Cron and at are tools for automating jobs and scripts on Linux systems. Cron is used for recurring jobs run on a schedule, while at is used for jobs that need to run only once at a specific time. The cron daemon crond handles cron jobs, while the at daemon atd handles at jobs. Commands like crontab -e and crontab -l are used to edit and view cron job schedules. Examples show how to set up jobs to run backups, reports, scripts, and other tasks on a variety of schedules using cron and at.

1. Job Automation Using
Linux
Cron and At

2.  Cron is a deamon called "crond" used to schedule and execute jobs
or scripts automatically without user intervention. Cron, also
referred to as crontab, can help with automated log rotation,
scheduled reporting and running of scripts at off times of the day.
Cron is primarily used for jobs needing to be executed over and over
like log rotation every week or a report email sent out every
morning.
 An addtitional tool one can use is called "at" and is used to execute
a job only once. "at" is very useful, for example if you want run a
backup job starting at 8pm and you expect to be leaving at 5:30pm.
 In OpenBSD and FreeBSD the daemon cron can handel cron jobs as
well at "at" jobs. In Linux the "crond" daemon is used for cron jobs
only and a separate daemon "atd" is used for at jobs. Make sure the
correct daemon is running for the job scheduler you are looking to
use.

3. To use cron tab there are two important commands:
 crontab -e edit your crontab entries
 crontab -l print the entries from crontab

4.  Here is an example of a very easy to reference header for your
crontab. You have the descriptions for every time slot and what every
slot will accept. This example also specifies the shell and the path
making sure the binaries you run can be found. The last line is an
example of running "newsyslog" Sunday at midnight. You are welcome
to cut/paste this block to the top of your cron tab.

5. Lets take a look at some examples in order of simple to alittle more
complex. Notice all of the binaries are using their absolute paths. Cron
uses its own PATH variable and it is a safe practice to always use absolute
paths in your crontab. This is to avoid confusion.
Rotate logs weekly at 12midnight. (just like the example above)
00 0 * * 0 /usr/bin/newsyslog
Rotate logs weekly at 12midnight. (instead of 0 for the day of the week
we can use Sun for Sunday)
00 0 * * Sun /usr/bin/newsyslog
Mail a report to root everyday at 11:59pm (23:59).
59 23 * * * /usr/local/bin/pflogsumm -d today /var/log/maillog | mail -s "mail
report" root

6.  Run the backup scripts at 5am on the 3rd (Wed) and 5th (Fri) day of the week.
Send any errors to /dev/null
 00 5 * * 3,5 /tools/BACKUP_script.sh >> /dev/null 2>&1
 Compress backup files at 6am on the 1st and 15th of the month.
 00 6 1,15 * * /tools/BACKUP_compress.sh
 Refresh the Squid ad blocker server list every 3 days at 12:05am.
 05 0 * * */3 /tools/ad_servers_newlist.sh
 Clear the blocked hosts list at 3:23pm (15:23) every Monday only on even
numbered months.
 23 15 * */2 1 /tools/clear_blocked_hosts.sh

7.  Run a script at 8:45pm (20:45) on 2nd and the 16th only in the months of
January and April.
45 20 2,16 1,4 * /tools/a_script.sh
 Run a script every day at 8:45pm (20:45) and add a random sleep time
between 0 and 300 seconds.
45 20 * * * sleep $(($RANDOM % 300)); /tools/a_script.sh
 Run the script at 23:59 (11:59pm) on the last day of the month.
59 23 28-31 * * [ $(date -d +1day +%d) -eq 1 ] && /tools/a_script.sh

8.  To run jobs only once it is easier to use "at" than to setup and cron job and
then go back and remove it once the job has ran. Remember you need to
have the "atd" daemon running on Linux systems to run "at" jobs. On OpenBSD
or FreeBSD system the "crond" daemon will handle "cron" and "at" jobs.
 To run an "at" job you need to fist tell "at" what time to run the job.
Remember to use absolute paths to avoid confusion. Once to execute att with
the time and date you will be put into an "at" shell. This is where you will
enter the commands you want to execute, one command per line to make it
simple.
 In this example we will be executing a set of commands at 5am on January
23rd. The backup script will run and then we will send out mail to root. To
close the "at" shell and save the job you must type Ctrl-d (the control key
with the lowercase d).