Mastering in Cron Tab yogeshdixit29@gmail.
com
Article by – Yogesh dixit
Crontab is a Unix-based utility that allows users to schedule jobs to run automatically at
specified intervals. It's particularly useful for automating system maintenance tasks,
backups, and other repetitive activities.
### Crontab Format###
A crontab file contains lines with the following format:
* * * * * command_to_execute
The five asterisks represent:
1. Minute (0-59)
2. Hour (0-23)
3. Day of Month (1-31)
4. Month (1-12)
5. Day of Week (0-7) (Sunday is both 0 and 7)
You can set cronjob on multi specific minutes,hour, DOM and month using comma separated (,) or
using hyphen (-).
Suppose want to run cron on 25 or 31 min in every hour then cron will be like "25,31 * * * *".
If 25 to 31 minutes then "25-31 * * * *"
### Example####
- 0 5 * * * /path/to/script.sh - Runs the script at 5:00 AM every day.
- */10 * * * * /path/to/script.sh - Runs the script every 10 minutes.
### Answer the below MCQs###
1. What does the first asterisk in a crontab entry represent?
- A) Hour
- B) Minute
- C) Day of Month
- D) Day of Week
2. Which of the following crontab entries runs a job every day at midnight?
- A) 0 0 * * *
- B) * * * * *
- C) 0 12 * * *
- D) 0 0 * * 1
3. What does */5 * * * * mean in a crontab?
- A) Runs every 5 hours
- B) Runs every 5 days
- C) Runs every 5 minutes
- D) Runs every 5 seconds
4. In crontab, what does the entry 0 22 * * 1-5 do?
- A) Runs at 10 PM on weekends
- B) Runs at 10 PM on weekdays (Monday to Friday)
- C) Runs at 10 AM on weekdays
- D) Runs at midnight on weekdays
5. Which command is used to edit the crontab for the current user?
- A) crontab -e
- B) crontab -l
- C) crontab -r
- D) crontab -x
6. Which command is used to remove the crontab for the current user?
- A) crontab -e
- B) crontab -l
- C) crontab -r
- D) crontab –x
7. Suppose If Cron is like "*/3 */8 * * 1-2,2 command to execute" how many times it will run in a
week?
- A) 320 times
- B) 120 times
- C) 60 times
- D) 160 times
Write the answer of 7th question in comments.
8. What does the crontab entry 15 14 1 * 5 mean?
- A) Run at 2:15 PM on the first day of each month, but only if it's a Friday.
- B) Run at 2:15 PM every Friday.
- C) Run at 2:15 PM on the first day of each month, regardless of the day of the week.
- D) Run every Friday at 2:15 PM.
9. Which of the following entries will run a job every day at 6:00 AM and 6:00 PM?
- A) 0 6,18 * * *
- B) 0 6 * * */18
- C) 0 6,18 * * 1-5
- D) 0 6-18 * * *
10. How many times will the job */5 2,3 * * 1-5 run in a week?
- A) 560
- B) 568
- C) 576
- D) 10
11. What will the crontab entry 0 0 1 1 * do?
- A) Run at midnight every day.
- B) Run at midnight on the first day of each month.
- C) Run at midnight on the first day of the year.
- D) Run at midnight every January.
12. What does the entry @reboot /path/to/script.sh do?
- A) Runs the script every hour.
- B) Runs the script at system startup.
- C) Runs the script when the cron daemon starts.
- D) Runs the script at midnight.
12. Which of the following entries will run a job every 15 minutes during business hours (9 AM to 5
PM) on weekdays?
- A) */15 9-17 * * 1-5
- B) 0,15,30,45 9-17 * * 1-5
- C) Both A and B
- D) */15 9-17 * * *
Note- Crontab does not support specifying seconds in its scheduling syntax. If you need to schedule
tasks at the second level. You can use sleep or systemd timer.
Answer1: B) Minute
Answer2: A) 0 0 * * *
Answer3:C) Runs every 5 minutes
Answer4: B) Runs at 10 PM on weekdays (Monday to Friday)
Answer5: A) crontab -e
Answer6: C) crontab -r
Answer8: A) Run at 2:15 PM on the first day of each month, but only if it's a Friday
Answer9: A) 0 6,18 * * *
Answer10: C) 576
Answer11:C) Run at midnight on the first day of the year.
Answer12: B) Runs the script at system startup.
Answer13:C) Both A and B are correct
