Schedule tasks using at and cron

– 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
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: –
– 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.

Crontab utility

– 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 :
# the maximal random delay added to the base delay of the jobs
# the jobs will be started during the following hours only

#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 :
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.

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