Abstract ：Linux Cron A utility is a program that works at a specific time and / Or date is an effective method to continuously schedule routine background jobs . This article tells about 15 A very useful one crontab Work scheduling example .
This article is shared from Huawei cloud community 《Linux Crontab：15 A very useful one Cron Examples of work 》, author ：Tiamo_T.
Experienced Linux The system administrator knows the importance of automatically running daily maintenance jobs in the background .
Linux Cron A utility is a program that works at a specific time and / Or date is an effective method to continuously schedule routine background jobs .
This article tells about 15 A very useful one crontab Work scheduling example .
Linux Crontab Format
MIN HOUR DOM MON DOW CMD
1. Schedule work for a specific time
cron The basic usage of is to execute a job at a specific time , As shown below . This will be 6 month 10 The morning of 08:30 Perform a full backup shell Script （ Full backup ）.
Please note that , The time field uses 24 Hour format . therefore , For the morning 8 Point use 8, For the night 8 Point use 20.
30 08 10 06 * /home/ramesh/full-backup
- 30 – 30 minute
- 08 – 08 In the morning
- 10 – The first 10 God
- 06 – The first 6 Months （ June ）
- * – Every day of the week
2. Scheduling jobs for multiple instances （ for example , Twice a day ）
The following script performs incremental backups twice a day .
This example is displayed on a daily basis 11:00 and 16:00 Perform the specified incremental backup shell Script （incremental-backup）. The comma separated value in the field specifies that the command needs to be executed at all times mentioned .
00 11,16 * * * /home/ramesh/bin/incremental-backup
- 00 – The first 0 minute （ The top of an hour ）
- In the morning 11,16 – 11 And in the afternoon 4 spot
- * – Every day
- * – Every month
- * – Every day of the week
3. Schedule work for a specific time frame （ For example, only on weekdays ）
If you want to schedule an assignment every hour within a specific time frame , Please use the following .
The number of working hours per day Cron Job
This example is used every morning 9 Point to afternoon 6 During working hours at （ Including weekends ） Check the status of the database
00 09-18 * * * /home/ramesh/bin/check-db-status
- 00 – The first 0 minute （ The top of an hour ）
- 09-18 – 9 am, 10 am, 11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm
- * – Every day
- * – Every month
- * – Every day of the week
Cron Job Working hours per working day
This example occurs every working day （ That is, excluding Saturdays and Sundays ） Our working hours are in the morning 9 Point to afternoon 6 Check the status of the database
00 09-18 * * 1-5 /home/ramesh/bin/check-db-status
- 00 – The first 0 minute （ The top of an hour ）
- 09-18 – 9 am, 10 am, 11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm
- * – Every day
- * – Every month
- 1-5 - Monday 、 Tuesday 、 Wednesday 、 Thursday and Friday （ Every working day ）
4. How to view Crontab entry ？
View the of the currently logged in user Crontab entry
To view your crontab entry , Please start from your unix Account type crontab -l, As shown below .
ramesh@dev-db$ crontab -l @yearly /home/ramesh/annual-maintenance */10 * * * * /home/ramesh/check-disk-space [Note: This displays crontab of the current logged in user]
View root Crontab entry
With root user (su – root) Log in and execute crontab -l , As shown below .
root@dev-db# crontab -l no crontab for root
Crontab HowTo： See more Linux User Crontab entry
To view other Linux User crontab entry , Please log in to root And use -u {username} -l, As shown below .
root@dev-db# crontab -u sathiya -l @monthly /home/sathiya/monthly-backup 00 09-18 * * * /home/sathiya/check-db-status
5. How to edit Crontab entry ？
Edit the name of the currently logged in user Crontab entry
To edit crontab entry , Please use crontab -e, As shown below . By default , This will edit the currently logged in user crontab.
ramesh@dev-db$ crontab -e @yearly /home/ramesh/centos/bin/annual-maintenance */10 * * * * /home/ramesh/debian/bin/check-disk-space ~ "/tmp/crontab.XXXXyjWkHw" 2L, 83C [Note: This will open the crontab file in Vim editor for editing. Please note cron created a temporary /tmp/crontab.XX... ]
When you use :wq When saving the above temporary files , It will preserve crontab And display the following message , indicate crontab Modification successful .
