选题: 20191106 How to Schedule and Automate tasks in Linux using Cron Jobs

sources/tech/20191106 How to Schedule and Automate tasks in Linux using Cron Jobs.md
2024-12-29 21:41:00 +08:00 · 2019-11-07 00:53:51 +08:00 · 2019-11-07 00:53:51 +08:00 · 4a00f91a6c
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+[#]: collector: (lujun9972)
+[#]: translator: ( )
+[#]: reviewer: ( )
+[#]: publisher: ( )
+[#]: url: ( )
+[#]: subject: (How to Schedule and Automate tasks in Linux using Cron Jobs)
+[#]: via: (https://www.linuxtechi.com/schedule-automate-tasks-linux-cron-jobs/)
+[#]: author: (Pradeep Kumar https://www.linuxtechi.com/author/pradeep/)
+
+How to Schedule and Automate tasks in Linux using Cron Jobs
+======
+
+Sometimes, you may have tasks that need to be performed on a regular basis or at certain predefined intervals. Such tasks include backing up databases, updating the system, performing periodic reboots and so on. Such tasks are referred to as **cron jobs**. Cron jobs are used for **automation of tasks** that come in handy and help in simplifying the execution of repetitive and sometimes mundane tasks. **Cron** is a daemon that allows you to schedule these jobs which are then carried out at specified intervals. In this tutorial, you will learn how to schedule jobs using cron jobs.
+
+[![Schedule -tasks-in-Linux-using cron][1]][2]
+
+### The Crontab file
+
+A crontab file, also known as a **cron table**, is a simple text file that contains rules or commands that specify the time interval of execution of a task. There are two categories of crontab files:
+
+**1)  System-wide crontab file**
+
+These are usually used by Linux services &amp; critical applications requiring root privileges. The system crontab file is located at **/etc/crontab** and can only be accessed and edited by the root user. It’s usually used for the configuration of system-wide daemons. The crontab file looks as shown:
+
+[![etc-crontab-linux][1]][3]
+
+**2) User-created crontab files**
+
+Linux users can also create their own cron jobs with the help of the crontab command. The cron jobs created will run as the user who created them.
+
+All cron jobs are stored in /var/spool/cron (For RHEL and CentOS distros) and /var/spool/cron/crontabs (For Debian and Ubuntu distros), the cron jobs are listed using the username of the user that created the cron job
+
+The **cron daemon** runs silently in the background checking the **/etc/crontab** file and **/var/spool/cron** and **/etc/cron.d*/** directories
+
+The **crontab** command is used for editing cron files. Let us take a look at the anatomy of a crontab file.
+
+### The anatomy of a crontab file
+
+Before we go further, it’s important that we first explore how a crontab file looks like. The basic syntax for a crontab file comprises 5 columns represented by asterisks followed by the command to be carried out.
+
+*    *    *    *    *    command
+
+This format can also be represented as shown below:
+
+m h d moy dow command
+
+OR
+
+m h d moy dow /path/to/script
+
+Let’s expound on each entry
+
+  * **m**: This represents minutes. It’s specified from 0 to 59
+  * **h**: This denoted the hour specified from 0 to 23
+  * **d**:  This represents the day of the month. Specified between 1 to 31`
+  * **moy**: This is the month of the year. It’s specified between 1 to 12
+  * **doy**:  This is the day of the week. It’s specified between 0 and 6 where 0 = Sunday
+  * **Command**: This is the command to be executed e.g backup command, reboot, &amp; copy
+
+
+
+### Managing cron jobs
+
+Having looked at the architecture of a crontab file, let’s see how you can create, edit and delete cron jobs
+
+**Creating cron jobs**
+
+To create or edit a cron job as the root user, run the command
+
+# crontab -e
+
+To create a cron job or schedule a task as another user, use the syntax
+
+# crontab -u username -e
+
+For instance, to run a cron job as user Pradeep, issue the command:
+
+# crontab -u Pradeep -e
+
+If there is no preexisting crontab file, then you will get a blank text document. If a crontab file was existing, The  -e option allows  to edit the file,
+
+**Listing crontab files**
+
+To view the cron jobs that have been created, simply pass the -l option as shown
+
+# crontab -l
+
+**Deleting a  crontab file**
+
+To delete a cron file, simply run crontab -e and delete or the line of the cron job that you want and save the file.
+
+To remove all cron jobs, run the command:
+
+# crontab -r
+
+That said, let’s have a look at different ways that you can schedule tasks
+
+### Crontab examples in Scheduling tasks.
