Cron – Create a cron job to run redis every 30 minutes if it is not running

cronredis

I want to create a cron job that will run redis every 30 minutes if it is not already running, but I don't know what I should do.

I know that the beginning should look like this */30 * * * */<some path to redis-server file>.

I want to create this cron job because its get a little tedious running redis via ./redis-server every time it crashes.

I figured out it was crashing because my memory usage was a bit over the limit. Thanks for your help.

Best Answer

Redis shouldn't be crashing. I'd focus on that first. Is it running out of RAM?
Use something like Supervisor or Monit, or run Redis through something like Upstart.

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Cron – Not running a cron job when the prev run was not finished

Use flock(1):

NAME
       flock - manage locks from shell scripts

SYNOPSIS
       flock [options] <file> -c <command>
       flock [options] <directory> -c <command>
       flock [options] <file descriptor number>
[...]

You can have it block or immediately exit. Pre-existence (or absence) of the lock file doesn't make a difference. It creates the file if needed and uses a flock() system call to lock the file. This lock is automatically released at process death.

For example, in cron:

*/5 * * * * /usr/bin/flock /tmp/my.lock /usr/local/bin/myjob

If your jobs don't always finish in 5 minutes, you might consider using --timeout so that your jobs don't queue up:

flock --timeout=300

Or use --nonblock to exit immediately -- it would be similar to --timeout=0

If your script is a shell script, you can use some clever redirection tricks to use flock within your script itself:

(
  flock -n 9 || exit 1
  # ... commands executed under lock ...
) 9>/var/lock/mylockfile

Or, the manual also suggests making your script recursive by putting this at the top (it uses the script itself as its lock file):

[ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :

See the flock(1) man page for more information

Linux – Don’t run cron job if already running

Please see Process Management. You will probably want to use a lock file. You can use flock for example.

Why can't you let your jobs complete?

Your questions says "I either didn't understand the answer or it didn't apply to me", but you don't say in what way. Please be more specific.

Best Answer

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Cron – Not running a cron job when the prev run was not finished

Linux – Don’t run cron job if already running

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