Cron and Anacron

cron and anacron are scheduled task systems. cron runs tasks at specific times on always-on systems. anacron runs missed tasks on systems that aren’t on 24/7.

cron — Time-Based Scheduling

cron runs as a daemon (crond) that wakes up every minute, reads schedule files, and executes matching jobs.

Crontab Format

┌───────────── minute (0-59)
│ ┌───────────── hour (0-23)
│ │ ┌───────────── day of month (1-31)
│ │ │ ┌───────────── month (1-12)
│ │ │ │ ┌───────────── day of week (0-7, 0 and 7 = Sunday)
│ │ │ │ │
* * * * * command

Field	Values	Specials
minute	0-59	* = any, */n = every n
hour	0-23	* = any, */2 = every 2h
day	1-31	* = any
month	1-12	* = any
weekday	0-7	* = any, 0,7=Sunday

Examples

# Every minute
* * * * * /usr/local/bin/backup.sh
 
# Every hour at minute 15
15 * * * * /usr/local/bin/check-alerts.sh
 
# Every day at 3:00 AM
0 3 * * * /usr/local/bin/daily-report.sh
 
# Every Monday at 9:00 AM
0 9 * * 1 /usr/local/bin/weekly-review.sh
 
# Every 15 minutes
*/15 * * * * /usr/local/bin/monitor.sh
 
# 2:30 AM on weekdays
30 2 * * 1-5 /usr/local/bin/weekday-task.sh
 
# Every 6 hours
0 */6 * * * /usr/local/bin/every-six-hours.sh
 
# Midnight on the 1st of every month
0 0 1 * * /usr/local/bin/monthly.sh
 
# Run once at boot (per user crontab — systemd timers preferred)
/reboot /usr/local/bin/startup-task.sh

Managing Crontabs

# Edit current user's crontab
crontab -e
 
# List current user's crontab
crontab -l
 
# Remove current user's crontab
crontab -r
 
# Edit another user's crontab (requires root)
crontab -u darshan -e
crontab -u nginx -l
 
# Cron log (depends on distro)
#   Debian/Ubuntu: /var/log/syslog (grep CRON)
#   CentOS/RHEL: /var/log/cron
journalctl -u cron
journalctl -u crond

System crontab (root’s crontab, /etc/crontab)

cat /etc/crontab
# SHELL=/bin/bash
# PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
# MAILTO=root
 
# minute hour day month dow user  command
  0      3    *   *    *   root  /usr/local/bin/backup.sh

System crontab includes a user field before the command.

cron.d and cron.daily/hourly

# /etc/cron.d/ — for package-installed cron jobs
ls /etc/cron.d/
# This is where packages (e.g., logrotate, apt) put their cron entries
 
# /etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly/, /etc/cron.monthly/
# run-parts executes scripts in these directories
ls /etc/cron.daily/
# logrotate  apt-compat  man-db  ...

@reboot — Boot-Time Jobs

# In any crontab:
@reboot /usr/local/bin/startup.sh
@reboot sleep 60 && /usr/local/bin/delayed-start.sh

Note: @reboot runs after cron daemon starts (usually ~30s after boot). For more precise timing, use systemd timers with AccuracySec=1us.

Anacron — For Non-Always-On Systems

cron misses jobs if the system is off. anacron tracks when jobs last ran and executes any that are overdue:

# Anacron config
cat /etc/anacrontab
 
# Format: period delay identifier command
#   period:    how often to run (in days, or @daily/@weekly/@monthly)
#   delay:     minutes to wait after boot before running
#   identifier: unique name (for logging)
#   command:    what to run
 
@daily    10  cron.daily   /usr/bin/logger "Daily job"
@weekly   20  cron.weekly  /usr/bin/logger "Weekly job"
@monthly  30  cron.monthly /usr/bin/logger "Monthly job"

Example: If cron.daily is scheduled to run but the machine was off:

1. Machine boots on Tuesday
2. anacron sees /etc/cron.daily wasn’t run since Friday
3. Waits 10 minutes (the delay field), then runs it

Anacron Timestamps

# Anacron stores last-run timestamps here:
ls /var/spool/anacron/
# cron.daily  cron.weekly  cron.monthly
 
# View:
cat /var/spool/anacron/cron.daily
# 20250606   ← last ran on this date

systemd Timers — Modern Alternative

systemd timers are more powerful than cron and are preferred on modern systems:

# /etc/systemd/system/mytask.timer
[Unit]
Description=Run mytask every day at 3am
 
[Timer]
OnCalendar=*-*-* 03:00:00
Persistent=true        # run missed jobs if system was off
RandomizedDelaySec=300 # add up to 5min jitter

# /etc/systemd/system/mytask.service
[Unit]
Description=My Scheduled Task
 
[Service]
Type=oneshot
ExecStart=/usr/local/bin/mytask.sh

systemctl enable --now mytask.timer
systemctl list-timers

cron vs systemd Timers

Feature	cron	systemd timers
Resolution	minute	second (or better)
Boot-time jobs	@reboot	OnBootSec=
Random jitter	no	RandomizedDelaySec=
Manual run	run-parts	systemctl start service
Dependencies	separate	PartOf=, After=
On-demand	no	yes (Path=, Socket=)
Persistence	file	unit files
Syslog	syslog	journald

Gotchas

# SHELL is NOT /bin/bash by default in crontab — it's /bin/sh
# So: cron uses /bin/sh, not bash
# Variables work differently (no arrays, no local, etc.)
 
# Workaround: explicitly call bash or source bash profile
0 3 * * * /bin/bash -c 'source ~/.bashrc && /usr/local/bin/mytask.sh'
 
# PATH is minimal in cron
# Always use absolute paths in crontab
0 3 * * * /usr/local/bin/backup.sh  # YES
0 3 * * * backup.sh                 # NO (might not be found)
 
# % has special meaning in crontab (newline in command output)
# Escape it:
0 3 * * * /usr/local/bin/report.sh "date: %Y-%m-%d"   # WRONG
0 3 * * * /usr/local/bin/report.sh "date: $(date +\%Y-\%m-\%d)"  # YES
 
# Output goes to email by default (MAILTO=)
# Disable output: redirect to /dev/null
0 3 * * * /usr/local/bin/mytask.sh > /dev/null 2>&1
 
# MAILTO="" in crontab suppresses email