As a system administrator some tasks are respective like backup, monitoring, log files.

To automate them with the help of
1)at
2)batch
3)crontab

Crontab:to run some tasks automatically

to set a crontab for an user
#crontab -e -u
* * * * * /bin/echo “hello”

A crontab file contains instructions to the cron deamon of the general form
“run this command at this time on this date”

Cron examines entries once every minute

field                allowed values
– – – –            – – – – – – – – – – –
minute              : 0-59
hour                  : 0-23
day of month    : 1-31
month               : 1-12 (or names, see below)
day of week      : 0-7 (0 or 7 is Sun, or use names)

A field may be an asterisk (*), which always stands for ‘‘first-last’’.

Ranges  of  numbers  are allowed.  Ranges are two numbers separated with a
hyphen.  The specified range is  inclusive.   For  example,  8-11  for  an
‘‘hours’’ entry specifies execution at hours 8, 9, 10 and 11.

Lists  are  allowed.   A list is a set of numbers (or ranges) separated by
commas.  Examples: ‘‘1,2,5,9’’, ‘‘0-4,8-12’’.

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”

To set a crontab for an user

#crontab -e -u
ex:crontab -e -u alex

To remove a crontab for an user

#crontab -r -u
ex:crontab -r -u alex

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