Task Scheduling

Scheduling Overview

Linux provides three complementary tools for running tasks at specific times:

Tool	When to use
cron	Recurring tasks on always-on servers (runs at fixed times)
anacron	Recurring tasks on desktops/laptops that may be off when cron fires
at	One-off tasks to run once at a specific future time
sleep	Delay within a script or shell session

---

at - One-Off Scheduled Commands

at schedules a command to run once at a specific future time. Jobs are run by the atd daemon.

# Basic scheduling
at 15:00                    # at 3 PM today (or tomorrow if past)
at 15:00 2025-12-31         # at 3 PM on a specific date
at 'August 20 2025'         # on a specific date
at '2:30 August 20 2025'    # date and time

# Human-friendly time expressions
at noon
at midnight
at teatime                  # 4 PM
at tomorrow
at 'next Monday'
at 'now + 30 minutes'
at 'now + 3 hours'
at 'now + 3 days'
at 'now + 3 weeks'
at 'now + 3 months'

After typing at <time>, you enter an interactive prompt. Type your commands, then press CTRL-D to save:

at 15:00
warning: commands will be executed using /bin/sh
at> /usr/bin/touch /tmp/file_created_by_at
at> <CTRL-D>
job 20 at Wed Nov 17 15:00:00 2021

Managing at Jobs

atq                         # list pending at jobs
# 20 Wed Nov 17 15:00:00 2021 a aaron
# (20 is the job ID)

at -c 20                    # show what job 20 will run

atrm 20                     # remove job 20

Access Control

at obeys access control files:

• /etc/at.allow - if this exists, only listed users can use at
• /etc/at.deny - list of users denied access; if neither file exists, only root can use at

---

cron - Recurring Scheduled Tasks

cron runs jobs on a recurring schedule. It is a daemon that wakes up every minute, checks all crontab files, and runs any jobs whose time has come.

Note

A daemon is a background process that runs without direct user interaction, waiting to be triggered by a specific event or condition. cron (also crond) is one of the most fundamental system daemons.

How cron Works

• cron is driven by crontab files - simple text files listing jobs and their schedules
• There are system-wide crontab files (/etc/crontab, /etc/cron.d/) and per-user files (managed with crontab -e)
• Each cron job is defined by a CRON expression (5 time fields) + the command to run

Cron Limitations

Limitation	Impact
Minimum interval is 60 seconds	Cannot schedule sub-minute jobs
Centralized on one machine	If the machine is off, jobs are missed (no make-up)
No retry on failure	A failed job won’t run again until next scheduled time
Misses jobs if machine is powered off	Solution: use anacron instead

Crontab Syntax

Forget the syntax? Run cat /etc/crontab to see an example with comments.

MIN  HOUR  DOM  MON  DOW  CMD
0    2     *    *    *    /usr/local/bin/backup.sh

Field	Name	Range
1	Minute	0-59
2	Hour	0-23
3	Day of Month	1-31
4	Month	1-12 (or Jan-Dec)
5	Day of Week	0-6 (0=Sunday)
6	Command	Full path recommended

Special characters:

Symbol	Meaning	Example
*	Every value	* * * * * = every minute
,	Multiple values	15,45 = at 15 and 45 minutes
-	Range	2-4 = hours 2, 3, 4
/	Step	*/4 = every 4 hours; 0-23/2 = every 2 hours

Example Crontab Entries

# Every minute
* * * * * /path/to/script.sh

# Every day at 2:30 AM
30 2 * * * /usr/local/bin/backup.sh

# Every Monday at 8 AM
0 8 * * 1 /usr/local/bin/weekly-report.sh

# 10th of every month at midnight
0 0 10 * * /usr/local/bin/monthly-cleanup.sh

# Every 15 minutes
*/15 * * * * /usr/local/bin/check-status.sh

# Weekdays only at 9 AM
0 9 * * 1-5 /usr/local/bin/workday-start.sh

# Full backup at 8:30 AM on June 10
30 8 10 6 * /home/sysadmin/full-backup

Managing Crontabs

crontab -e                  # edit your crontab (opens in $EDITOR)
crontab -l                  # list your crontab
crontab -r                  # remove your crontab (careful - no confirmation)

sudo crontab -l             # view root's crontab
sudo crontab -e -u username # edit another user's crontab
sudo crontab -r -u username # remove another user's crontab

System Crontab Directories

Drop scripts directly into these directories for automatic scheduling (no crontab syntax needed):

/etc/cron.hourly/           # run every hour
/etc/cron.daily/            # run every day
/etc/cron.weekly/           # run every week
/etc/cron.monthly/          # run every month

# Add a script to run hourly
sudo cp myscript.sh /etc/cron.hourly/
sudo chmod +rx /etc/cron.hourly/myscript.sh

# Remove it
sudo rm /etc/cron.hourly/myscript.sh

Scripts in these directories must be executable and should not have a file extension (or have at most a .sh extension).

