Common cron expressions in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to schedule a job to run every minute.

Linux CLI

* * * * [1] /path/to/script.sh

Drag options to blanks, or click blank then click option'

A*

B0

C1

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' in the last field runs only on Sunday.

Using '1' or '5' restricts the job to specific weekdays.

The last field in a cron expression is the day of the week. Using '*' means every day of the week, so the job runs every minute of every day.

2fill in blank

medium

Complete the code to schedule a job to run at 3 AM every day.

Linux CLI

0 [1] * * * /path/to/script.sh

Drag options to blanks, or click blank then click option'

A0

B23

C3

D12

Attempts:

3 left

💡 Hint

Common Mistakes

Setting hour to 0 runs at midnight, not 3 AM.

Using 12 runs at noon, not 3 AM.

The second field is the hour. Setting it to '3' means the job runs at 3 AM.

3fill in blank

hard

Fix the error in the cron expression to run a job every Sunday at midnight.

Linux CLI

0 0 * * [1] /path/to/script.sh

Drag options to blanks, or click blank then click option'

A*

B7

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' runs the job every day, not just Sunday.

Using '1' runs the job on Monday.

In cron, Sunday can be represented as 0 or 7. Using '0' is standard and avoids confusion.

4fill in blank

hard

Fill both blanks to schedule a job to run every 15 minutes between 9 AM and 5 PM on weekdays.

Linux CLI

[1] 9-17 * * [2] /path/to/script.sh

Drag options to blanks, or click blank then click option'

A*/15

B0

C1-5

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' in minutes runs every minute, not every 15 minutes.

Using '0' in day of week runs only on Sunday.

Using '*/15' in the minutes field runs the job every 15 minutes. '1-5' in the day of week field means Monday to Friday.

5fill in blank

hard

Fill all three blanks to schedule a job to run at 30 minutes past every hour from 8 AM to 10 AM on the 1st and 15th of each month.

Linux CLI

[1] [2] 1,15 * [3] /path/to/script.sh

Drag options to blanks, or click blank then click option'

A30

B8-10

C*

D1-5

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' in hours runs every hour, not just 8 to 10.

Using '1-5' in day of week restricts to weekdays, which is not needed here.

The minutes field is '30' for 30 minutes past the hour. The hour field is '8-10' for 8 AM to 10 AM. The day of week field is '*' to run on any day of the week since days are restricted by day of month.

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