To use Linux cron to set up periodic tasks, first open the crontab editor and enter the command crontab -e (use sudo crontab -e for system-level tasks); secondly, master the five-segment cron expression, such as 30 6 * to execute tasks at 6:30 every day; then optionally set environment variables and logging to avoid execution problems, such as defining SHELL, PATH and LOGFILE, and redirecting the output to the log; finally view the task through crontab -l and clear the task. If the task is not executed, you can view the system log and troubleshoot the problem.
Timed execution of tasks is very common in server maintenance and automated script operation, and the cron tool that comes with Linux systems is specially used to do this. As long as you set the rules, it can automatically execute commands or scripts at the time you specify.
The following are a few key points to explain how to set up periodic tasks using cron.
1. Open the crontab editor
Each user has its own crontab file, which you can edit with the following command:
crontab -e
The first time you run it, you may be asked to choose an editor. It is recommended to choose nano, which is simple and easy to use.
If you want to set system-level tasks, such as root, add sudo:
sudo crontab -e
2. Understand and write cron expressions
The time format of cron is a five-segment formula, in the following order:
* * * * * Time-sharing day and day
Each paragraph can be filled with numbers, asterisks (*), commas (,) or slashes (/). for example:
-
* * * * *means every minute is executed -
0 3 * * *Perform every day at 3 am -
0 0 * * 0Execute every Sunday at 0:00 -
*/5 * * * *Execute every 5 minutes
To give a practical example: If you want to execute a script /home/user/backup.sh every morning at 6:30, then this line is written as:
30 6 * * * /home/user/backup.sh
Notice:
- The path must be written in full, not just
backup.sh - If the script depends on environment variables, it is best to set it separately in cron or use absolute paths
3. Set environment variables and logging (optional but recommended)
The execution environment of cron is different from your shell. Sometimes the script will not run due to environment variable problems. You can add a few sentences to the beginning of the crontab file:
SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin LOGFILE=/var/log/mycron.log
Then you can record the output when executing the command to facilitate troubleshooting:
30 6 * * * /home/user/backup.sh >> $LOGFILE 2>&1
In this way, standard output and errors will be recorded in the log file.
4. View and manage set tasks
View the cron task of the current user:
crontab -l
Clear all tasks (use with caution):
crontab -r
If the task does not execute as expected, you can check the system log:
grep CRON /var/log/syslog
Or on CentOS/RHEL:
journalctl
Basically that's it. After setting it once, cron will keep silently helping you work. As long as you don't write the wrong time expression, there will generally be no problem.
The above is the detailed content of How to schedule a recurring task using cron?. For more information, please follow other related articles on the PHP Chinese website!
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