Scheduling in Unix

Unix provides two main tools for scheduling jobs:

• cron → for repeated/periodic tasks (like every day, every hour, every Monday at 2 AM, etc.)

• at → for one-time tasks (run once at a specific date/time)

1. cron – Periodic Jobs

How cron works

• Runs the cron daemon (crond) in the background.

• Jobs are defined in a file called the crontab (cron table).

Crontab Format

A cron job entry has 6 fields:

MIN  HOUR  DAY  MONTH  WEEKDAY  COMMAND

• MIN → 0–59

• HOUR → 0–23

• DAY → 1–31

• MONTH → 1–12

• WEEKDAY → 0–6 (0 = Sunday)

• COMMAND → the script/command to execute

Examples

crontab -e         # Edit your crontab file
crontab -l         # List your cron jobs

Jobs:

0 2 * * * /home/user/backup.sh        # Run backup script every day at 2:00 AM
30 9 * * 1 /home/user/report.sh       # Run every Monday at 9:30 AM
*/5 * * * * echo "Hello" >> /tmp/hi   # Run every 5 minutes

2. at – One-Time Jobs

How at works

• You schedule a job to run once at a specific time.

• The atd daemon must be running.

Syntax

at TIME

Then type the commands, and press Ctrl+D to finish.

Examples

echo "echo Hello > /tmp/test.txt" | at 5pm     # Run at 5 PM today
at 11:00pm                                     # Start interactive mode
> echo "Backup started" >> /tmp/log
> Ctrl+D                                       # End input

View scheduled jobs:

atq          # List pending at jobs
atrm <jobid> # Remove a scheduled job

3. Comparison

Summary:

• Use cron when you need repeated scheduling (daily, weekly, monthly).

• Use at when you need a one-time scheduled task.