2 Understand a cron job syntax<\/a>
3 How to handle an error on your cron job<\/a>
\u2022 Send output to a specific file<\/a>
\u2022 Use \/dev\/null<\/a>
4 Write a simple cron automation script<\/a>
5 Conclusion<\/a>
6 References<\/a><\/pre>\n\n\n\n
What are cron, cron job, and crontab?<\/h2>\n\n\n\n
Cron is a system that helps Linux users to schedule any task. However, a cron job is any defined task to run in a given time period. It can be a shell script or a simple bash command. Cron job helps us automate our routine tasks, it can be hourly, daily, monthly, etc.<\/p>\n\n\n\n
\nNote:<\/strong>\u00a0in most of Linux system, we must get a permission of system administrator before defining a spesific cron job that is listed on\u00a0
crontab<\/strong><\/code><\/p>\n<\/blockquote>\n\n\n\nMeanwhile, the crontab stands for cron table. It is a Linux system file that contains a list of the cron job. We define our task \u2014 bash command, shell script, Python script, etc scheduled in crontab.<\/p>\n\n\n\n
# Check cron service on Linux system<\/strong>
sudo systemctl status cron.service<\/pre>\n\n\n\nCheck the cron service on the Linux system (Image by Author)<\/figcaption><\/figure>\n\n\n\n Understand a cron job syntax<\/h2>\n\n\n\n
It\u2019s time to learn about cron job syntax on
crontab<\/strong><\/code>.<\/p>\n\n\n\n\n
crontab -a <filename><\/strong><\/code>: create a new\u00a0<filename><\/strong><\/code>\u00a0as crontab file<\/li>\n\n\n\ncrontab -e<\/strong><\/code>: edit our crontab file or create one if it doesn\u2019t already exist<\/li>\n\n\n\ncrontab -l<\/strong><\/code>: show up our crontab file<\/li>\n\n\n\ncrontab -r<\/strong><\/code>: delete our crontab file<\/li>\n\n\n\ncrontab -v<\/strong><\/code>: show up the last time we have edited our crontab file<\/li>\n<\/ul>\n\n\n\nminute(s) hour(s) day(s) month(s) weekday(s) command(s)<\/strong><\/pre>\n\n\n\nDescription of the six columns on crontab file (Image by Author)<\/figcaption><\/figure>\n\n\n\n \nNote:<\/strong>\u00a0day names 0\u20136 begin with Sunday. We can easily determine our schedule on\u00a0https:\/\/crontab.guru\/<\/a><\/p>\n<\/blockquote>\n\n\n\n
How to handle an error on your cron job<\/h2>\n\n\n\n
If the cron job encounters an error, the default, it will send an email to the system administrator. Instead, we will find out two common ways when we encounter the error.<\/p>\n\n\n\n
1 Send output to a specific file<\/strong><\/h2>\n\n\n\n
It\u2019s a common way and I always use it on my cron job. It\u2019s simple. We just need to create a file that will save our cron job logs. It will print out the output just in case the job is accomplished properly or print out an error if it fails.<\/p>\n\n\n\n
In this tutorial, it just created a
log.out<\/strong><\/code> file. The output will be redirected tolog.out<\/strong><\/code>.<\/p>\n\n\n\n* * * * * cd \/home\/audhi && \/bin\/bash shell-script.sh >> log.out<\/pre>\n\n\n\nThe description of the above syntax on a crontab file is as follows.<\/p>\n\n\n\n
\n
- The\u00a0
* * * * *<\/strong><\/code>\u00a0means that a task will be executed every minute of every hour of every day of every month and every day of the week<\/li>\n\n\n\n- The directory will be switched to\u00a0
