Cron is the next program in the Linux system that automatically executes a specified task.
The cron service is a built-in service for Linux, but it does not boot automatically. You can start and stop the service with the following command:
Linux Code
- /sbin/service Crond Start
- /sbin/service Crond Stop
- /sbin/service Crond Restart
- /sbin/service Crond Reload
The above 1-4 lines start, stop, restart the service, and reload the configuration, respectively.
To set cron to start automatically when booting, in the/etc/rc.d/rc.local script can be added /sbin/service crond start
.
View, delete, modify
Cron saves the command line in the crontab (cron table) file, which is usually in the/etc directory. Each system user can have their own crontab (under/var/spool/cron/).
- File uses the contents of the file as a cron service configuration for a user, and the contents are copied to the Cron service's configuration file
- -U sets a user's cron service configuration, which is typically required by the root user when executing this command
- -l lists a user cron service configuration
- -R Delete a user's cron service configuration
- -e Edit a user's cron service configuration, when specified crontab does not exist, new, will use VI to open the configuration file for editing save after the exit, file syntax see after
- Crontab–v show The last edit time (available on some operating systems only)
Example: sudo crontab-lGrammareach line represents an instruction, which represents a scheduled task, each line consists of six parts separated by a space, the first 5 represents the time of the scheduled task execution, and the remainder is the command to be executed by the timed task, in the following format:
minute Hour Sun Moon Week command
minute hour day month dayofweek Command
- minute the first few minutes of each hour to perform the task, 0-59
- command specifies the program to execute
The first five items above can be used with the following four special symbols:
- * Indicates all values, such as the first bit using * indicates every minute
- /denotes each, as the first bit uses */5 to denote every 5 minutes
- -Represents a range of values, as the second bit uses 2-4 to indicate 2 to 4 points
- That represents a discrete number of values, such as the 2nd bit using 6,8 to represent 6 points and 8 points
- Specify "step": 8-14/2 for 8,10,12,14
- Specify a list: such as "1,2,3,4″," 0-4,8-12″
In the six fields of each row, except that the command is a field that must be specified each time, the other fields are optional fields, which can be determined visually. For a field that is not specified, "*" is used to fill its position.
Example: Execute the following command every 10 minutes
- 5 * * * ls Specify the Daily 5 : Span class= "number" style= "Color:rgb (192,0,0)" >30 execute ls command
- 7 8 * * ls Specify monthly 8 " 7 : 30 performs the ls command
- 5 8 6 * ls Specify annual 6 month 8 day 5 : 30 execute ls command
- 6 * * 0 ls specified per Sunday of 6: Execute ls command [note:0 means Sunday, 1 said the week 1, and so on, can also be expressed in English, Sun said Sunday, Mon said Monday and so on. ]
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How to use the crontab command under Linux: sudo crontab-l