Common cron expressions in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this cron expression schedule?

Given the cron expression 0 0 * * 0, what does it schedule?

A Runs the job every Sunday at midnight

B Runs the job every day at midnight

C Runs the job every hour on Sundays

D Runs the job every Sunday at noon

💡 Hint

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

The cron expression '0 0 * * 0' means minute 0, hour 0 (midnight), every day of month, every month, and day 0 which is Sunday in cron. So it runs every Sunday at midnight.

💻 Command Output

intermediate

What does this cron expression do?

What is the schedule for the cron expression */15 9-17 * * 1-5?

A Runs every 15 minutes from 9 AM to 5 PM on weekends

B Runs every 15 minutes all day on weekdays

C Runs every 15 minutes from 9 AM to 5 PM, Monday to Friday

D Runs every 15 minutes from 9 AM to 5 PM every day

💡 Hint

Look at the hour and day of week fields carefully.

The expression '*/15' in minutes means every 15 minutes. '9-17' in hours means from 9 AM to 5 PM. '1-5' in day of week means Monday to Friday. So it runs every 15 minutes during working hours on weekdays.

💻 Command Output

advanced

What is the output of this cron expression?

What schedule does the cron expression 30 23 1 * * represent?

A Runs at 11:30 PM on the first day of every month

B Runs at 11:30 AM on the first day of every month

C Runs at 11:30 PM every day

D Runs at 11:30 PM on the first Monday of every month

💡 Hint

Check the day of month and hour fields.

The expression means minute 30, hour 23 (11 PM), day 1 of the month, every month, every day of week. So it runs once a month at 11:30 PM on the first day.

💻 Command Output

advanced

What error or output does this cron expression produce?

What happens if you use the cron expression 60 24 * * *?

A Runs at midnight every day

B Runs at 24:00 every day (valid time)

C Runs at 00:00 on the 60th day of the month

D It causes an error because 60 is invalid for minutes

💡 Hint

Minutes range from 0 to 59, hours from 0 to 23.

Minute field cannot be 60 (valid range 0-59). Hour field cannot be 24 (valid range 0-23). So this expression is invalid and causes an error.

🧠 Conceptual

expert

How many times does this cron expression run in a week?

How many times will the job run with the cron expression 0 12 */2 * 1 in one week?

A 4 times

B 3 times

C 1 time

D 0 times

💡 Hint

Cron uses OR logic for day of month and day of week fields interaction.

In standard Linux cron, the job runs when either the day-of-month OR day-of-week matches (minute, hour, month also match). '*/2' matches even days of month (2,4,6,...30), '1' matches Monday. It runs at 12:00 on even days of the month OR Mondays. A week typically has 3 even days, so 3 times.

Practice

1. What does the cron expression 0 0 * * * do?

easy

A. Runs a task every minute

B. Runs a task every hour

C. Runs a task every day at midnight

D. Runs a task every Sunday at midnight

Solution

1. Step 1: Understand the cron fields

   The expression has five fields: minute=0, hour=0, day=*, month=*, weekday=*. This means minute 0, hour 0, every day, every month, every weekday.

2. Step 2: Interpret the schedule

   Since minute and hour are fixed at 0, and others are '*', the task runs once daily at 00:00 (midnight).

3. Final Answer:

   Runs a task every day at midnight -> Option C

4. Quick Check:

   0 0 * * * = daily at midnight [OK]

Hint: Minute and hour 0 means daily at midnight [OK]

Common Mistakes:

• Thinking * means no schedule
• Confusing hour and minute fields
• Assuming it runs every minute

2. Which of the following is the correct cron syntax to run a job every 15 minutes?

easy

A. 15 * * * *

B. */15 * * * *

C. * 15 * * *

D. 0 */15 * * *

Solution

1. Step 1: Understand the minute field syntax

   Using '*/15' in the minute field means every 15 minutes (0,15,30,45).

2. Step 2: Check other fields

   All other fields are '*', meaning every hour, day, month, and weekday.

3. Final Answer:

   */15 * * * * -> Option B

4. Quick Check:

   */15 in minutes means every 15 minutes [OK]

Hint: */15 in minute field means every 15 minutes [OK]

Common Mistakes:

• Using 15 in minute field runs only at minute 15
• Putting 15 in hour field runs at 3 PM only
• Using 0 */15 runs every 15 hours, not minutes

3. What is the output behavior of this cron expression: 30 6 * * 1?

medium

A. Runs at 6:30 AM every Monday

B. Runs at 6:30 AM every day

C. Runs at 6:00 AM every Monday

D. Runs every minute on Mondays

Solution

1. Step 1: Analyze the fields

   Minute=30, Hour=6, Day=*, Month=*, Weekday=1 (Monday).

2. Step 2: Interpret the schedule

   The task runs at 6:30 AM only on Mondays.

3. Final Answer:

   Runs at 6:30 AM every Monday -> Option A

4. Quick Check:

   30 6 * * 1 = Monday 6:30 AM [OK]

Hint: Weekday 1 means Monday, time 6:30 is hour:minute [OK]

Common Mistakes:

• Confusing weekday with day of month
• Assuming it runs daily instead of weekly
• Mixing hour and minute fields

4. Identify the error in this cron expression: 60 24 * * *

medium

A. Minute value 60 is invalid

B. Hour value 24 is invalid

C. No error, this is valid syntax

D. Both minute and hour values are invalid

Solution

1. Step 1: Check minute field validity

   Minute field must be 0-59; 60 is invalid.

2. Step 2: Check hour field validity

   Hour field must be 0-23; 24 is invalid.

3. Final Answer:

   Both minute and hour values are invalid -> Option D

4. Quick Check:

   Minute max 59, hour max 23 [OK]

Hint: Minutes max 59, hours max 23 in cron [OK]

Common Mistakes:

• Assuming 60 is valid minute
• Assuming 24 is valid hour
• Thinking cron allows 24:00 time

5. You want to schedule a backup script to run at 2:15 AM on the 1st day of every month. Which cron expression should you use?

hard

A. 15 2 1 * *

B. 2 15 1 * *

C. 15 2 * * 1

D. 0 2 15 * *

Solution

1. Step 1: Map time and date to cron fields

   Minute=15, Hour=2, Day=1 (first day), Month=*, Weekday=* (any day).

2. Step 2: Verify expression correctness

   Expression '15 2 1 * *' matches 2:15 AM on day 1 of every month.

3. Final Answer:

   15 2 1 * * -> Option A

4. Quick Check:

   Minute=15, Hour=2, Day=1 means 2:15 AM on 1st [OK]

Hint: Minute Hour Day Month Weekday order in cron [OK]

Common Mistakes:

• Swapping hour and minute fields
• Using weekday instead of day of month
• Setting wrong day or time values