Linux Cron Scheduling Task

Cron Syntax

A B C D E USERNAME /path/to/command arg1 arg2
OR
A B C D E USERNAME /root/backup.sh

  • A: Minutes range: 0 – 59

  • B: Hours range: 0 – 23

  • C: Days range: 0 – 31

  • D: Months range: 0 – 12

  • E: Days of the week range: 0 – 7. Starting from Monday, 0 or 7 represents Sunday

  • USERNAME: replace this with your username

  • /path/to/command – The name of the script or command you want to schedule

11 Cron Scheduling Task Examples

1. List Crontab Entries

# crontab -l

2. Edit Crontab Entries

# crontab -e

3. List Scheduled Cron Jobs

# crontab -u root -l

4. Remove Crontab Entry

# crontab -r

5. Prompt Before Deleting Crontab

# crontab -i -r

6. Allowed special character (*, -, /, ?, #)

  • Asterik(*) – Match all values in the field or any possible value.

  • Hyphen(-) – To define range.

  • Slash (/) – 1st field /10 meaning every ten minute or increment of range.

  • Comma (,) – To separate items.

7. System Wide Cron Schedule

  • /etc/cron.d

  • /etc/cron.daily

  • /etc/cron.hourly

  • /etc/cron.monthly

  • /etc/cron.weekly

8. Schedule a Jobs for Specific Time

# crontab -e
30 0 * * * root find /tmp -type f -empty -delete

9. Special Strings for Common Schedule

Strings Meanings
@reboot Command will run when the system reboot.
@daily Once per day or may use @midnight.
@weekly Once per week.
@yearly Once per year. we can use @annually keyword also.

10. Multiple Commands with Double amper-sand(&&)

# crontab -e
@daily <command1> && <command2>

11. Disable Email Notification

# crontab -e
* * * * * >/dev/null 2>&1

Cron Job Examples

Run /root/backup.sh at 3 am every day:

0 3 * * * /root/backup.sh

Run script.sh at 4:30 pm on the second of every month:

30 16 2 * * /path/to/script.sh

Run /scripts/phpscript.php at 10 pm during the week:

0 22 * * 1-5 /scripts/phpscript.php

Run perlscript.pl at 23 minutes after midnight, 2am and 4am, everyday:

23 0-23/2 * * * /path/to/perlscript.pl

Run Linux command at 04:05 every Sunday:

5 4 * * sun /path/to/linuxcommand

Cron Options

Delete Cron job for a specific user.

# crontab -r -u username

Strings in Crontab

  • @hourly: Run once every hour i.e. β€œ0 * * * *β€œ

  • @midnight: Run once every day i.e. β€œ0 0 * * *β€œ

  • @daily: same as midnight

  • @weekly: Run once every week, i.e. β€œ0 0 * * 0β€œ

  • @monthly: Run once every month i.e. β€œ0 0 1 * *β€œ

  • @annually: Run once every year i.e. β€œ0 0 1 1 *β€œ

  • @yearly: same as @annually

  • @reboot: Run once at every startup

References