Ubuntu Tutorials - Herong's Tutorial Examples

∟Process Management

∟Cron Daemon, Table and Jobs

This section describes what is cron daemon and its related cron tables/jobs.

What Is a Cron Job? - A cron job is a task scheduled to be executed at specific times.

On a Ubuntu system, there are 4 components involved in managing and executing cron jobs.

1. Cron Daemon, "crond" - The process that runs cron jobs according their scheduled times.

2. System Cron Table, "/etc/crontab" - The system file that defines cron jobs and their execution times.

3. User Cron Tables, "/var/spool/cron/*" - User files that defines cron jobs and their execution times for each user.

4. Cron Table Manager, "crontab" - The command to manage user cron tables.

Here is what I did to view the Cron Daemon and the Cron Table on my Ubuntu 18 computer.

1. The cron daemon, "crond", should be launched at the system startup time and runs forever.

herong$ ps -elf | grep crond
F S UID    PID  PPID  C PRI  STIME TTY      TIME CMD
4 S root  1724     1  0  80  13:46 ?    00:00:00 /usr/sbin/crond -n

2. The system cron table, "/etc/crontab", should be empty on a new computer.

herong$ sudo more /etc/crontab

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# For details see man 4 crontabs

# 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,...
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed

3. Add a cron job in my own cron table by running the "crontab -e" command in a system default editor.

herong$ crontab -e

# Run it at 06:00 every day
0 6 * * * echo "Time to wake up!" > /dev/null

4. View the cron table of a given user.

herong$ sudo crontab -l -u herong

# Run it at 06:00 every day
0 6 * * * echo "Time to wake up!" > /dev/null

Cron Job Scheduling Syntax - As you can see from the system cron table file, the cron job scheduling syntax supports 5 parameters for 5 time components: minute, hour, date, month, day-of-week.

Cron job scheduling parameters work together based on the following rules:

• Scheduling parameters are evaluated very minute to determine if the cron job should be launched or not.
• Each parameter is evaluated independently to result a Boolean value.
• If all 5 resulting Boolean values are TRUE, the cron job will be launched.
• If all 5 resulting Boolean values are TRUE, the cron job will be launched.

Each scheduling parameter supports 5 syntaxes:

• A number - The current time component must match.
• "*" - The wildcard number to be replaced by the current time component.
• "," - The Boolean OR operator to combine neighboring elements.
• "-" - The RANGE operator to create a list of numbers joined with OR operators.
• "/" - The DIVISIBLE operator, resulting TRUE if the left number is divisible by the right number.

Here are some nice examples of cron job schedules:

0 6 * * * echo "Time to wake up!" > /dev/null
0 0 1 * * echo "Happy first day of the month!" > /dev/null
0 8,19 * * * echo "Take it twice a day!" > /dev/null
*/10 * * * * echo "Check Website every 10 minutes..." > /dev/null
*/5 7-9,16-18 * * 1-5 echo "Every 5 minutes in rush hour ..." > /dev/null

Table of Contents

 About This Book

 Introduction to Ubuntu Systems

 GNOME - Desktop Interface and Environment

 Shell - The Command-Line Interpreter

►Process Management

 "ps" - Display Current Processes

 "jobs" - Manage Background Jobs

►Cron Daemon, Table and Jobs

 Memory Management

 Files and Directories

 APT (Advanced Package Tool)

 Network Connection on Ubuntu

 Internet Networking Tools

 SSH Protocol and ssh/scp Commands

 Administrative Tasks

 References

 Full Version in PDF/EPUB

Cron Daemon, Table and Jobs - Updated in 2025, by Herong Yang