Cron

cron is a time-based job scheduler. A scheduled job is known as a cron job. Although typically used to automate system maintenance and administration it can be used to automate any task. is most suitable for scheduling repetitive tasks, as scheduling a one-time task can be accomplished via at . The daemon name originates from Chronos, the Greek word for time. cron is generally available on Unix-like operating systems.

cron is a time-based job scheduler. A scheduled job is known as a cron job. Although typically used to automate system maintenance and administration it can be used to automate any task. is most suitable for scheduling repetitive tasks, as scheduling a one-time task can be accomplished via at . The daemon name originates from Chronos, the Greek word for time. cron is generally available on Unix-like operating systems. 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. 45 23 * * 6 /home/oracle/scripts/export_dump.sh Note: On some systems 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 line 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). /5 1,2,3 * * * echo hello world The configuration file for a user can be edited by calling crontab -e regardless of where the actual implementation stores this file. Some cron implementations, such as the popular 4th BSD edition written by Paul Vixie and included in many Linux distributions, add a sixth field: an account username that runs the specified job (subject to user existence and permissions). This is allowed only in the system crontabs—not in others, which are each assigned to a single user to configure. The sixth field is alternatively sometimes used for year instead of an account username—the nncron daemon for Windows does this. The Amazon EventBridge implementation of cron does not use 0 based day of week, instead it is 1-7 SUN-SAT (instead of 0-6), as well as supporting additional expression features such as first-weekday and last-day-of-month. Most cron implementations simply interpret crontab entries in the system time zone setting that the cron daemon runs under. This can be a source of dispute if a large multi-user machine has users in several time zones, especially if the system default time zone includes the potentially confusing DST. Thus, a cron implementation may as a special case recognize lines of the form "CRON_TZ=<time zone>" in user crontabs, interpreting subsequent crontab entries relative to that time zone. ==Cron expression== A cron expression is a string comprising five, six, or seven fields separated by white space that represents a set of times, normally as a schedule to execute some routine. Comments begin with a comment mark #, and must be on a line by themselves. The month and weekday abbreviations are not case-sensitive. In the particular case of the system crontab file (/etc/crontab), a user field inserts itself before the command. It is generally set to 'root'. Asterisk Asterisks (also known as wildcard) represents "all". For example, using "* * * * *" will run every minute. Using "* * * * 1" will run every minute only on Monday. Using six asterisks means every second when seconds are supported. Comma : Commas are used to separate items of a list. For example, using "MON,WED,FRI" in the 5th field (day of week) means Mondays, Wednesdays and Fridays. Hyphen ( - ): Hyphen defines ranges. For example, "2000-2010" indicates every year between 2000 and 2010, inclusive. Percent ( % ): Percent-signs (%) in the command, unless escaped with backslash (\), are changed into newline characters, and all data after the first % are sent to the command as standard input.

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