How to write and schedule a cron job?
Jul 16, 2025 am 02:19 AM
Cron job is a tool used to perform timing tasks in Linux systems, and its core lies in writing the correct time expressions. 1. The Cron expression consists of 5 fields, representing minutes, hours, days, months and days of the week. For example, when running the command at 2 a.m. every day, you can write "0 2 your_command". 2. To add a task, you need to use the "crontab -e" command to edit the task list of the current user and add an expression and a full path command at the end, such as "0 1 /home/user/backup.sh". 3. Check the existing tasks to use "crontab -l" and delete all tasks to use "crontab -r". 4. During debugging, you should check whether the script permissions, paths are correct, and whether the environment variables are set, and troubleshoot problems through the system log. It is recommended to test a simple task to confirm whether cron works normally.
Timed tasks are very common in server operations and maintenance, such as automatic backup of data every day, regular cache cleaning, or regular email sending. Cron job is a tool used in Linux systems to do timing tasks. Let’s take a look at how to write a cron job and arrange it.
1. How to edit Cron expressions?
The core of Cron job is to write time expressions. It consists of 5 fields representing minutes, hours, days, months and days of the week:
* * * * * command_to_execute │ │ │ │ │ │ │ │ └── Day of the week (0-6) Sunday is 0 │ │ └────Month (1-12) │ │ └───────What day of each month (1-31) │ └───────────Hours (0-23) └─────────────── Minutes (0-59)
Let me give you a few common examples:
- Run at 2 am every day:
0 2 * * * your_command - Run every hour:
0 * * * * your_command - Run every Monday at 9
0 9 * * 1 your_command - Run every 10 minutes:
*/10 * * * * your_command
2. How to add your own Cron job?
To add a task, you need to use the crontab -e command to open the current user's scheduled task list. If you are using it for the first time, the system may let you choose an editor, either nano or vim is OK.
After opening, add your time expression and command at the end of the file, save and exit to take effect.
For example, suppose you want to execute a script /home/user/backup.sh at 1 a.m. every day, then add a line:
0 1 * * * /home/user/backup.sh
Notes:
- The path needs to be written in the complete path, and no relative path can be used.
- If the command depends on environment variables, it is recommended to set explicitly in the script or use absolute paths
- The output will send an email to the user by default. If you don't want to receive an email, you can add
> /dev/null 2>&1
3. How to view and manage existing tasks?
- View the cron list of the current user:
crontab -l - Delete all tasks of the current user:
crontab -r - Did you accidentally correct the mistake? You can use
crontab -eto delete or modify
If you have multiple users, each user's crontab is independent. For example, if you want the root user to perform a task, you have to use sudo crontab -e to edit it.
4. FAQs and debugging tips
Sometimes the task is not executed, don't rush to doubt cron, check these places first:
- ? Do scripts have executable permissions? Try
chmod x your_script.sh - ? Are the paths in the script correct? Especially when citing other files
- ? Are environment variables set? The environment of cron is much simpler than that of shell
- ? Where to read the log? Generally, you can find it in the system log, such as
/var/log/syslogorjournalctl(depending on the system)
You can first test the simplest task, such as outputting a sentence to a file every minute:
* * * * * echo "test cron" >> /tmp/crontest.log 2>&1
Wait for a few minutes to see if the file is updated, so that you can quickly determine whether cron is working normally.
Basically that's it. It is not difficult to write a cron job well, but you need to pay attention to details, especially the issues of paths and permissions. Just try a few more times and you will be able to master it.
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