2.5 A Deep Dive into Task Scheduling with Crontab, Anacron, and At

Introduction

Task scheduling is a fundamental aspect of system administration, allowing users to automate routine processes on Unix-like systems. In this technical blog, we'll explore three powerful tools for scheduling tasks: Crontab, Anacron, and At. Each tool serves a unique purpose in orchestrating automated processes, providing users with flexibility and control over when tasks are executed.

Crontab

Crontab, derived from "cron table," is a time-based job scheduler in Unix-like operating systems. It allows users to schedule recurring tasks at fixed intervals.

Purpose:

Crontab is designed for executing repetitive tasks automatically, such as system maintenance, backups, and periodic script executions.

Syntax

cron [-f] [-l] [-L loglevel]

-f : Used to stay in foreground mode, and don’t daemonize.
-l : This will enable the LSB compliant names for /etc/cron.d files.
-n : Used to add the FQDN in the subject when sending mails.
-L loglevel : This option will tell the cron what to log about the jobs with the following values: 
1 : It will log the start of all cron jobs.
2 : It will log the end of all cron jobs.
4 : It will log all the failed jobs. Here the exit status will not equal to zero.
8 : It will log the process number of all the cron jobs.

Examples:

• Schedule a backup script to run every day at 3:00 AM:

    0 3 * * * /path/to/backup_script.sh
  

• Run a cleanup task every Sunday at midnight:

    0 0 * * 0 /path/to/cleanup_script.sh
  

• Execute a custom script every weekday at 8:30 AM:

    30 8 * * 1-5 /path/to/custom_script.sh
  

• Set up a monthly report generation task on the 15th at 6:00 PM:

    0 18 15 * * /path/to/report_generation.sh
  

• Run a task every 15 minutes:

    */15 * * * * /path/to/task.sh
  

Anacron

Anacron is a task scheduler designed for systems that may not be running continuously. It ensures that scheduled tasks are executed, even if the system is powered off during the specified time.

Purpose:

Anacron is ideal for scenarios where tasks need to be performed regularly, but the system might not be online all the time, such as on laptops or intermittently used servers.

Syntax

anacron [-s]  [-f]  [-n] [-d] [-q] [-t anacrontab] [-S spooldir] [job]
anacron [-S spooldir] -u [-t anacrontab] [job] ...
anacron [-V|-h]
anacron -T [-t anacrontab]

f : Used to force execution of the jobs, ignoring the timestamps.
u : Only update the timestamps of the jobs, to the current date, but don’t run anything.
s : Serialize execution of jobs. Anacron will not start a new job before the previous one finished.
n : Run jobs now.Ignore any delay.
d : Don’t fork to the background. In this mode, Anacron will output informational messages to standard error, as well as to syslog. The output of jobs is mailed as usual.
q : Suppress messages to standard error. Only applicable with -d.
V (Use specified anacrontab) : Print version information and exit.
h (Use specified anacrontab) : Print short usage message, and exit.

Examples:

• Schedule a weekly system update check:

    @weekly /path/to/update_check.sh
  

• Run a monthly backup on the 1st day of each month:

    @monthly /path/to/backup_script.sh
  

• Execute a task every 12 hours:

    0 */12 * * * /path/to/task.sh
  

• Set up a task to run every day at 8:00 AM:

    @daily /path/to/daily_task.sh
  

• Ensure a task is executed every 10 days:

    0 0 */10 * * /path/to/ten_day_task.sh
  

CRON vs ANACRON

The major difference between cron and anacron is that cron works effectively on machines that will run continuously while anacron is intended for machines that will be powered off in a day or week.

At

• at command is a command-line utility that is used to schedule a command to be executed at a particular time in the future.

• Jobs created with at command are executed only once.

• It executes commands at a particular time and accepts times of the form HH:MM to run a job at a specific time of day. The following expression like noon, midnight, teatime, tomorrow, next week, next Monday, etc. could be used with at command to schedule a job.

Purpose:

At is useful for scheduling one-time tasks or jobs that need to be executed at a specific moment in the future.

Syntax:

at [OPTION...] runtime
#press Ctrl+D to save job

at 15:00
warning: commands will be executed using /bin/sh
at> /usr/bin/touch file_created_by_at

#at a specific date
at 'August 20 2022'
#at a specific date and time
at '2:30 August 20 2022'

#using relative dates and time
# run 30 min later
at 'now + 30 minutes'

#run 3 hours later
at 'now + 3 hours'

#run 3 days later
at 'now + 3 days'

#run 3 weeks later
at 'now + 3 weeks'

#run 3 months later
at 'now + 3 months'

#to see which jobs are scheduled to run
atq
20 Wed Nov 17 08:30:00 2021 a aaron
# 20 is job ID

#if we forget what job is supposed to do use -c option and job ID
at -c 20
LESSOPEN=\|\|/usr/bin/lesspipe.sh\ %s; export LESSOPEN
cd /home/aaron || {
echo 'Execution directory inaccessible' >&2
exit 1
}
${SHELL:-/bin/sh} << 'marcinDELIMITER1d46213b'
command1
command2
marcinDELIMITER1d46213b

#to remove a command use atrm with job id
atrm 20

Examples:

Schedule a job to run at a specific time tomorrow:

•     at 2:30 PM tomorrow
  

• Run a script in two hours:

    at now + 2 hours < /path/to/script.sh
  

• Schedule a task for midnight:

    at midnight
  

• Execute a command at a specific date and time:

    at 2024-02-01 15:45 < /path/to/task.sh
  

• Set up a job to run at 3:00 AM next Monday:

    at 3:00 AM next Monday
  

Verify completion of scheduled jobs

• Every cron, anacron and at jobs are logged in system.

sudo cat /var/log/cron
sudo grep CMD /var/log/cron
sudo grep anacron /var/log/cron
sudo grep atd /var/log/cron

Conclusion

In conclusion, mastering the use of Crontab, Anacron, and At provides system administrators with powerful tools for automating tasks on Unix-like systems. Whether scheduling recurring jobs, accommodating intermittent system availability, or planning one-time executions, these tools offer flexibility and precision in managing routine processes.