not been modified so we delete it.
find /var/tmp -mmin +240 -name "BOOT_LOCK" -delete

# Ok, so part of me wants to check the $? result of the /find to see if a file was deleted, that would
# be the much more elegant way but we'll just go retro / "old school" on this file's ass.

# If the lock file exists then we just exit doing nothing.
if [[ -f /var/tmp/BOOT_LOCK ]] ; then
  echo "--------------------"
  echo $(date) Lock File Found
  ls -l /var/tmp/BOOT_LOCK
  # Tiny Elvis Has Left The Server
  exit 1
fi

# So now if we get here there's a no lock file, we can start thinking about rebooting

touch /var/tmp/SMS_LOCK
sudo shutdown -r now

# Elvis Has Left The Server