Scheduling Tasks with cron

📝 Note: The majority of the following commands must be run by the superuser @root

Part 1: Using the crontab Command

The crontab command is used to schedule commands to be executed periodically. It allows tasks to be automatically run in the background at regular intervals.

That means that you can use crontab to automatically create backups, synchronize files, schedule updates, and much more.

The main configuration file for cron is /etc/crontab. If you view the content of it, it will display:

# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed

• List programmed tasks

  crontab -l

It will list the crontabs that are currently running on your environment, if you are a root user, you can list all the crons that the system has.

If you have not set any jobs, it will display a message such as crontab: no crontab for x0y.

• Edit the list of cronjobs

  crontab -e

The option e let you edit a list of tasks. You can set some tasks using this format:

  # Every hour, at the minute 37, a copy will take place
  37 * * * *  root  cp a /tmp/b

  # Every day at 5:23 compression will occur
  23 5 * * *  root  zip f1.zip f1

  #Every week on Sunday at 03:19 a copy will be made
  19 3 * * 0  root  scp or-condo-login01.ornl.gov:/tmp/files .

  #Every month on day 6 at 00:23 minutes, a script will be run
  23 0 6 * *  root  ./script

  #run a cron job from a script for every Monday, Wednesday and Friday at 7:00 pm
  0 19 * * 1,3,5 nohup /home/x0y/script.sh > /tmp/script.log 2>&1

Part 2: Using the at Command

Use at when you want to execute a command or multiple commands once at some future time.

at 4:55pm Friday
echo '5 p.m. meeting with Carol' | mail raithel
^D
Job c00ceb7fb.01 will be executed using /bin/sh

The at command takes input up to the end-of-file character (ctrl D while at the beginning of a line). It reports the job number and informs you that it will use /bin/sh to execute the command. An email to raithel will be sent at 4:55pm on Friday with the Subject: '5 p.m. meeting with Carol'.

To program a script from now, you may add hours, minutes, or seconds with the + symbol, e.g.:

at now + 25 minutes
echo ^G > /dev/ttyp4
^D
Job c00ceb7fb.00 will be executed using /bin/sh

This script will notify you with a beep in 25 minutes.

• To get a list of your pending at jobs, enter atq. If you are superuser, atq shows you the pending at jobs of all users.

• To delete a job, enter atrm job_number where job_number is the job number returned by atq. The superuser can also remove other user's jobs.