Cron Job Reference

The cron utility is a time-based job scheduler in Unix-like operating systems. Users can schedule jobs (commands or scripts) to run periodically at fixed times, dates, or intervals. This reference provides an overview of how to write cron expressions, manage cron jobs, and includes examples for common scheduling tasks.

Understanding the Cron Syntax

A typical cron job consists of a line in the crontab file with the following format:

<minute> <hour> <day_of_month> <month> <day_of_week> <command>

The fields are separated by spaces or tabs and have the following meanings:

Field	Allowed Values	Description
`minute`	0-59	Minute of the hour
`hour`	0-23	Hour of the day
`day_of_month`	1-31	Day of the month
`month`	1-12 or `JAN-DEC`	Month of the year
`day_of_week`	0-6 or `SUN-SAT`	Day of the week (0 or 7 is Sunday)
`command`	Any valid command or script	The task to execute

Special Characters in Cron Expressions

Character	Description	Example
`*`	Matches all possible values	`* * * * *` (Every minute)
`,`	Separates multiple values	`0,15,30,45 * * * *` (Every 15 minutes)
`-`	Defines a range of values	`1-5` (From 1 to 5)
`/`	Specifies step values	`*/5` (Every 5 units)
`#`	Specifies the nth occurrence of a weekday in a month (non-standard, Vixie cron)	`1#2` (Second Monday of the month)
`L`	Specifies the last day of the week or month (non-standard)	`LW` (Last weekday of the month)
`?`	No specific value (used in day_of_month and day_of_week fields, non-standard)
`%`	Newline in the command

Special Scheduling Strings

Cron also supports special strings in place of the five time fields:

String	Equivalent	Description
`@reboot`	N/A	Run once at startup
`@yearly` or `@annually`	`0 0 1 1 *`	Once a year at midnight on January 1st
`@monthly`	`0 0 1 * *`	Once a month at midnight on the first day
`@weekly`	`0 0 * * 0`	Once a week at midnight on Sunday
`@daily` or `@midnight`	`0 0 * * *`	Once a day at midnight
`@hourly`	`0 * * * *`	Once an hour at the beginning of the hour

Common Examples

Cron Expression	Description
`0 * * * * command`	Every hour at minute 0 (e.g., 1:00, 2:00)
`/15 * * * command`	Every 15 minutes
`0 6 * * * command`	Every day at 6:00 AM
`30 21 * * 1-5 command`	At 9:30 PM every weekday (Monday through Friday)
`0 0 1 * * command`	At midnight on the first day of every month
`0 0 * * 0 command`	Every Sunday at midnight
`0 8-18/2 * * * command`	Every 2 hours between 8 AM and 6 PM
`0 0 1 1 * command`	Once a year on January 1st at midnight
`@reboot command`	Run once at system startup

Managing Cron Jobs

crontab -e - Edit your user’s crontab file.
crontab -l - List the contents of your crontab file.
crontab -r - Remove your crontab file.
sudo crontab -e - Edit the root user's crontab file.
crontab -u username -e - Edit the crontab file for a specific user (requires appropriate permissions).

Crontab File Format

The crontab file can include environment variable settings and comments:

# This is a comment
SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin

# Run backup script every day at 2 AM
0 2 * * * /home/user/backup.sh

# Run cleanup script on Sundays at 3 AM
0 3 * * SUN /home/user/cleanup.sh

Tips for Writing Cron Jobs

Specify Absolute Paths: Always use full paths for commands and scripts (e.g., /usr/bin/python3).
Set Environment Variables: The environment in cron may be different from your shell. Set necessary variables in the crontab file.
Redirect Output: By default, cron emails output to the user. Redirect output to a file or to /dev/null if not needed.
```
0 * * * * command > /dev/null 2>&1
```
Use a Wrapper Script: For complex tasks, call a script that handles the logic and environment setup.
Test Your Commands: Run your commands manually to ensure they work before scheduling them with cron.

Advanced Scheduling Examples

Cron Expression	Description
`0 0 * * MON command`	Every Monday at midnight
`0 9-17 * * MON-FRI command`	Every hour from 9 AM to 5 PM on weekdays
`/10 6-18 * * command`	Every 10 minutes between 6 AM and 6 PM every day
`0 0 1 /2 command`	At midnight on the first day of every even-numbered month
`0 0 * * 1 command`	Every Monday at midnight
`0 0 1 1-6/2 * command`	At midnight on the first day of every other month from January through June
`0 12 * * * [ "$(date '+\%d')" -le 07 ] && command`	At noon on the first seven days of the month
`0 0 29 2 * command`	At midnight on February 29th (leap year)

Cron Job Examples

Backup Home Directory

# Backup home directory to /backup at 2 AM every day
0 2 * * * tar -zcf /backup/home_$(date +\%Y\%m\%d).tar.gz /home/user/ > /dev/null 2>&1

Clear Temporary Files

# Clear /tmp directory every week on Sunday at 3 AM
0 3 * * SUN rm -rf /tmp/* > /dev/null 2>&1

Renew Let's Encrypt SSL Certificates

# Renew SSL certificates on the first day of every month at 4 AM
0 4 1 * * /usr/bin/certbot renew --quiet

Run a Python Script

# Execute Python script every day at 6 PM
0 18 * * * /usr/bin/python3 /home/user/scripts/daily_report.py >> /var/log/daily_report.log 2>&1

Special Environment Variables

Cron recognizes several special environment variables:

MAILTO - Specifies the email address to send output to. Set to an empty string to disable email.
```
MAILTO="[email protected]"
```
PATH - Specifies the search path for commands.
```
PATH=/usr/local/bin:/usr/bin:/bin
```
SHELL - Specifies the shell to use (default is /bin/sh).
```
SHELL=/bin/bash
```
HOME - Specifies the home directory to use for the cron job.
```
HOME=/home/user
```

Editing System-Wide Cron Jobs

System-wide cron jobs can be found in the following directories and files:

/etc/crontab - System-wide crontab file.
/etc/cron.d/ - Directory for system cron jobs.
/etc/cron.hourly/ - Scripts to run hourly.
/etc/cron.daily/ - Scripts to run daily.
/etc/cron.weekly/ - Scripts to run weekly.
/etc/cron.monthly/ - Scripts to run monthly.

The system-wide crontab file and files in /etc/cron.d/ have an additional field specifying the user:

<minute> <hour> <day_of_month> <month> <day_of_week> <user> <command>

Cron Allow and Deny Files

Access to cron is controlled by the files /etc/cron.allow and /etc/cron.deny:

If /etc/cron.allow exists, only users listed in it can use cron.
If /etc/cron.allow does not exist but /etc/cron.deny exists, users listed in cron.deny cannot use cron.
If neither file exists, only the superuser can use cron.

Troubleshooting Cron Jobs

Check Logs: Cron logs are typically found in /var/log/cron, /var/log/syslog, or /var/log/messages.
Verify Command Paths: Ensure you are using absolute paths to commands and scripts.
Environment Variables: Remember that cron runs with a limited environment. Set necessary variables in the crontab.
Permissions: Ensure the user running the cron job has permission to execute the commands and access files.
Test Commands Manually: Run your command or script in the terminal to verify it works as expected.
Script Shebang: Ensure your scripts have the correct shebang line (e.g., #!/bin/bash).
Output Redirection: Redirect output to a file for debugging purposes.

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