Setting up cronjobs in Unix and Solaris :-
Cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs in unix , solaris.
Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.
1. Crontab Restrictions 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
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
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
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/*
Changing the parameter values as below will cause this command to run at different time schedule below :
30 0 1 1,6,12 * -- 00:30 Hrs on 1st of Jan, June & Dec.
0 20 * 10 1-5 --8.00 PM every weekday (Mon-Fri) only in Oct.
0 0 1,10,15 * * -- midnight on 1st ,10th & 15th of month
5,10 0 10 * 1 -- At 12.05,12.10 every Monday & on 10th of every month
Note : If you inadvertently enter the crontab command with no argument(s), do not attempt to get out with Control-d. This removes all entries in your crontab file. Instead, exit with Control-c.
5. Crontab Environment
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:
Users who desire to have their .profile executed must explicitly do so in the crontab entry or in a script called by the entry.
6. Disable Email
By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line .
ORACLE_BASE=/data/u01/app/oracle; export ORACLE_BASE
ORACLE_HOME=$ORACLE_BASE/product/10.2.0/db_1; export ORACLE_HOME
ORACLE_SID=orcl; export ORACLE_SID
ORACLE_TERM=xterm; export ORACLE_TERM
PATH=$ORACLE_HOME/bin:$PATH; export PATH
LD_LIBRARY_PATH=$ORACLE_HOME/lib:/lib:/usr/lib; export LD_LIBRARY_PATH
CLASSPATH=$ORACLE_HOME/JRE:$ORACLE_HOME/jlib:$ORACLE_HOME/rdbms/jlib; export CLASSPATH
if [ $USER = "pccuser" ]; then
if [ $SHELL = "/bin/ksh" ]; then
ulimit -p 16384
ulimit -n 65536
ulimit -u 16384 -n 65536
exp PCPCC_O/LEEO920@orcl file=/data/backup/dump_orcl/PCPCC_O_`date +%d%m%H%M`.DMP log=/data/backup/dump_orcl/PCPCC_O_`date +%d%m%H%M`.LOG statistics=NONE;
exp PCPCC_PUB_O/LEEO920@orcl file=/data/backup/dump_orcl/PCPCC_PUB_O_`date +%d%m%H%M`.DMP log=/data/backup/dump_orcl/PCPCC_PUB_O_`date +%d%m%H%M`.LOG statistics=NONE;
zip -r dump_orcl.zip dump_orcl
#scp command to send the file from one server to another server
scp dump_orcl.zip email@example.com:/data/backup_orcl
rm -rf dump_orcl.zip
rm -rf *
8. Generate log file To collect the cron execution execution log in a file :
30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log
9. Next Steps
This article covered a significant aspect of system administration of setting up cronjobs . Unix administration involves lots of different tasks and some of these tasks are covered in this website but still there are many areas not covered here .
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