Computing/Linux/Cron

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Contents

General

  • Cron searches /var/spool/cron for crontab files which are named after accounts in /etc/passwd; crontabs found are loaded into memory.
  • Cron also searches for /etc/crontab and the files in the /etc/cron.d/ directory, which are in a different format
  • When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists).
  • Example cron file
# 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"
  • /etc/crontab - quick reference

Syntax

*     *     *     *     *  command to be executed
-     -     -     -     -
|     |     |     |     |
|     |     |     |     +----- day of week (1 - 7) (monday = 1)
|     |     |     +------- month (1 - 12)
|     |     +--------- day of month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

Commands

export EDITOR=vi - to specify a editor to open crontab file

  • crontab -e - Edit your crontab file, or create one if it doesn't already exist.
  • crontab -l - Display your crontab file.
  • crontab -r - Remove your crontab file.
  • crontab -v - Display the last time you edited your crontab file. (This option is only available on a few systems.)

cron invokes the command from the user's HOME directory with the shell, (/usr/bin/sh).

Environment

cron supplies a default environment for every shell, defining:

  • HOME=user's-home-directory
  • LOGNAME=user's-login-id
  • PATH=/usr/bin:/usr/sbin:.
  • SHELL=/usr/bin/sh
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