daemon to execute scheduled commands


How to run the cron daemon: Cron should be started from /etc/rc or /etc/rc.local. It will return immediately, so you don't need to start it with '&'.

What cron does
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.

Cron then wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute.

Modifying a cron job
To edit a users crontab entry, log into your system for that particular user and type crontab -e.
The default editor for the 'crontab -e' command is vi.
Change the default editor by running: export VISUAL='editor'

cron checks each minute to see if its spool directory's modtime (or the modtime on /etc/crontab) has changed, and if it has, cron will then examine the modtime on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modfied. Note that the crontab command updates the modtime of the spool directory whenever it changes a crontab.

Mailing output
Cron will email to the user all output of the commands it runs, to silence this, redirect the output to a log file or to /dev/null
You can also redirect email to the user named in the MAILTO environment variable in the crontab, if such exists.

Cron comes from the word chronos, the Greek word for time.

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crontab - Schedule a command to run at a later time
chroot - Run a command with a different root directory
exec - Execute a command
if - Conditionally perform a command
nohup - Run a command immune to hangups
su - Run a command with substitute user and group id
watch - Execute/display a program periodically
.period - Run commands from a file
Equivalent Windows commands: schtasks - Schedule a command to run at a later time

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