The crontab file has a specific syntax for defining the schedule of a task. Each line in the file represents a single task, and the line is made up of six fields separated by spaces, representing the schedule for when the task should run. The fields are as follows:
For example, to run a task every day at 3:00 AM, the line in the crontab file would be:
To schedule a task to run every day at 3:00 PM, the line in the crontab file would be:
You can also use the special characters , /n, a-b, , to configure the schedule.
/5 * * * * <command> # run every 5 minutes
0 3,6,9 * * * <command> # run at 3:00 AM, 6:00 AM, 9:00 AM
To create or edit your crontab
list current crontab
remove current crontab
crontab can run only shell commands, not scripts!
sudo apt-get update
sudo apt-get install cron
crontab -l
#**If you see the output of** "*/30 * * * * python /var/www/py/testingpython.py"
#**Then no need to adjust**
#**Otherwise**
sudo nano crontab -e
1
#hit page down a few times then paste and save changes
*/30 * * * * python3 /var/www/py/testingpython.py
#IF CRONTAB NO WORKY, DO THIS
cat /etc/cron.d/speedtest_cron
#/**************** TEST! *********************/
cd /var/www/py/
python3 testingpython.py
Basic Cron Syntax
There's a useful site called Crontab Generator that will calculate the setup of a cron job for you, but first we'll go through a brief overview of what a syntax will look like.
Parts of a cron command
cron command parts: - timing - the command to execute
In this example, the command includes three parts:
| Timing | Execute | Path to script | Output |
|---|---|---|---|
| * * * * * | /usr/bin/php | /var/www/html/crontest/cron.php | > /dev/null 2>&1 |
- Timing - set the minutes, hours, days, months, and weekday settings (more below).
- Execute - the cron job needs to call upon PHP to run, which is located at /usr/bin/php.
- Path to script - the full path of the file you plan to run.
- Output - (optional) you can write the output to a file or discard it - > /dev/null 2>&1 will discard.
In the above example, the cron job is set to send every minute - or more specifically, every minute of every hour of every day of every month, every day of the week. An asterisk is a wildcard that stands for "all".
- Minutes - set the minutes, from 0-59
- Hours - set the hour, from 0-24
- Days - set the day, from 1-31
- Months - set the month, from 1-12 (January - December)
- Week - set the day of the week, from 0-6 (Sunday - Saturday)
Here are a few basic examples to get an idea of how it works.
| Syntax | Explanation |
|---|---|
| 0 * * * * | run once an hour (every hour at minute zero) |
| 0 0 * * * | run once a day (every day at midnight and minute zero) |
| 0 0 1 * * | run once a month (on the first day of every month at midnight and minute zero) |
| 0 0 1 1 * | run once a year (on the first day of the first month every year at midnight and minute zero) |
There are all sorts of settings, such as for odd/even days, every 5 minutes, etc.
Setting Up a Cron Job
If cron is set up under root, you'll need to run sudo before your code.
example of a cron job sending out an email once a day
list all crontabs
edit the crontab
save your changes and exit the file.