The cron command-line utility, also known as cron job, is a job scheduler on a Unix-like operating system. Users who set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals. It typically automates system maintenance or administration—though its general-purpose nature makes it useful for things like downloading files from the internet and downloading email at regular intervals.
A cron job is defined by using a series of asterisks (*****) which denotes different timing as indicated below.
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday)
# │ │ │ │ │
# │ │ │ │ │
# │ │ │ │ │
# * * * * *
This is very useful when you perform repetitive tasks that can be done programmatically for instance clearing logs, downloading files from the internet regularly or sending SMS to your spouse regularly from a Love SMS API ****😓
Examples of cron-job in a GNU system
The following command runs the ./clean_file.sh script file regularly at 1 minutes past midnight everyday
1 0 * * * ./clean_file.sh
More Examples of cron job notation
-
45 23 * * 6- runs on Saturdays at 23:45 (11:45pm) -
0 0 25 12 *- runs at midnight on 25th of December (Christmas day) -
0 0 * * *- runs at midnight everyday -
* * * * *- runs every minute -
* 10,14 * * *- runs everyday at 10:00 (10am) and 14:00 (2pm) -
0 0 14 2 *- runs every 14th day in February and at midnight
To use the cron notation to schedule tasks in our application, we will install the node package node-cron running the command below in our terminal.
npm install node-cron
Bree is another package with support for workers threads and cron syntax. But for the purpose of this article, we will stick to node-cron. So let's run a simple example:
const cron = require('node-cron');
cron.schedule('* * * * *', () => {
console.log('running a task every minute');
});
So you could perform basically any function at different scheduled dates by passing the function as a second argument.
Running in background
on Linux you can run the program in the background by using the ampersand & sign behind the command:
node app .js &
And use the command jobs to see the running processes in the background.
A similar command on Powershell is known as Start-Job
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Top comments (6)
If for scaling purpose i run my application in multiple server with multiple instances....How do i ensure that only one instance run the scheduled job?
I was faced with a similar situation. What I did is I added all the jobs to a queue. I made 2 queues one for pending job and another for on-going jobs. when a job is being ran in an instance, it's moved to the ongoing queue so other instances pick tasks from the pending queue. You can use AWS simple queue service (SQS) to implement the queues.
Thank you very much!
What would be the simplest cron job notation to run every last day of every month including a leap-year?
There's no single cron notation to run a task every last day of the month. You can run the task at the first day of the month at midnight or make use of npm module like bree to run the task at the last day of the month or have mutiple cron jobs for a single task.
But if you want to stick to cron, a simple solution is to run the tasks from 28 - 31 every month but make use of any robust date framework (that what date tomorrow is) to determine if any of the date is the last date. For instance
Nicely explained 🙂
Hey! You may want to check our new platform to schedule and execute NodeJS scripts without a dedicated server or VPS at: crontask.io
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