Learning--crontab for Linux

Prior to understand a little crontab, the previous period of time more idle, familiar with a bit, today summary record.

The crontab command is commonly used in Unix-and Unix-like operating systems to set instructions that are executed periodically. The command reads the instruction from the standard input device and stores it in a "crontab" file for later reading and execution.

When do I need to use crontab? Here are a few good scenarios.

• A program needs to be executed every minute to check the system running status
• Need to count business data for the past day in the wee hours of the day
• Log files need to be backed up every week
• Need to back up the database every month

Installing crontab

The installation in Ubuntu system is as follows:

sudo Install cron

Time setting

# File Format Description #  --minutes (0|  --Hours (0| |  --Day   (1|  | | --month   (1|  | | | --Week (07) (Sunday= | | | | |  * * * * * Command executed

Note:

1. In the "Day of the Week" (fifth field), 0 and 7 are considered Sunday.
2. Not very intuitive to use: if the date and week are set simultaneously, then one of the conditions is satisfied, the instruction will be executed.
3. The first 5 fields are called the time of day and week , can be convenient for personal memory.

From the sixth field, indicate the command to execute.

crontab command

    crontab [-u user] [-i] {-e |-L |-r        }1003.2)    -E    (edit users CR Ontab)-    l    (list user's crontab)    -R    (delete user's crontab)    -I (Prompt before deleting user's crontab)

crontab Example

Restart Nginx every 21:30

* * * Service Nginx restart

1, 10, 22nd of 4:45 restart Nginx

4 1,10,22 * * Service Nginx restart

1-10 days a month 4:45 restart Nginx

4 1-10 * * Service Nginx restart

Restart the Nginx server every 2 minutes

*/2 * * * * Service Nginx restart

1-59/2 * * * * Service Nginx restart

From 11 o'clock to 7 in the morning, restart nginx every hour.

0 23-7/1 * * * Service Nginx restart

Restart Nginx every 30 minutes from 18:00 to 23:00 every day

0,30 18-23 * * * Service Nginx restart

0-59/30 18-23 * * * Service Nginx restart

Summary

• * Indicates any time to match
• You can use "A,b,c" to represent A or B or C when executing A command
• You can use "A-B" to represent the command
• You can use "*/a" to indicate that every A-minute (hour, etc.) command is executed

Learning--crontab for Linux