Cron in Linux — Crontab Syntax, Examples and Calculator

Cron is the standard task scheduler in Linux: it runs commands on a schedule (every minute, hour, day and so on). The schedule is defined by five fields in a crontab file. Here is the syntax plus ready-made examples.

Crontab syntax

Every job line consists of five time fields and a command:

* * * * * command
│ │ │ │ │
│ │ │ │ └─ day of week (0–7, where 0 and 7 are Sunday)
│ │ │ └─── month (1–12)
│ │ └───── day of month (1–31)
│ └─────── hour (0–23)
└───────── minute (0–59)

Special characters:

• * — any value;
• */5 — every 5 units (every 5 minutes, for example);
• 1,15 — a list of values;
• 9-18 — a range.

How to edit the crontab

To open your own job list for editing:

crontab -e

To view the current jobs:

crontab -l

Changes apply automatically once you save — nothing needs restarting.

Schedule examples

Ready-made job examples:

# Database backup every day at 2:30 a.m.
30 2 * * * /usr/local/bin/backup.sh

# Clean up temporary files every Sunday at 4:00
0 4 * * 0 find /tmp -type f -mtime +7 -delete

# Check the website every 10 minutes
*/10 * * * * curl -s https://example.com > /dev/null

Special strings

Instead of five fields you can use convenient aliases:

@reboot   /path/script.sh   # when the server boots
@daily    /path/script.sh   # once a day at midnight
@hourly   /path/script.sh   # once an hour
@weekly   /path/script.sh   # once a week

Logging and debugging

Cron does not print anything to your screen. To see what is happening, redirect the output to a file:

0 2 * * * /path/backup.sh >> /var/log/backup.log 2>&1

2>&1 sends both errors and normal output to the same log.

To check that cron runs jobs at all, look at the system log:

grep CRON /var/log/syslog

Common reasons a job does not run

• Relative paths. Cron runs in a stripped-down environment. Use full paths to commands and files (/usr/bin/php, not php).
• Environment variables. PATH inside cron is minimal. Set the variables you need at the top of the crontab or inside the script.
• No execute permission. Make the script executable: chmod +x /path/script.sh.
• Missing newline. The last line of a crontab must end with a newline.

Frequently asked questions

How does a user crontab differ from the system one? crontab -e edits the current user’s jobs. System jobs live in /etc/crontab and /etc/cron.d/, where you also specify the user the command runs as.

Why does a job work manually but not from cron? Usually because of the environment: cron does not know your PATH and variables. Use absolute paths and set the variables explicitly in the script.

How do I run a job as root? Edit root’s crontab with sudo crontab -e, or add the job to /etc/cron.d/ with the user set to root.

Is there an alternative to cron? Yes, systemd timers are a more flexible and better-logged alternative. But cron is simpler and present on virtually any server, so it remains the standard.

Summary

Cron covers almost any scheduled automation: backups, cleanups, checks. Learn the five syntax fields, always use absolute paths and log the output — and your jobs will run reliably.

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