\/home\/audhi<\/strong><\/code>\u00a0where the shell-script.sh is located<\/li>\n\n\n\n\/bin\/bash<\/strong><\/code>\u00a0is the path and executable of the\u00a0Bash shell<\/em><\/li>\n\n\n\n- The\u00a0
>><\/strong><\/code>\u00a0<\/strong>symbol\u00a0<\/strong>will append the output to an existing file (log.out<\/strong><\/code>), while a single\u00a0><\/strong><\/code>\u00a0symbol will overwrite the file<\/li>\n\n\n\n- The\u00a0
shell-script.sh<\/strong><\/code>\u00a0is a certain shell script<\/li>\n<\/ul>\n\n\n\nThe crontab file on Linux system (Image by Author)<\/figcaption><\/figure>\n\n\n\n \nNote:<\/strong>\u00a0we need to write the complete and clear command in a crontab. It needed to specify the file location using\u00a0
cd<\/strong><\/code><\/p>\n<\/blockquote>\n\n\n\n2 Use \/dev\/null<\/h2>\n\n\n\n
We can easily send our cron job logs and error to the
dev\/null<\/strong><\/code> instead of an alert via email. Whatever we send or write todev\/null<\/strong><\/code>, it will be discarded.<\/p>\n\n\n\n* * * * * cd \/home\/audhi && \/bin\/bash shell-script.sh > \/dev\/null 2>&1<\/pre>\n\n\n\nA little description of the commands:<\/p>\n\n\n\n
\n
- The\u00a0
> \/dev\/null<\/strong><\/code>\u00a0tells the cron to redirect the output (STDOUT) to\u00a0\/dev\/null<\/strong><\/code><\/li>\n\n\n\n2<\/strong><\/code>\u00a0is the file descriptor for\u00a0Standard Error\u00a0<\/em>(STDERR)<\/li>\n\n\n\n&<\/strong><\/code>\u00a0is the symbol for file descriptor (without it, the following\u00a01<\/strong><\/code>\u00a0will be a filename)<\/li>\n\n\n\n1<\/strong><\/code>\u00a0is the file descriptor for\u00a0Standard Out\u00a0<\/em>(STDOUT)<\/li>\n<\/ul>\n\n\n\n\nNote:<\/strong>\u00a0The 2>&1 tells the cron to redirect all errors (STDERR) to same as standard out (STDOUT)<\/p>\n<\/blockquote>\n\n\n\n
Write a simple cron automation script<\/h2>\n\n\n\n
To complete this article, I have created a Python script to demonstrate how to use a cron job. This Python script will collect the Covid-19 data from one of the largest Indonesian online news, Kompas News<\/em><\/a>. You can find out my Python script for Covid-19 data web scraping at my GitHub repo<\/strong><\/a>. Its filename is
Web Scraping Covid-19 Kompas News.py<\/strong><\/code>.<\/p>\n\n\n\nOpen our terminal and type
crontab -e<\/strong><\/code> to open a crontab file. Then, scroll down and type the following command.<\/p>\n\n\n\n5 16 * * * cd 'covid19 data' && \/usr\/bin\/python3 'Web Scraping Covid-19 Kompas News.py' >> test.out<\/pre>\n\n\n\nThe description of the above syntax on a crontab file is as follows.<\/p>\n\n\n\n
\n
- The crontab is located in\u00a0
\/home<\/em><\/strong><\/code>\u00a0<\/strong>and my script is in\u00a0\/home\/covid19 data<\/em><\/strong><\/code>\u00a0<\/strong>,\u00a0<\/strong>so we need to switch to the\u00a0\/home\/covid19 data<\/em><\/strong><\/code>\u00a0<\/em><\/strong>first<\/li>\n\n\n\n- The\u00a0
python3<\/strong><\/code>\u00a0interpreter is located in\u00a0\/usr\/bin\/python3<\/strong><\/code><\/li>\n\n\n\n- The output will be redirected to\u00a0
test.out<\/strong><\/code>\u00a0file in\u00a0\/home\/covid19 data<\/em><\/strong><\/code><\/li>\n<\/ul>\n\n\n\n\n
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