– Linux can run tasks automatically, and comes with automated tasks utilities: cron, anacron, at, batch.
– cron jobs can run as often as every minute.
– A scheduled cron job is skipped if the system is down.
– anacron can run a job only once a day.
– Scheduled jobs are remembered and run the next time that the system is up.
– crond daemon searches multiple files and directories for scheduled jobs:
1. /var/spool/cron/ 2. /etc/anacrontab 3. /etc/cron.d
Configuring cron jobs
cron jobs are defined in /etc/crontab.
The crontab entries are of the form:
Minutes Hours Date Month Day-of-Week command
where: Minutes = [0 to 59] Hours = [0 to 23] Date = [1 to 31] Month = [1 to 12] Day-of-Week = [0 to 6] 0=Sunday - 6=Saturday command = a script file or a shell command. Other special characters can be used: - An asterisk (*) can be used to specify all valid values. - A hyphen (-) between integers specifies a range of integers. - A list of values separated by commas (,) specifies a list. - A forward slash (/) can be used to specify step values.
Other cron Directories and Files
– Contains files with same syntax as the /etc/crontab – accessible by root privileges only
– Other cron directories in /etc: –
cron.hourly cron.daily cron.weekly cron.monthly
– Scripts in these directories run hourly, daily, weekly, or monthly, depending on the name of the directory.
– The /etc/cron.allow and /etc/cron.deny files restrict user access to cron. If neither file exists, only root can use cron.
– Users other that root can also configure cron using the crontab utility.
– user defined crontabs are stored in /var/spool/cron/[username].
– To create or edit a crontab entry :
# crontab -e
– To list the entries in the user defined crontab :
# crontab -l
Configuring anacron jobs
– anacron jobs are defined in /etc/anacrontab.
– Jobs are defined by :
Period in days : frequency of execution in days Delay in minutes - Minutes to wait before executing the job job-identifier - A unique name used in logfiles command : a shell script or command to execute
example anacron file :
SHELL=/bin/sh PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root # the maximal random delay added to the base delay of the jobs RANDOM_DELAY=45 # the jobs will be started during the following hours only START_HOURS_RANGE=3-22 #period in days delay in minutes job-identifier command 1 5 cron.daily nice run-parts /etc/cron.daily 7 25 cron.weekly nice run-parts /etc/cron.weekly @monthly 45 cron.monthly nice run-parts /etc/cron.monthly
at and batch
– at and batch utilities are used for scheduling one-time tasks.
– the at command executes a task at a specific time.
– the batch command executes a task when system load average is below 0.8.
– the atd service must be running to run at or batch jobs
– at command syntax :
# at time
– The time argument accept multiple formats :
HH:MM MMDDYY,MM/DD/YY or MM.DD.YY month-name day year midnight: At 12:00 AM teatime: At 4:00 PM now + time -- here time can be minutes, hours, days or weeks
– batch command syntax :
# batch (at> promp is displayed)
– The /etc/at.allow and /etc/at.deny files restrict user access to at. If neither file exists, only root can use cron.