Cron Syntax Reference

Cron syntax reference — minute, hour, day, month, weekday fields. * * * * *, @daily, @hourly, @reboot. Every cron expression pattern with examples.

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What it is

A cron daemon utility for scheduling commands and scripts to run automatically at specified intervals.

Installation

Cron is typically pre-installed on most Linux and macOS systems.

Linux:

sudo apt update && sudo apt install cron # Debian/Ubuntu
sudo yum install cronie # CentOS/RHEL/Fedora

macOS: Cron is included by default.

Windows: Windows Task Scheduler is the equivalent. Cron for Windows is available but not standard.

Core Concepts

  • Cron Job: A scheduled task.
  • Crontab: A file that contains the schedule of cron jobs to be run. Each user can have their own crontab file.
  • Cron Daemon: The background process that reads crontab files and executes jobs.

Commands / Usage

Managing Crontab Files

  • crontab -e Edit the current user’s crontab file. If no crontab exists, a new one is created.
  • crontab -l List the current user’s crontab entries.
  • crontab -r Remove the current user’s crontab file.
  • crontab -i Remove the current user’s crontab file with a confirmation prompt.
  • crontab -u <username> -e Edit the crontab file for a specific user (requires root privileges).
  • crontab -u <username> -l List the crontab entries for a specific user (requires root privileges).
  • crontab -u <username> -r Remove the crontab file for a specific user (requires root privileges).

Cron Syntax

A cron job line has five time-and-date fields, followed by the command to be executed.

┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of week (0 - 6) (Sunday to Saturday;
│ │ │ │ │                                   7 is also Sunday on some systems)
│ │ │ │ │
* * * * * command_to_execute

Special Characters:

  • * (Asterisk): Wildcard. Matches any value.
    • * * * * * : Run every minute.
  • , (Comma): List separator. Specifies a list of values.
    • 0,15,30,45 * * * * : Run at minutes 0, 15, 30, and 45 of every hour.
    • 0 9,17 * * 1-5 : Run at 9 AM and 5 PM on weekdays (Monday to Friday).
  • - (Hyphen): Range of values.
    • 0 9-17 * * 1-5 : Run at the start of every hour from 9 AM to 5 PM on weekdays.
  • / (Slash): Step values.
    • */15 * * * * : Run every 15 minutes.
    • 0 */2 * * * : Run every 2 hours, on the hour.

Predefined Schcedules (Shortcuts):

These are convenient shortcuts for common schedules.

  • @reboot Run once after reboot.
    • @reboot /home/user/scripts/startup.sh
  • @yearly or @annually Run once a year. Equivalent to 0 0 1 1 *.
    • @yearly /home/user/scripts/yearly_report.sh
  • @monthly Run once a month. Equivalent to 0 0 1 * *.
    • @monthly /home/user/scripts/monthly_backup.sh
  • @weekly Run once a week. Equivalent to 0 0 * * 0.
    • @weekly /home/user/scripts/weekly_cleanup.sh
  • @daily or @midnight Run once a day. Equivalent to 0 0 * * *.
    • @daily /home/user/scripts/daily_sync.sh
  • @hourly Run once an hour. Equivalent to 0 * * * *.
    • @hourly /home/user/scripts/hourly_check.sh

Example Crontab Entries:

  • Run a script every day at 2:30 AM:
    30 2 * * * /usr/local/bin/my_daily_script.sh
  • Run a backup script every Sunday at 11:00 PM:
    0 23 * * 0 /opt/scripts/backup.sh
  • Run a command every 10 minutes:
    */10 * * * * /usr/bin/ping -c 1 google.com > /dev/null
  • Run a command at 8 AM, 12 PM, and 4 PM on weekdays:
    0 8,12,16 * * 1-5 /usr/bin/send_notification.sh
  • Run a cleanup script on the 1st of every month at 3 AM:
    0 3 1 * * /home/user/scripts/cleanup.sh

Environment Variables and PATH:

Cron jobs run with a very minimal environment. The PATH variable is often limited. It’s best practice to:

  1. Use absolute paths for commands and scripts.
  2. Source your environment if your script depends on specific variables.
    0 * * * * . /home/user/.bashrc; /home/user/scripts/my_script_with_env.sh
    Or define variables within the crontab:
    PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/home/user/bin
MY_VAR="some_value"

0 * * * * /home/user/scripts/my_script.sh

Output Redirection:

By default, cron sends the output (stdout and stderr) of a job via email to the user who owns the crontab. To manage this:

  • Discard all output:
    * * * * * /path/to/command > /dev/null 2>&1
    (>/dev/null redirects stdout, 2>&1 redirects stderr to stdout).
  • Log output to a file:
    * * * * * /path/to/command >> /var/log/my_command.log 2>&1
    (Appends output to the log file).

Common Patterns

  • Running a script daily at midnight:
    @daily /home/myuser/scripts/daily_task.sh
  • Running a script every 5 minutes:
    */5 * * * * /home/myuser/scripts/frequent_task.sh
  • Running a script on the first day of the month at 5 AM:
    0 5 1 * * /home/myuser/scripts/monthly_report.sh
  • Running a script at specific times on weekdays:
    0 9,13,17 * * 1-5 /home/myuser/scripts/work_hours_alert.sh
  • Running a script on reboot and logging output:
    @reboot /home/myuser/scripts/startup_service.sh >> /var/log/startup.log 2>&1
  • Running a command that requires a specific PATH:
    PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
0 3 * * * /usr/local/bin/my_custom_tool --process

Gotchas

  • Environment Variables: Cron jobs run with a minimal set of environment variables. PATH is particularly limited. Always use absolute paths for commands and scripts, or explicitly set the PATH in your crontab.
  • Permissions: Ensure the user whose crontab you are editing has the necessary permissions to execute the script and access any files/directories it uses.
  • Working Directory: Cron jobs do not necessarily run from your home directory. Scripts might fail if they expect to be run from a specific directory. Use cd within your script or specify the full path to files.
    * * * * * cd /home/user/myproject && /usr/bin/python manage.py run_scheduled_tasks
  • Output Email: By default, any output (stdout or stderr) from a cron job is emailed to the user. If you don’t want these emails, redirect output to /dev/null or a log file.
  • Special Characters: Be careful with characters like %. In a crontab, % is treated as a newline, and everything after it is passed as standard input to the command. If you need a literal %, escape it with \%.
    # This will run 'echo hello%world'
* * * * * echo hello\%world

# This will echo 'hello' and pass 'world' as stdin to echo
* * * * * echo hello%world
  • System Time: Cron uses the system’s clock. Ensure your system time is accurate, especially if scheduling time-sensitive tasks.
  • Root Crontab: Editing the root crontab (sudo crontab -e) requires root privileges and affects system-wide tasks. Be cautious.
  • @reboot Execution Time: @reboot jobs run after the cron daemon starts, which might be before all other system services are fully initialized. If your job depends on other services, you might need a delay or a more robust startup mechanism.