LPIC-1 107.2 — Automate System Administration Tasks by Scheduling Jobs

Exam weight: 4 — LPIC-1 v5, Exam 102

What You Need to Know

From the official LPIC-1 objectives:

• Manage cron and at jobs.
• Configure user access to job scheduling.
• Use systemd timer units as an alternative to cron.

Key files and commands: /etc/cron.d/, /etc/cron.{daily,hourly,monthly,weekly}/, /etc/at.deny, /etc/at.allow, /etc/crontab, /etc/cron.allow, /etc/cron.deny, /var/spool/cron/, crontab, at, atq, atrm, systemctl, systemd-run.

Cron — Recurring Jobs

The cron daemon wakes every minute and checks crontab files for jobs to run. User crontabs are stored in /var/spool/cron/.

User Crontab Format

A user crontab has 5 time fields followed by the command:

min  hour  dom  month  dow  command

Time Field Operators

Crontab Examples

# Run daily at 10:00 am
0 10 * * * /home/frank/foo.sh

# Run every Tuesday at 08:00, 08:15, 08:30, 08:45
0,15,30,45 08 * * 2 /home/frank/bar.sh

# Run Mon-Fri Jan and June, days 1-15, at 20:30
30 20 1-15 1,6 1-5 /home/frank/foobar.sh

# Every 20 minutes
*/20 * * * * /path/to/script.sh

Crontab Shortcuts

Crontab Variables

Set at the top of a crontab file:

crontab Command

System Crontabs

System crontabs have a 6th field (username) before the command, since they run as root:

# /etc/crontab or files in /etc/cron.d/
30 01 * * * root /root/barfoo.sh >>/root/output.log 2>>/root/error.log

Drop-in directories for scripts (no crontab format needed — just executable scripts):

• /etc/cron.hourly/
• /etc/cron.daily/
• /etc/cron.weekly/
• /etc/cron.monthly/

Cron Access Control

/etc/cron.allow and /etc/cron.deny control which non-root users may use crontab:

• If /etc/cron.allow exists: only listed users may schedule jobs.
• If only /etc/cron.deny exists: listed users may not schedule jobs; an empty file means all users are allowed.
• If neither exists: access depends on the distribution.

Both files contain one username per line.

systemd Timers — Cron Alternative

Systemd timers are unit files with the .timer suffix. Each timer activates a matching .service unit (same name, different suffix).

Timer File Structure

[Unit]
Description=Run the foobar service

[Timer]
OnCalendar=Mon *-*-1..7 05:30:00
Persistent=true

[Install]
WantedBy=timers.target

OnCalendar Syntax

DayOfWeek Year-Month-Day Hour:Minute:Second

DayOfWeek is optional. Use * for any, , for list, .. for range (not -), / for step.

OnCalendar Examples

OnCalendar Shortcuts

Managing Timers

systemctl enable foobar.timer
systemctl start foobar.timer
systemctl daemon-reload          # after editing timer files
systemctl list-timers            # active timers, sorted by next trigger
systemctl list-timers --all      # include inactive timers

Timer logs go to the systemd journal; review with journalctl. Ordinary users need --user with systemctl and journalctl.

at — One-Time Jobs

at schedules a command to run once at a specified future time. The atd daemon must be running.

$ at now +5 minutes
warning: commands will be executed using /bin/sh
at> date
at> Ctrl+D
job 12 at Sat Sep 14 09:15:00 2019

at Options

Time Specifications for at

• HH:MM — 24h or 12h with AM/PM; if time has passed, next day is assumed
• midnight, noon, teatime (4 PM)
• now +N minutes/hours/days/weeks
• today, tomorrow
• Date formats: month-name day, MMDDYY, MM/DD/YY, DD.MM.YY, YYYY-MM-DD

atq and atrm

atq                    # list pending at jobs (as root: all users)
atrm 14                # delete job 14
atrm 50 51 52          # delete multiple jobs
at -l                  # alias for atq
at -d 14               # alias for atrm 14

at Access Control

Same logic as cron:

• If /etc/at.allow exists: only listed users may schedule at jobs.
• If only /etc/at.deny exists: listed users may not; empty file = all allowed.
• If neither exists: access depends on the distribution.

systemd-run — Alternative to at

# Run date at a specific calendar time
systemd-run --on-calendar='2019-10-06 11:30' date

# Run a script after 2 minutes
systemd-run --on-active="2m" ./foo.sh

Ordinary users need --user. Review logs with journalctl --user.

