Scheduling with cron and at#

Concepts#

cron — Recurring Scheduled Tasks#

cron runs commands on a schedule — every minute, every day, every Monday, etc. The cron daemon (cron or crond) checks schedules once per minute.

Crontab Syntax#

Each line in a crontab defines one scheduled task:

┌───────── minute (0-59)
│ ┌─────── hour (0-23)
│ │ ┌───── day of month (1-31)
│ │ │ ┌─── month (1-12 or jan-dec)
│ │ │ │ ┌─ day of week (0-7, 0 and 7 = Sunday, or sun-sat)
│ │ │ │ │
* * * * *  command to execute

Examples#

# Every minute
* * * * *  /usr/local/bin/check-health.sh

# Every day at 3:30 AM
30 3 * * *  /home/kmiguel/backup.sh

# Every Monday at 8 AM
0 8 * * 1  echo "Week started" | mail -s "Monday" kmiguel

# Every 15 minutes
*/15 * * * *  /usr/local/bin/sync.sh

# First day of every month at midnight
0 0 1 * *  /home/kmiguel/monthly-report.sh

# Weekdays at 6 PM
0 18 * * 1-5  /home/kmiguel/eod-report.sh

# Every 6 hours
0 */6 * * *  /usr/local/bin/cleanup.sh

Special Strings#

@reboot    command     # run once at startup
@hourly    command     # 0 * * * *
@daily     command     # 0 0 * * *
@weekly    command     # 0 0 * * 0
@monthly   command     # 0 0 1 * *
@yearly    command     # 0 0 1 1 *

Managing Your Crontab#

# Edit your crontab
crontab -e

# List your crontab
crontab -l

# Remove your entire crontab (careful!)
crontab -r

# Edit another user's crontab (root only)
sudo crontab -u username -e

Cron Tips#

Output handling: By default, cron emails any output to the user. Since most systems don’t have a local mail setup, redirect output:

# Redirect stdout and stderr to a log
30 3 * * * /home/kmiguel/backup.sh >> /home/kmiguel/backup.log 2>&1

# Discard output entirely
*/5 * * * * /usr/local/bin/healthcheck.sh > /dev/null 2>&1

Environment: cron runs with a minimal environment (limited PATH, no .bashrc). If your script needs specific tools:

# Use full paths in cron commands
30 3 * * * /usr/bin/python3 /home/kmiguel/script.py

# Or set PATH in the crontab
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
30 3 * * * backup.sh

Permissions: Scripts must be executable (chmod +x script.sh).

System Cron Directories#

Besides user crontabs, the system has directories for system-wide scheduled tasks:

ls /etc/cron.d/           # custom system cron files
ls /etc/cron.daily/       # scripts run daily
ls /etc/cron.hourly/      # scripts run hourly
ls /etc/cron.weekly/      # scripts run weekly
ls /etc/cron.monthly/     # scripts run monthly

Scripts placed in cron.daily/, cron.hourly/, etc. are run by anacron (which handles machines that aren’t always on — it runs missed jobs after boot).

System cron files in /etc/cron.d/ have an extra field for the user:

# /etc/cron.d/example
*/10 * * * * root /usr/local/bin/system-check.sh
#                 ^^^^ user field

at — One-Time Scheduled Tasks#

at runs a command once at a specific time (unlike cron which repeats).

# Install (may not be pre-installed)
sudo apt install -y at
sudo systemctl enable --now atd

# Schedule a command
at 14:30
# Type commands, then Ctrl+D to submit:
echo "Reminder: meeting!" > /tmp/reminder.txt

# Various time formats
at now + 5 minutes
at now + 2 hours
at 3:00 PM
at midnight
at noon tomorrow
at 10:00 AM 2026-05-15

# One-liner
echo "/home/kmiguel/task.sh" | at now + 1 hour

# List pending jobs
atq

# View a job's commands
at -c JOB_NUMBER

# Remove a pending job
atrm JOB_NUMBER

systemd Timers#

systemd can also schedule tasks using timer units. They are more powerful than cron (support dependencies, logging via journalctl, randomized delays) but more verbose to set up.

# List active timers
systemctl list-timers

# Common system timers
systemctl list-timers --all | head -10

A systemd timer consists of two unit files: a .timer (the schedule) and a .service (the action). For most user-level tasks, cron is simpler. Timers are worth learning when you need systemd integration.

Example Timer#

# /etc/systemd/system/backup.service
[Unit]
Description=Daily backup

[Service]
Type=oneshot
ExecStart=/home/kmiguel/backup.sh
User=kmiguel

# /etc/systemd/system/backup.timer
[Unit]
Description=Run backup daily

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

sudo systemctl enable --now backup.timer
systemctl list-timers | grep backup

cron vs at vs systemd Timers#

Lab#

Exercise 1: View Existing Cron Jobs#

# Your crontab
crontab -l

# System cron
ls /etc/cron.d/
ls /etc/cron.daily/

# Active timers
systemctl list-timers --no-pager | head -10

Exercise 2: Create a Cron Job#

# Add a cron job that logs the date every minute
crontab -e
# Add this line:
# * * * * * echo "$(date): cron ran" >> /tmp/crontest.log

# Wait 2-3 minutes, then check
cat /tmp/crontest.log

# Remove the test job
crontab -e
# Delete the line you added

# Clean up
rm -f /tmp/crontest.log

Exercise 3: Use at for a One-Time Task#

# Schedule a task 1 minute from now
echo 'echo "at ran at $(date)" > /tmp/attest.txt' | at now + 1 minute

# List pending jobs
atq

# Wait, then check
cat /tmp/attest.txt

# Clean up
rm -f /tmp/attest.txt

Exercise 4: Explore System Timers#

# List all timers
systemctl list-timers --all --no-pager

# Check a specific timer
systemctl status apt-daily.timer 2>/dev/null
systemctl cat apt-daily.timer 2>/dev/null

Review#

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