Knowledge Base

What Is a Cron Job on Linux? How to Schedule Tasks on Your Server

In short: A cron job Linux server teams rely on is a command the cron daemon runs from a crontab schedule. Five fields (minute, hour, day of month, month, day of week) describe when it fires — for example 0 2 * * * runs daily at 02:00. This guide stays OS-generic and follows crontab(5); we do not invent CWP menu paths that we have not verified.

1. What Cron and crontab Are For

Cron runs repeating work (backups, log cleanup, certificate hooks, maintenance scripts) without a human at the keyboard. Each user can have a crontab; jobs run as that user. System-wide jobs use /etc/crontab and /etc/cron.d/, which need a username field after the five time fields.

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2. Crontab Fields (Verified Standard)

A user crontab line is: minute hour day-of-month month day-of-week command. Per crontab(5), allowed values are:

• * means “every value” in that field.
• Ranges: 8-11 (hours 8, 9, 10, 11).
• Lists: 1,15 or mon,wed,fri.
• Steps: */15 or 0-23/2.

Note: If both day-of-month and day-of-week are restricted (neither is *), the job runs when either field matches. Example: 30 4 1,15 * 5 → 04:30 on the 1st and 15th or every Friday at 04:30.

3. Editing a User Crontab

crontab(1) manages per-user tables stored under /var/spool/cron/ (or /var/spool/cron/crontabs/ on some distros). Edit via these commands, not by hand-editing spool files:

• crontab -e — opens your editor (VISUAL or EDITOR); saving installs the table.
• crontab -l — lists the current table.
• crontab -r — removes the table (use care; some builds offer -i to confirm).
• crontab -u user -e — as root, edit another user's table.

Example lines:

# Daily backup at 02:00
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

# Top of every hour on weekdays
0 * * * 1-5 /usr/local/bin/hourly-check.sh

# Every Sunday at 05:05
5 4 * * sun /usr/local/bin/weekly.sh

Some cron builds also accept nicknames such as @daily, @hourly and @reboot (@daily ≈ 0 0 * * *). Check your local crontab(5) for support.

4. System Crontab and /etc/cron.d/

System jobs use: minute hour dom month dow user command — a username sits between the schedule and the command. Files:

• /etc/crontab — main system crontab
• /etc/cron.d/ — drop-in system jobs from packages or admins

User crontabs have no username field; system files do. Do not mix the formats.

5. Environment, PATH and Output

Cron does not load your interactive shell profile. Per the man page, SHELL defaults to /bin/sh; HOME and LOGNAME come from /etc/passwd. If your script lives under /usr/local/bin, set an explicit PATH=... at the top of the crontab or use absolute paths. Job output is usually mailed to the crontab owner; set MAILTO=address or MAILTO="" to change that. Redirecting to a log file is practical: ... >> /var/log/job.log 2>&1.

6. Safe Usage Tips

• Run the command once by hand over SSH and check the exit code before scheduling it.
• Write logs only to directories you own; do not scatter files in /.
• cron.allow / cron.deny control who may use the crontab command (not whether existing jobs keep running).
• Never put secrets in a crontab line; use an env file or a proper secret store.

If you need help with scheduled tasks on your VPS or server plan, contact our support team.

Frequently Asked Questions