- 1. Crontab Restrictions
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You can execute crontab if your name appears in the file /usr/lib/cron/cron.allow. If that file does not exist, you can use
crontab if your name does not appear in the file /usr/lib/cron/cron.deny.
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab. The allow/deny files consist of one user name per line.
2. Crontab Commands
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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.)
3. Crontab file
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Crontab syntax :-
A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval.
| * * * * * command to be executed - - - - - | | | | | | | | | +----- day of week (0 - 6) (Sunday=0) | | | +------- month (1 - 12) | | +--------- day of month (1 - 31) | +----------- hour (0 - 23) +------------- min (0 - 59) |
* in the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).
Note: The specification of days can be made in two fields: month day and weekday. If both are specified in an entry, they are cumulative meaning both of the entries will get executed .
4. Crontab Example
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A line in crontab file like below removes the tmp files from /home/someuser/tmp each day at 6:30 PM.
30 18 * * * rm /home/someuser/tmp/*
sebuah kasus contoh, saya akan membackup tiap jam 13.10, maka saya menseeting di /etc/crontab
10 13 * * * root /bin/backup.sh
Sebelumnya saya telah membuat sebuah file yang harus bisa di eksekusi, yang berada di /bin. Nama filenya backup.sh, file ini saya berikan perintah chmod +x backup.sh, isi dari file backup.sh adalah :
jam=`date +%Y%m%d`
tar zcf /home/bafrin/data-$jam.tgz /home/bafrin/data/coba
pada /home/bafrin adalah posisi file ektrak yang saya taruh, dan file2 yang saya ektrak adalah di /home/bafrin/data/coba (folder coba).
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