The software utility
cron also known as
cron job is a time-based job scheduler in Unix-like computer operating systems. Users that set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals.
The actions of cron are driven by a
crontab (cron table) file, a configuration file that specifies shell commands to run periodically on a given schedule. The crontab files are stored where the lists of jobs and other instructions to the cron daemon are kept. Users can have their own individual crontab files and often there is a system-wide crontab file (usually in
/etc or a subdirectory of
/etc) that only system administrators can edit.
Each line of a crontab file represents a job, and looks like this:
Code:
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │ 7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * <command to execute>
The syntax of each line expects a cron expression made of five fields which represent the time to execute the command, followed by a shell command to execute.
While normally the job is executed when the time/date specification fields all match the current time and date, there is one exception: if both "day of month" (field 3) and "day of week" (field 5) are restricted (not "*"), then one or both must match the current day.
For example, the following clears the Apache error log at one minute past midnight (00:01) every day, assuming that the default shell for the cron user is Bourne shell compliant:
Code:
1 0 * * * printf "" > /var/log/apache/error_log
This example runs a shell program called export_dump.sh at 23:45 (11:45 PM) every Saturday:
Code:
45 23 * * 6 /home/oracle/scripts/export_dump.sh
Note: It is also possible to specify
*/n to run for every n-th interval of time. Also, specifying multiple specific time intervals can be done with commas (e.g.,
1,2,3). The below would output "hello world" to the command line every 5th minute of every first, second and third hour (i.e., 01:00, 01:05, 01:10, up until 03:55):
Code:
*/5 1,2,3 * * * echo hello world
