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Automating Tasks with at and cron

Learn how to automate tasks in Unix system administration using the at and cron utilities. Execute commands at a specific time or repeatedly, control usage, and manage access.

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Automating Tasks with at and cron

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  1. COP 4343Unix System Administration Unit 8: at cron

  2. Automating Tasks • at, batch • execute command in future • controlled by atd daemon • crontab • execute command repeatedly • controlled by crond daemon

  3. at • syntax: at time • executes commands • from standard input • file (-f option) • output arrives via email

  4. at examples • at noon • at now + 5 minutes • at 4pm + 5 days • at noon July 31 • at 10am 10/5/05 • at 3:13am tomorrow

  5. batch • similar to at • command is executed when system load is below 0.8 • output arrives via email

  6. at utilities • directory /var/spool/at • atq • list at jobs • atrm • remove at jobs • control usage: • /etc/at.allow • /etc/at.deny

  7. cron • automate recurring tasks • crond daemon is started in run level • /etc/crontab • /etc/cron.d • /var/spool/cron/username

  8. using crontab • edit /etc/crontab • crontab -e • crontab -u username -e • controlling access • /etc/cron.allow • /etc/cron.deny

  9. crontab file • fields • minute: 00 - 59 • hour: 00 - 23 • day of month: 1 - 31 • month: 1 - 12 • day of week: 0 - 7 (0 is Sunday) • username (only in /etc/crontab file) • command to execute • run-parts /etc/cron.daily

  10. crontab example # use /bin/sh to run commands, no matter what /etc/passwd says SHELL=/bin/sh # mail any output to “paul”, no matter whose crontab this is MAILTO=paul # # run five minutes after midnight, every day 5 0 * * * $HOME/bin/daily.job >> $HOME/tmp/out 2>&1 # run at 2:15pm on the first of every month -- output mailed to paul 15 14 1 * * $HOME/bin/monthly # run at 10 pm on weekdays, annoy Joe 0 22 * * 1-5 mail -s "It’s 10pm" joe%Joe,%%Where are your kids?% 23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday" 5 4 * * sun echo "run at 5 after 4 every sunday"

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