Quick Reference

User crontab: 5 time fields + command (min hour dom month dow cmd)
System crontab: 6 fields (adds username before command)
  /etc/crontab, /etc/cron.d/

Drop-in script dirs (no crontab format):
  /etc/cron.hourly/  /etc/cron.daily/  /etc/cron.weekly/  /etc/cron.monthly/

User crontabs stored in: /var/spool/cron/

crontab flags:
  -e  edit    -l  list    -r  remove    -u user  (root only)

Cron shortcuts (in crontab):
  @reboot  @hourly  @daily/@midnight  @weekly  @monthly  @yearly/@annually

Cron output: mailed by default; MAILTO="" suppresses; MAILTO="user" redirects

Cron access:
  /etc/cron.allow   whitelist (only listed users allowed)
  /etc/cron.deny    blacklist (listed users denied; empty = all allowed)

systemd timer:
  OnCalendar=DayOfWeek Year-Month-Day Hour:Minute:Second
  Persistent=true   catch up missed runs
  systemctl enable/start NAME.timer
  systemctl list-timers [--all]

at — one-time scheduling (requires atd):
  at HH:MM / at now +N minutes / at midnight / at tomorrow
  atq                 list pending jobs
  atrm ID             delete job
  at -c ID            show job commands
  at -l = atq    at -d = atrm

at access:
  /etc/at.allow   /etc/at.deny   (same logic as cron)

systemd-run (alternative to at):
  systemd-run --on-calendar='YYYY-MM-DD HH:MM' cmd
  systemd-run --on-active="Nm" cmd

Exam Questions

1. What is the format of a user crontab entry? → Five time fields (min hour dom month dow) followed by the command.
2. What does */20 * * * * mean in crontab? → Run every 20 minutes.
3. What shortcut replaces 0 0 * * * in crontab? → @daily or @midnight
4. What is the difference between /etc/crontab and user crontabs? → System crontabs have a 6th field specifying the username to run the job as.
5. Where are user crontabs stored? → /var/spool/cron/
6. What does crontab -e do? → Opens the current user’s crontab for editing in $EDITOR.
7. What does the MAILTO="" variable in a crontab do? → Suppresses email output from cron jobs.
8. What is /etc/cron.allow? → Whitelist — only listed non-root users may use crontab; takes precedence over cron.deny.
9. What command runs a job once in 30 minutes? → at now +30 minutes
10. What is atq? → Lists pending at jobs; as root, shows jobs for all users.
11. What is atrm? → Deletes pending at jobs by job ID; at -d is an alias.
12. What daemon must be running for at to work? → atd
13. What is the OnCalendar syntax for every Friday in September–December at 16:20? → Fri *-09..12-* 16:20:00
14. How do you view active systemd timers? → systemctl list-timers
15. What systemd-run option schedules a one-time job at a calendar time? → --on-calendar=
16. What does Persistent=true do in a systemd timer? → Runs the timer immediately at next start if the last scheduled run was missed.
17. How do you schedule a cron job for the 1st and 15th of each month at 08:30? → 30 08 1,15 * * command
18. What OnCalendar shortcut means “every Monday at midnight”? → weekly (equivalent to Mon *-*-* 00:00:00)

Exercises

Exercise 1 — Crontab Shortcuts

What crontab time specification corresponds to @weekly?

Exercise 2 — Reading Cron Expressions

Explain what the following crontab entries do:

30 13 * * 1-5
00 09-18 * * *
0,20,40 11 * * Sun
*/20 * * * *

Exercise 3 — Send Cron Output by Email

How do you redirect all cron job output to user emma via email? And how do you prevent any mail from being sent?

MAILTO="emma"      # send output to emma
MAILTO=""          # suppress all mail

Exercise 4 — One-Time at Job

Schedule a job to run ./backup.sh at 10:30 AM on October 31st as an ordinary user.

at 10:30 AM October 31
at> ./backup.sh
at> Ctrl+D

Exercise 5 — OnCalendar Expressions

Write an OnCalendar expression that runs a timer every 5 minutes.

LPIC-1 Study Notes | Topic 107: Administrative Tasks