crontab syntax in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this crontab entry?

Given the crontab line below, what time(s) will the command run?

30 2 * * 1 /usr/bin/backup.sh

Linux CLI

30 2 * * 1 /usr/bin/backup.sh

A Runs every Monday at 2:30 AM

B Runs every day at 2:30 AM

C Runs every Monday at 12:30 PM

D Runs every day at 12:30 PM

💡 Hint

Remember the order: minute, hour, day of month, month, day of week.

The first field is minutes (30), second is hour (2), and the last field '1' means Monday. So it runs at 2:30 AM every Monday.

💻 Command Output

intermediate

What does this crontab entry do?

What is the effect of this crontab line?

0 */4 * * * /usr/bin/cleanup.sh

Linux CLI

0 */4 * * * /usr/bin/cleanup.sh

A Runs every 4 hours at the start of the hour

B Runs every 4 minutes

C Runs every 4 days at midnight

D Runs every day at 4 AM

💡 Hint

The hour field uses '*/4' which means every 4 hours.

The minute is 0, hour is every 4 hours (0,4,8,12,16,20), so it runs at the start of those hours daily.

📝 Syntax

advanced

Which crontab entry runs a command every weekday at 6:15 PM?

Select the correct crontab line to run a script at 6:15 PM Monday through Friday.

A 15 6 * * 1-5 /usr/bin/script.sh

B 18 15 * * 1-5 /usr/bin/script.sh

C 15 18 * * 1-5 /usr/bin/script.sh

D 15 18 * * 0-6 /usr/bin/script.sh

💡 Hint

Remember the order: minute, hour, day of month, month, day of week.

Minute is 15, hour is 18 (6 PM), and days 1-5 are Monday to Friday.

🔧 Debug

advanced

Why does this crontab entry not run at midnight as expected?

A user wants to run a script at midnight every day but wrote:

0 24 * * * /usr/bin/midnight.sh

What is the problem?

A Minute field cannot be 0

B Hour field '24' is invalid; valid range is 0-23

C Day of month field is missing

D Command path is incorrect

💡 Hint

Check the valid ranges for hour in crontab.

Hours must be between 0 and 23. '24' is invalid and causes the job not to run.

🚀 Application

expert

How many times will this crontab entry run in a week?

Given this crontab line:

*/15 9-17 * * 1-5 /usr/bin/task.sh

How many times does the command run in one week?

A 240 times

B 200 times

C 160 times

D 180 times

💡 Hint

Calculate runs per hour, hours per day, and days per week.

Runs every 15 minutes (4 times per hour) from 9 to 17 hours (9 hours) Monday to Friday (5 days). 4 * 9 * 5 = 180. But 9-17 means hours 9 through 17 inclusive, which is 9 hours (9,10,11,12,13,14,15,16,17). So 4 runs/hour * 9 hours/day * 5 days = 180 runs. The correct answer is 180.

Practice

1. What does the asterisk (*) symbol mean in a crontab time field like * * * * *?

easy

A. It means 'run only on weekends'

B. It means 'no value' or 'skip this field'

C. It means 'only once at midnight'

D. It means 'every' possible value for that field

Solution

1. Step 1: Understand the asterisk in crontab

   The asterisk (*) in crontab means to match every possible value for that time field, like every minute or every hour.

2. Step 2: Apply to the example

   In * * * * *, each asterisk means every minute, every hour, every day, every month, and every weekday.

3. Final Answer:

   It means 'every' possible value for that field -> Option D

4. Quick Check:

   Asterisk = every value [OK]

Hint: Asterisk means 'every' in crontab fields [OK]

Common Mistakes:

• Thinking * means 'no value' or skip
• Confusing * with specific times
• Assuming * means only weekends

2. Which of the following is the correct crontab syntax to run a script every day at 3:30 AM?

easy

A. 30 3 * * * /path/to/script.sh

B. 3 30 * * * /path/to/script.sh

C. * 3 30 * * /path/to/script.sh

D. 30 * 3 * * /path/to/script.sh

Solution

1. Step 1: Understand crontab field order

   The fields are: minute, hour, day of month, month, day of week.

2. Step 2: Match time 3:30 AM

   3:30 AM means minute=30, hour=3, so the first two fields must be 30 and 3 respectively.

3. Final Answer:

   30 3 * * * /path/to/script.sh -> Option A

4. Quick Check:

   Minute=30, Hour=3 correct [OK]

Hint: Minute is first, hour second in crontab [OK]

Common Mistakes:

• Swapping hour and minute fields
• Using day or month fields incorrectly
• Misplacing asterisks

3. Given the crontab entry 15 14 1 * * running /usr/bin/backup.sh, when will the script run?

medium

A. At 2:15 PM on the first day of every month

B. At 2:15 AM on the first day of every month

C. At 2:15 PM every day

D. At 2:15 AM every Monday

Solution

1. Step 1: Decode the crontab fields

   Fields: minute=15, hour=14 (2 PM), day=1, month=*, weekday=* (any day).

2. Step 2: Interpret the schedule

   The script runs at 14:15 (2:15 PM) on the 1st day of every month, regardless of weekday.

3. Final Answer:

   At 2:15 PM on the first day of every month -> Option A

4. Quick Check:

   Hour=14 means 2 PM, day=1 means first day [OK]

Hint: Hour is 24-hour format; 14 means 2 PM [OK]

Common Mistakes:

• Confusing 14 as 2 AM instead of 2 PM
• Ignoring day of month field
• Assuming runs every day

4. Identify the error in this crontab entry: 60 12 * * 1 /usr/bin/script.sh

medium

A. Day of week cannot be 1; must be 0 or 7 for Monday

B. Hour field cannot be 12; valid range is 0-11

C. Minute field cannot be 60; valid range is 0-59

D. Command path is invalid; must be absolute path

Solution

1. Step 1: Check minute field range

   Minute field must be between 0 and 59. 60 is invalid.

2. Step 2: Verify other fields

   Hour=12 is valid (12 PM), day of week=1 is valid (Monday), command path looks absolute.

3. Final Answer:

   Minute field cannot be 60; valid range is 0-59 -> Option C

4. Quick Check:

   Minute max is 59, 60 is invalid [OK]

Hint: Minute max is 59, not 60 [OK]

Common Mistakes:

• Using 60 in minute field
• Confusing hour ranges
• Misunderstanding day of week numbers

5. You want to schedule a cleanup script to run every Monday and Friday at 11:45 PM. Which crontab entry is correct?

hard

A. 23 45 * * 1,5 /usr/local/bin/cleanup.sh

B. 45 23 * * 1,5 /usr/local/bin/cleanup.sh

C. 45 11 * * 1,5 /usr/local/bin/cleanup.sh

D. 45 23 1,5 * * /usr/local/bin/cleanup.sh

Solution

1. Step 1: Understand time and day fields

   11:45 PM means hour=23, minute=45. Monday=1, Friday=5 in day of week field.

2. Step 2: Match fields to crontab syntax

   Minute=45, hour=23, day of month=*, month=*, day of week=1,5 means Monday and Friday.

3. Final Answer:

   45 23 * * 1,5 /usr/local/bin/cleanup.sh -> Option B

4. Quick Check:

   Minute=45, Hour=23, Days=1,5 correct [OK]

Hint: Day of week uses numbers; Monday=1, Friday=5 [OK]

Common Mistakes:

• Swapping hour and minute
• Using day of month instead of day of week
• Wrong day numbers for weekdays