~ "crontab.XXXXyjWkHw" 2L, 83C written crontab: installing new crontab
Edit root Crontab entry
With root user (su – root) Log in and execute crontab -e, As shown below .
root@dev-db# crontab -e
Edit other Linux User Crontab File entry
To edit other Linux User crontab entry , Please log in to root And use -u {username} -e, As shown below .
root@dev-db# crontab -u sathiya -e @monthly /home/sathiya/fedora/bin/monthly-backup 00 09-18 * * * /home/sathiya/ubuntu/bin/check-db-status ~ ~ ~ "/tmp/crontab.XXXXyjWkHw" 2L, 83C
6. Use Cron One assignment per minute .
Ideally , You may not need to schedule your homework every minute . But understanding this example will help you understand the other examples mentioned below in this article .
* * * * * CMD
* Represents all possible units —— That is, every minute of every hour throughout the year . In addition to using this directly *, You will find it very useful in the following situations .
- When you specify... In the minutes field */5 when , each 5 Minutes at a time .
- When you specify... In the minutes field 0-10/2 when , It means before 10 Every... In minutes 2 Minutes at a time .
- therefore , The above agreement can be used for all other purposes 4 A field .
7. Every time 10 Arrange a backstage in minutes Cron Homework .
If you want every 10 Check disk space every minutes , Please use the following methods .
*/10 * * * * /home/ramesh/check-disk-space
It every year 10 Execute the specified command once per minute check-disk-space. However, you may need to execute commands only during office hours , vice versa . The above example shows how to do these things .
We can use a single keyword to specify it , Not in 5 Specify a value in a field , As follows .
In some special cases , You can use @ Followed by keywords to replace the above 5 A field , For example, restart 、 The middle of the night 、 Every year, 、 Every hour .
8. Use @yearly Arrange work at the first minute of each year
If you want to do a job at the first minute of each year , Then you can use @yearly cron keyword , As shown below .
This will happen every year 1 month 1 Japanese 00:00 Use annual maintenance shell Script execution system annual maintenance .
@yearly /home/ramesh/red-hat/bin/annual-maintenance
9. Use @monthly Schedule the start of each month Cron Homework
It's with the @yearly be similar . But use @monthly cron Keyword execute the command once a month .
This will happen every month 1 Japanese 00:00 perform shell Script tape backup .
@monthly /home/ramesh/suse/bin/tape-backup
10. Daily use @daily Schedule background jobs
Use @daily cron keyword , This will happen every day 00:00 Use cleanup-logs shell The script executes daily log file cleanup .
@daily /home/ramesh/arch-linux/bin/cleanup-logs "day started"
11. How to use... After each restart @reboot perform Linux command ？
Use @reboot cron keyword , This will execute the specified command once every time the machine starts .
@reboot CMD
12. How to use MAIL Keyword disable / Redirect Crontab Mail output ？
By default ,crontab Send the job output to the user who scheduled the job . If you want to redirect the output to a specific user , Please be there. crontab Add or update MAIL Variable , As shown below .
ramesh@dev-db$ crontab -l MAIL="ramesh" @yearly /home/ramesh/annual-maintenance */10 * * * * /home/ramesh/check-disk-space [Note: Crontab of the current logged in user with MAIL variable]
If you don't want to send mail anywhere , Stop sending via email crontab Output , Please be there. crontab Add or update MAIL Variable , As shown below .
MAIL=""
13. How to use Crontab Once a second Linux Cron Homework .
You can't schedule... Once a second cronjob. Because in cron in , The minimum unit you can specify is minutes . In a typical scenario , Most of us have no reason to run any jobs in the system every second .
14. stay Crontab It is specified in PATH Variable
In all the above examples, we have specified the Linux Order or shell-script The absolute path of .
for example , If you just want to specify tape-backup, Instead of specifying /home/ramesh/tape-backup, The path /home/ramesh Add to crontab Medium PATH variable , As shown below .
ramesh@dev-db$ crontab -l PATH=/bin:/sbin:/usr/bin:/usr/sbin:/home/ramesh @yearly annual-maintenance */10 * * * * check-disk-space [Note: Crontab of the current logged in user with PATH variable]
15. from Cron Files installed Crontab
In addition to direct editing crontab Outside the document , You can also add all entries to cron In file . After all these entries are included in the file , You can upload or install them to cron, As shown below .
ramesh@dev-db$ crontab -l no crontab for ramesh $ cat cron-file.txt @yearly /home/ramesh/annual-maintenance */10 * * * * /home/ramesh/check-disk-space ramesh@dev-db$ crontab cron-file.txt ramesh@dev-db$ crontab -l @yearly /home/ramesh/annual-maintenance */10 * * * * /home/ramesh/check-disk-space
Be careful ： This will cron-file.txt Install to your crontab, This will also delete your old cron entry . therefore , from cron-file.txt Upload cron Please note when entering .
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