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RHCSA - Deploy, Configure, & Maintain Systems: Schedule Tasks Using at & cron

Both cron and at are used for scheduling tasks to be executed at specific times or intervals. They are essential tools for automating various administrative and repetitive tasks.

cron

cron is a time-based job scheduler allowing users to schedule tasks (commands or scripts) to run at specific intervals or times. These tasks can be set to run daily, weekly, monthly, or even at custom intervals. cron uses a configuration file called the crontab to define scheduled tasks and every user can have their own crontab.

To access the crontab file you use the crontab command with either the -e option to edit the crontab or -l option to list entries in the crontab to screen. by default it will open the crontab of the current user but root or sudo can access other user crontabs by specifying the -u <user> option.

To remove a crontab file you use the crontab -r command.

The -e for edit and -r for remove options are next to each other on the keyboard. Maybe one of your cron jobs could be making a backup of the cron itself (Trying to recreate it from memory is not fun).

The table below shows the fields, the allowed values, and their meanings for a crontab entry:

Field	Allowed Values	Meaning
Minute	0-59	Minute of the hour when the task should run (0-59)
Hour	0-23	Hour of the day when the task should run (0-23)
Day of Month	1-31	Day of the month when the task should run (1-31)
Month	1-12 or JAN-DEC	Month of the year when the task should run (1-12 or JAN-DEC)
Day of Week	0-6 or SUN-SAT	Day of the week when the task should run (0-6 or SUN-SAT)
Command	Any valid command	The command that should be executed at the specified time

An example user cron entry may look like:

# Run a backup script every day at 2:30 AM
30 2 * * * /path/to/backup_script.sh

# Send a daily email reminder at 8:15 AM
15 8 * * * echo "Don't forget your daily tasks!" | mail -s "Daily Reminder" user@example.com

# Generate a monthly report on the 1st of each month
0 0 1 * * /path/to/weekly_report.sh

# Clean up temporary files every Monday, Wednesday & Friday at 10 PM
0 22 * * 1,3,5 /path/to/cleanup_script.sh

# Update a Git repository Monday to Friday, in working hours, every 15 minutes 
*/15 9-17 * * 1-5 cd /path/to/repo && git pull

# Send Christmas reminder to do shopping at 8AM on 24th Dec
0 8 24 12 * echo "Don't forget it is Christmas tomorrow!" | mail -s "Daily Reminder" user@example.com

The following exercises will get you working with cron:

Create a cron entry and observe it run:

Open your crontab file for editing:

crontab -e

Add an entry that will append the current date & time to a file every minute:

# Print date and time to file /tmp/cron-executed.log
* * * * * echo "$(date)" >> /tmp/cron-executed.log

Tail the log file /tmp/cron-executed.log to see the date & time print to file every minute:

tail -f /tmp/cron-executed.log

Use Ctrl+C to stop the tail command.

List your crontab file now that it has an entry present:

crontab -l

The crontab should display to screen:

# Print date and time to file /tmp/cron-executed.log
* * * * * echo "$(date)" >> /tmp/cron-executed.log

at

The at command allows users to schedule tasks to run once at a specific time in the future. It is particularly useful for one-time tasks that need to be executed at a specific moment.

The at command takes a time arguments, for example at 17:00, and then drops you into the at prompt where you specify the command you wish to run at that time. To submit the job, you then press Ctrl+D. It can also take relative time arguments such as at now + 1 minute.

There are 2 other useful commands related to at which are; atq to show the list of scheduled jobs and atrm to remove a scheduled job.

Schedule a job with an exact and relative time:

To schedule a job to run at a specific time, run:

at 17:00

Then in the at prompt, type:

echo 'echo command scheduled for 17:00' > /tmp/at-1700.job

Then press Ctrl+D to submit the job.

To schedule a job with a relative time, run:

at now + 10 minutes

Then in the at prompt, type:

echo 'echo command that was scheduled for 10 minutes time' > /tmp/at-5mins.job

Then press Ctrl+D to submit the job.

Display and remove a scheduled job:

To list scheduled jobs:

atq

This will display the time of when the 2 scheduled jobs are due to run:

1   Thu Aug 10 17:00:00 2023 a user1
2   Thu Aug 10 08:59:00 2023 a user1

Remove job 1 as it is no longer required:

atrm 1

When listing the at queue again, it will only display job 2 (unless it took you over 10 minutes to complete the exercise, in which case no jobs will be displayed):

atq

2   Thu Aug 10 08:59:00 2023 a user1

Check at job run successfully:

After 10 minutes the scheduled job should have run. Check file /tmp/at-5mins.job to see if it has the expected contents:

cat /tmp/at-5mins.job

You should receive output:

echo command that was scheduled for 10 minutes time

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