cron

Unix time-based job scheduler. Reads crontab files and runs commands at specified times.

Schedule syntax

┌───────────── minute        (0–59)
│ ┌─────────── hour          (0–23)
│ │ ┌───────── day of month  (1–31)
│ │ │ ┌─────── month         (1–12 or JAN–DEC)
│ │ │ │ ┌───── day of week   (0–7 or SUN–SAT; 0 and 7 = Sunday)
│ │ │ │ │
* * * * *  command_to_run

Operators

Symbol	Meaning	Example	Matches
*	every value	* * * * *	every minute
,	list	0,15,30,45 * * * *	:00, :15, :30, :45
-	range	0 9-17 * * *	every hour 09:00–17:00
/	step	*/5 * * * *	every 5 minutes
combined	range + step	0 8-18/2 * * *	every 2h between 08–18

Common patterns

# Every minute
* * * * *  /path/to/script.sh
 
# Every 5 minutes
*/5 * * * *  /path/to/script.sh
 
# Top of every hour
0 * * * *  /path/to/script.sh
 
# Daily at 02:30
30 2 * * *  /path/to/backup.sh
 
# Weekdays at 09:00
0 9 * * 1-5  /path/to/report.sh
 
# First day of every month at midnight
0 0 1 * *  /path/to/monthly.sh
 
# Every Sunday at 04:00
0 4 * * 0  /path/to/weekly.sh

Named shortcuts (non-POSIX, widely supported)

Shortcut	Equivalent	Meaning
@reboot	—	Run once at startup
@yearly	0 0 1 1 *	Once a year (Jan 1, 00:00)
@monthly	0 0 1 * *	Once a month
@weekly	0 0 * * 0	Once a week (Sunday)
@daily	0 0 * * *	Once a day at midnight
@hourly	0 * * * *	Once an hour at :00

Managing crontabs

crontab -e            # edit current user's crontab
crontab -l            # list current user's jobs
crontab -r            # remove current user's crontab (no confirmation!)
crontab -u alice -e   # edit alice's crontab (root only)
 
sudo systemctl status cron      # check cron daemon (Debian/Ubuntu)
sudo systemctl status crond     # (RHEL/Fedora)

System crontabs

• /etc/crontab — system-wide; extra user field between schedule and command
• /etc/cron.d/* — drop-in files, same format as /etc/crontab
• /etc/cron.{hourly,daily,weekly,monthly}/ — scripts executed at that interval (no schedule line needed)

# /etc/crontab format (note the user field)
0 3 * * *  root  /usr/local/bin/cleanup.sh

Environment

cron runs with a minimal environment. Common gotchas:

# Set env vars at top of crontab
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
MAILTO=ops@example.com   # mail stdout/stderr (empty = disable)
 
# Use absolute paths in commands — cron's $PATH is tiny
0 * * * *  /usr/bin/python3 /opt/app/job.py

Logging and output

# Discard all output
*/5 * * * *  /path/to/job.sh >/dev/null 2>&1
 
# Append to a logfile
*/5 * * * *  /path/to/job.sh >> /var/log/job.log 2>&1
 
# Capture stderr only
*/5 * * * *  /path/to/job.sh 2>> /var/log/job.err

System cron logs: /var/log/syslog (Debian) or journalctl -u cron.

Locking (prevent overlapping runs)

# flock — exits immediately if lock held
*/5 * * * *  flock -n /tmp/job.lock /path/to/job.sh

Gotchas

• Percent signs (%) are interpreted as newlines in commands — escape as \% or quote the command.
• DST can cause jobs to run twice or skip on transition days.
• Day-of-month and day-of-week are OR’d, not AND’d: 0 0 1 * 1 runs every 1st or every Monday.
• Jobs do not run if the machine is asleep — use anacron for laptops.
• The crontab file must end with a newline.

Validation

# Dry-run check (syntax)
crontab -l | crontab -
 
# Online: crontab.guru — paste expression, get human-readable preview