Cron, Anacron, at Cron

Scheduling Jobs: cron, anacron, at cron jboydt :: csci 26 | cron, anacron, at What is cron? ● a daemon ● a job scheduler ● a way to run both: ○ system cron jobs ■ run as root, for system-wide maintenance ○ user cron jobs ■ run as a specified user jboydt :: csci 26 | cron, anacron, at How do I use cron? ● /etc/crontab (system cron jobs) ● crontab (utility, user cron jobs) ○ crontab -u USER -l (list current crontab for USER) ○ crontab -u USER -e (edit current crontab for USER) ○ crontab -u USER -r (remove current crontab for USER) jboydt :: csci 26 | cron, anacron, at What does a cron entry look like? jboydt :: csci 26 | cron, anacron, at What does a cron entry look like? ● 7 fields minute hour day month day-of-week user command (0-59) (0-23) (1-31) (1-12)* (0-7) ● month and day-of-week can be specified using first three letters (e.g., Dec, Wed) ● 0 and 7 for day-of-week are both Sunday ● an asterisk (*) in a field means all available (e.g., a * for day field means every day) ● use a dash (-) to specifiy a range (e.g., 1-6 for month means January through February) ● use a slash (/) to specify increments (e.g., */15 for minutes means every 15 minutes) ● use commas to specify multiple values (e.g., 1,3,5 for day-of-week means Mon, Wed, Fri) jboydt :: csci 26 | cron, anacron, at What does a cron entry look like? ● some examples 0 12 * * * root SOME_SCRIPT (midnight every day) */30 * * * 1,3,5 janedoe SOME_SCRIPT >> /home/janedoe/LOGFILE (every 30 minutes on Mon, Wed, Fri) 0 0 1 * * root SOME_SCRIPT (midnight on the first day of every month) * note that a cron job should not require user inputs, and output statements do not work jboydt :: csci 26 | cron, anacron, at anacron jboydt :: csci 26 | cron, anacron, at What is anacron? ● a daemon ● a supplement to cron ○ good for systems that are not necessarily “always on” (cron handles those systems) ● a job scheduler ○ day, week, or month only (not by the minute or hour like cron) ○ tracks “past due” jobs and runs them jboydt :: csci 26 | cron, anacron, at +/- of anacron ● only administrator can schedule ● may end up running several “past due” jobs shortly after booting ○ the jobs are run sequentially (one after another) ○ may cause a system to be less responsive ● can only run a job once a day at most ● /etc/anacrontab ○ format is different from /etc/crontab jboydt :: csci 26 | cron, anacron, at at jboydt :: csci 26 | cron, anacron, at What is at? ● a daemon ● a “one-time” job scheduler ● examples * schedule non-interactively echo “cp IMPORTANT_FILE IMPORTANT_FILE.backup” | at 1200 apr 1 * schedule interactively at 2359 apr 1 at> who >> /home/janedoe/whowason at> CTRL-D jboydt :: csci 26 | cron, anacron, at jboydt :: csci 26 | cron, anacron, at .

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