---

anacron - Cron for Machines That Sleep

anacron executes jobs periodically (in days), but unlike cron, it checks timestamps to see if a job was missed. If the machine was off when a job was due, anacron runs it at the next boot (after a configurable delay).

This makes anacron ideal for desktops, laptops, and any machine that isn’t guaranteed to be running 24/7.

How anacron Works

1. On startup (or periodically), anacron reads /etc/anacrontab
2. For each job, it checks the timestamp in /var/spool/anacron/
3. If current_date - last_run >= period_in_days, it runs the job
4. A configurable delay in minutes staggers jobs to avoid all running at once immediately after boot

Key files:

• /etc/anacrontab - job definitions
• /var/spool/anacron/ - timestamp files per job (daily, weekly, monthly, etc.)

anacrontab Format

period_days   delay_minutes   job-identifier   command

sudo vim /etc/anacrontab

# Run every 3 days, 10 min delay after boot
3    10    test-job    /usr/bin/touch /root/anacron_was_here

# Weekly
7    10    weekly-job  /usr/local/bin/weekly.sh
@weekly  10    weekly-job  /usr/local/bin/weekly.sh   # same thing

# Monthly
@monthly  15   monthly-job  /usr/local/bin/monthly.sh

The delay (in minutes) is important: if a machine was off and missed 5 scheduled daily jobs, running all 5 simultaneously at boot would cause a CPU/I/O spike. The delay staggers them.

Verify anacrontab Syntax

anacron -T                   # test syntax (no output = valid)
# anacron: Invalid syntax in /etc/anacrontab on line 17 - skipping this line

anacron Flags

Flag	Effect
-f	Force run all jobs, ignoring timestamps
-n	Run now - ignore delay
-s	Serialize: don’t start a new job until previous one finishes
-u	Update timestamps only; don’t actually run jobs
-d	Debug mode: don’t fork, print messages to stderr
-T	Test syntax of /etc/anacrontab

---

cron vs anacron

Feature	cron	anacron
Type	Daemon (runs continuously)	Not a daemon (runs on trigger)
Best for	Servers (always on)	Desktops / laptops
Minimum interval	1 minute	1 day
Misses jobs when off?	Yes - they are lost	No - runs them on next boot
Who can use it	Root and regular users	Root only (unless specially configured)
Retry on failure	No	No

Rule of thumb:

• Server that’s always on: use cron
• Workstation or laptop that sleeps/hibernates: use anacron (or systemd timers)

---

sleep - Pause Execution

sleep suspends execution for a specified duration. After the time elapses (or a signal is received), execution resumes.

sleep 5               # wait 5 seconds
sleep 5s              # same (explicit suffix)
sleep 2m              # 2 minutes
sleep 1h              # 1 hour
sleep 1d              # 1 day
sleep 0.5             # 500 milliseconds

Suffixes: s (seconds, default), m (minutes), h (hours), d (days).

Common Uses in Scripts

# Retry loop with backoff
for i in 1 2 3 4 5; do
    command && break        # run command; if success, exit loop
    echo "Attempt $i failed, waiting 10s..."
    sleep 10
done

# Wait for a service to come up
until systemctl is-active --quiet nginx; do
    echo "Waiting for nginx..."
    sleep 2
done

# Throttle a loop (avoid hammering an API)
for item in "${items[@]}"; do
    process "$item"
    sleep 1
done

Tip

sleep and at serve different purposes:

• sleep pauses the current process for a duration
• at schedules a new process to start at an absolute time

---

Verifying Scheduled Jobs Ran

All cron, anacron, and at jobs are logged:

# All cron job activity
sudo cat /var/log/cron
journalctl -u crond --since "yesterday"

# Filter by type
sudo grep CMD /var/log/cron         # cron jobs that ran
sudo grep anacron /var/log/cron     # anacron activity
sudo grep atd /var/log/cron         # at jobs that ran

# Check for failed jobs (non-zero exit)
journalctl -u crond -p warning

Output from cron jobs is typically mailed to the crontab owner. To redirect it:

# Discard all output
* * * * * /path/to/script.sh > /dev/null 2>&1

# Log stdout and stderr to a file
* * * * * /path/to/script.sh >> /var/log/myscript.log 2>&1

# Log with timestamp
* * * * * date >> /tmp/job.log && /path/to/script.sh >> /tmp/job.log 2>&1