Lets learn "Cron job schedule notation"

-

In this tutorial, we'll learn about the notation that cron jobs use to describe the period in which the job needs to be run.

Notation

You must have seen the cron jobs scheduled defined like

"*/2 * * * *"

Let's learn what this means. The general meaning of each field in the cron job schedule is as follows.

# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │                                   7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * <command to execute>


Examples

Let's go through some examples.

45 23 * * 6 /home/user/test.sh

The above example means that the test.sh script should run at 23:45 ( or 11:45 PM ) of every Saturday.

*/2 * * * * /home/user/test.sh

NOTE: we can specify */n to run the program every nth interval of the time. Like in the above example, the script will run in every 2 minutes.

*/2 1,2,3 * * * /home/user/test.sh

 The above schedule means that the script should run in every 2 minutes of 1st, 2nd and 3rd hour (01:00, 01:02,...03:58)

Some predefined schedule definitions:

EntryDescriptionEquivalent to
@yearly (or @annually)Run once a year at midnight of 1 January0 0 1 1 *
@monthlyRun once a month at midnight of the first day of the month0 0 1 * *
@weeklyRun once a week at midnight on Sunday morning0 0 * * 0
@daily (or @midnight)Run once a day at midnight0 0 * * *
@hourlyRun once an hour at the beginning of the hour0 * * * *
@rebootRun at startupN/A

Cron Permissions

There are two files containing cron related permissions:
  1. /etc/cron.allow: If this file exists, It must contain the user's name for that user to be allowed to use cron jobs.
  2. /etc/cron.deny: If cron.allow file doesn't exist but cron.deny file exists then, to use cron jobs, users must not be listed in the /etc/cron.deny file.

HOPE YOU LEARNT SOMETHING NEW IN THIS TUTORIAL. FEEL FREE TO COMMENT BELOW IF YOU HAVE ANY DOUBTS. AND STAY TUNED FOR MORE TUTORIALS :)

Comments

Post a Comment

Popular posts from this blog

Lets learn "About kube proxy in iptables mode"

-
Image
In this tutorial, we'll learn about the Kube-proxy in iptables mode. Basically, we'll see how Kube-proxy create iptables that help in load balancing and service discovery. If you are not familiar with Linux IPTables I would recommend to check out my tutorial on IPTables here  first and then come back to read this tutorial. Kube-proxy Kube-proxy when running in iptables mode basically watch kube-apiserver for new endpoints and create corresponding iptables rules and iptables are fully responsible for service discover and proper routing of traffic to corresponding pods. Kube-proxy needs to run on all k8s worker nodes. Below is a small video describing the working of kube-proxy in iptables mode.  src:  https://www.slideshare.net/CJCullen/kubernetes-networking-55835829 Now that we know what Kube-proxy is and how it Kube-proxy works, let deep dive into the types of iptables Kube-proxy adds to the firewall. Kube-proxy added IPTables To learn this let's consider an example. Consid
Read more

Lets learn "System design for paste bin (or any text sharing website)"

-
Image
  In this tutorial, we'll learn about the system design for PasteBin. If you don't know what Pastebin is here goes its definition. Pastebin is a type of online content hosting service where users can store texts to share with others over the internet.  Pastebin system design requirements  Functional Basic - User should be able to paste the text, generate the link and share the link with other users. There should be the max size of the text that our system will support (say 10Mb) Users should be able to provide the custom URL path for the link created. Like if a user wants to create the links  www.pb.com/letlearn, he can give this URL and pastebin should create a link using this URL to share with others. The link or paste expiry feature should be there for each stored text. There can be default expiry time say 1 year but this should be configurable. Personalization means the user should be able to login to track all the pastes created by him and manage them. Non-Functional Durab
Read more

Lets learn "Factory design pattern"

-
 In this tutorial, we'll learn about the factory design pattern as in what it is, in which use cases this is being used and how it works. If you've are not familiar with what are software design patterns and what are its different types check out this tutorial. Introduction A factory design pattern is a creational design pattern that is related to object creation. In this pattern, we'll have a method to create an object without exposing its creation logic to the client. If a class method is creating an object it has to be static.  The idea is to create a static method ( factory method ) which instantiates a class without exposing its logic to the client and the client can use that method every time it needs to create an object of the class. The reason why the factory pattern is useful is that as it does not expose the logic of object creation, so if logic is changed, the client code need not to be changed. Usecases The place where this pattern is mostly used is library cod
Read more