แล้วเพิ่มข้อความ
00 00 * * * /sbin/reboot
หรือ
00 00 * * * /sbin/shutdown -r now
อันนี้ reboot ทุกเที่ยงคืน เปลี่ยนเวลาเอาเองนะครับ
-------------------------------------------
นาที ชั่วโมง วันที่ เดือน วันในสัปดาห์ คำสั่ง
* * * * * คำสั่ง
นาที 0-59
ชั่วโมง 0-23 0=เที่ยงคืน
วันที่ 1-31
เดือน 1-12
วันในสัปดาห์ 0-6 0=วันอาทิตย์
ดูรายละเอียด crontab
crontab -l
เพิ่มเติม
Howto Set-up a Crontab File
Yesterday I mainly worked on some PHP backup scripts [not very exciting, but necessary.] The scripts are run at regular intervals and perform various data backups across numerous domains. To accomplish this I set-up some cron jobs that will automatically execute the scripts at set intervals.
This is not the first time that I've had to set-up cron jobs [I've got several that date back a few years.] I think that having the ability to set-up scheduled tasks is really quite important. Therefore I thought I'd blog about it for future reference.
What is cron?
Cron is a daemon/service that executes shell commands periodically on a given schedule. Cron is driven by a crontab, a configuration file that holds details of what commands are to be run along with a timetable of when to run them.
Creating a crontab file
You can create a crontab file by entering the following terminal command:
crontab -e
Entering the above command will open a terminal editor [Ubuntu uses Nano by default] with a new blank crontab file [or it will open an existing crontab if you already have one.] You can now enter the commands to be executed [see syntax below] before saving the file and exiting the editor. As long as your entries were entered correctly your commands should now be executed at the times/dates you specified. You can see a list of active crontab entries by entering the following terminal command:
crontab -l
Crontab syntax
A crontab file has six fields for specifying minute, hour, day of month, month, day of week and the command to be run at that interval. See below:
* * * * * command to be executed
- - - - -
| | | | |
| | | | +----- day of week (0 - 6) (Sunday=0)
| | | +------- month (1 - 12)
| | +--------- day of month (1 - 31)
| +----------- hour (0 - 23)
+------------- min (0 - 59)
Crontab examples
Writing a crontab file can be a somewhat confusing for first time users [and the above table probably doesn't help much!] Therefore I've listed below some crontab examples:
* * * * *
30 * * * *
45 6 * * *
45 18 * * *
00 1 * * 0
00 1 * * 7
00 1 * * Sun
30 8 1 * *
00 0-23/2 02 07 *
As well as the above there are also special strings that can be used:
@reboot
@yearly
@annually
@monthly
@weekly
@daily
@midnight
@hourly
Multiple commands
A double-ampersand "&&" can be used to run multiple commands consecutively. The following example would run command_01 and then command_02 once a day:
@daily
Disabling email notifications
By default a cron job will send an email to the user account executing the cronjob. If this is not needed put the following command at the end of the cron job line:
>/dev/null 2>&1
Specifying a crontab file to use
As mentioned at the top of this post, you can create a new crontab file with the "crontab -e" command. However, you may already have a crontab file, if you do you can set it to be used with the following command:
crontab -u
Therefore the following command…
crontab -u tux ~/crontab
…would set Tux's crontab file to that of the file named "crontab" residing in Tux's home directory.
Removing a crontab file
To remove your crontab file simply enter the following terminal command:
crontab -r
Further information
Refer to the man page for further information about crontab. Enter the terminal command:
man crontab
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