- 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
* * * * * 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)
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).
cron supplies a default environment for every shell, defining: