Submit and manage periodic tasks for users
The crontab command is used to submit and manage tasks that need to be executed periodically by users, similar to Scheduled Tasks in Windows. After installing the operating system, this service tool is typically installed and the crond process starts automatically. The crond process checks for scheduled tasks every minute and executes them automatically if the criteria are met.
crontab [options] [arguments]
-e: Edit the current user's crontab entries;
-l: List the current user's crontab entries;
-r: Remove the current user's crontab entries;
-u <username>: Specify the user whose crontab is to be managed.
crontab file: Specify a file containing the crontab entries to be loaded.
Task scheduling in Linux is divided into two categories: System Task Scheduling and User Task Scheduling.
System Task Scheduling: Tasks that the system needs to perform periodically, such as writing cache data to disk or cleaning up logs. The configuration file for system tasks is located at /etc/crontab.
The /etc/crontab file typically includes the following lines:
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# run-parts
51 * * * * root run-parts /etc/cron.hourly
24 7 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly
The first four lines configure the environment for crond tasks. SHELL specifies the shell to use (bash), PATH specifies the search path for commands, MAILTO specifies that task execution info should be emailed to root (if empty, no email is sent), and HOME specifies the home directory for commands or scripts.
User Task Scheduling: Tasks that users need to perform periodically, such as data backups or email reminders. Users can use the crontab tool to customize their scheduled tasks. All user-defined crontab files are stored in the /var/spool/cron directory, with filenames matching the usernames. Relevant permission files include:
/etc/cron.deny List of users not allowed to use crontab.
/etc/cron.allow List of users allowed to use crontab.
/var/spool/cron/ Directory for user crontab files.
Meaning of crontab file fields: Each line in a crontab file represents a task. Each line has six fields: the first five are time settings, and the sixth is the command to be executed. The format is as follows:
minute hour day month week command
Special characters:
crond service commands:
/sbin/service crond start # Start the service
/sbin/service crond stop # Stop the service
/sbin/service crond restart # Restart the service
/sbin/service crond reload # Reload the configuration
Check service status:
service crond status
Start the service manually:
service crond start
Check if crontab is set to start on boot:
ntsysv
# or
chkconfig --level 35 crond on
Execute command every minute:
* * * * * command
Execute at the 3rd and 15th minute of every hour:
3,15 * * * * command
Execute at the 3rd and 15th minute between 8 AM and 11 AM:
3,15 8-11 * * * command
Execute at the 3rd and 15th minute between 8 AM and 11 AM every two days:
3,15 8-11 */2 * * command
Execute at the 3rd and 15th minute between 8 AM and 11 AM every Monday:
3,15 8-11 * * 1 command
Restart smb at 21:30 every night:
30 21 * * * /etc/init.d/smb restart
Restart smb at 4:45 on the 1st, 10th, and 22nd of every month:
45 4 1,10,22 * * /etc/init.d/smb restart
Restart smb at 1:10 every Saturday and Sunday:
10 1 * * 6,0 /etc/init.d/smb restart
Restart smb every 30 minutes between 18:00 and 23:00 every day:
0,30 18-23 * * * /etc/init.d/smb restart
Restart smb at 11:00 PM every Saturday:
0 23 * * 6 /etc/init.d/smb restart
Restart smb every hour:
0 */1 * * * /etc/init.d/smb restart
Restart smb every hour between 11 PM and 7 AM:
0 23-7/1 * * * /etc/init.d/smb restart
Restart smb at 11 AM on the 4th of the month and from Monday to Wednesday:
0 11 4 * mon-wed /etc/init.d/smb restart
Restart smb at 4 AM on January 1st:
0 4 1 jan * /etc/init.d/smb restart
Execute scripts in /etc/cron.hourly every hour:
01 * * * * root run-parts /etc/cron.hourly
观察