+
+All cron jobs being with a shebang header as shown
+
+#!/bin/bash
+
+This indicates the shell you are using, which, for this case, is bash shell.
+
+Next, specify the interval at which you want to schedule the tasks using the cron job entries we specified earlier on.
+
+To reboot a system daily at 12:30 pm, use the syntax:
+
+30  12 *  *  * /sbin/reboot
+
+To schedule the reboot at 4:00 am use the syntax:
+
+0  4  *  *  *  /sbin/reboot
+
+**NOTE:**  The asterisk * is used to match all records
+
+To run a script twice every day, for example, 4:00 am and 4:00 pm, use the syntax.
+
+0  4,16  *  *  *  /path/to/script
+
+To schedule a cron job to run every Friday at 5:00 pm  use the syntax:
+
+0  17  *  *  Fri  /path/to/script
+
+OR
+
+0 17  *  *  *  5  /path/to/script
+
+If you wish to run your cron job every 30 minutes then use:
+
+*/30  *  *  *  * /path/to/script
+
+To schedule cron to run after every 5 hours, run
+
+*  */5  *  *  *  /path/to/script
+
+To run a script on selected days, for example, Wednesday and Friday at 6.00 pm execute:
+
+0  18  *  *  wed,fri  /path/to/script
+
+To schedule multiple tasks to use a single cron job, separate the tasks using a semicolon for example:
+
+*  *  *  *  *  /path/to/script1 ; /path/to/script2
+
+### Using special strings to save time on writing cron jobs
+
+Some of the cron jobs can easily be configured using special strings that correspond to certain time intervals. For example,
+
+1)  @hourly timestamp corresponds to  0 * * * *
+
+It will execute a task in the first minute of every hour.
+
+@hourly /path/to/script
+
+2) @daily timestamp is equivalent to  0 0 * * *
+
+It executes a task in the first minute of every day (midnight). It comes in handy when executing daily jobs.
+
+  @daily /path/to/script
+
+3) @weekly   timestamp is the equivalent to  0 0 1 * mon
+
+It executes a cron job in the first minute of every week where a week whereby, a  week starts on Monday.
+
+ @weekly /path/to/script
+
+3) @monthly is similar to the entry 0 0 1 * *
+
+It carries out a task in the first minute of the first day of the month.
+
+  @monthly /path/to/script
+
+4) @yearly corresponds to 0 0 1 1 *
+
+It executes a task in the first minute of every year and is useful in sending New year greetings 🙂
+
+@monthly /path/to/script
+
+### Crontab Restrictions
+
+As a Linux user, you can control who has the right to use the crontab command. This is possible using the **/etc/cron.deny** and **/etc/cron.allow** file. By default, only the /etc/cron.deny file exists and does not contain any entries. To restrict a user from using the crontab utility, simply add a user’s username to the file. When a user is added to this file, and the user tries to run the crontab command, he/she will encounter the error below.
+
+![restricted-cron-user][1]
+
+To allow the user to continue using the crontab utility,  simply remove the username from the /etc/cron.deny file.
+
+If /etc/cron.allow file is present, then only the users listed in the file can access and use the crontab utility.
+
+If neither file exists, then only the root user will have privileges to use the crontab command.
+
+### Backing up crontab entries
+
+It’s always advised to backup your crontab entries. To do so, use the syntax
+
+# crontab -l &gt; /path/to/file.txt
+
+For example,
+
+```
+# crontab -l > /home/james/backup.txt
+```
+
+**Checking cron logs**
+
+Cron logs are stored in /var/log/cron file. To view the cron logs run the command:
+
+```
+# cat /var/log/cron
+```
+
+![view-cron-log-files-linux][1]
+
+To view live logs, use the tail command as shown:
+
+```
+# tail -f /var/log/cron
+```
+
+![view-live-cron-logs][1]
+
+**Conclusion**
+
+In this guide, you learned how to create cron jobs to automate repetitive tasks, how to backup as well as how to view cron logs. We hope that this article provided useful insights with regard to cron jobs. Please don’t hesitate to share your feedback and comments.
+
+--------------------------------------------------------------------------------
+
+via: https://www.linuxtechi.com/schedule-automate-tasks-linux-cron-jobs/
+
+作者：[Pradeep Kumar][a]
+选题：[lujun9972][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://www.linuxtechi.com/author/pradeep/
+[b]: https://github.com/lujun9972
+[1]: data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
+[2]: https://www.linuxtechi.com/wp-content/uploads/2019/11/Schedule-tasks-in-Linux-using-cron.jpg
+[3]: https://www.linuxtechi.com/wp-content/uploads/2019/11/etc-crontab-linux.png