crontab syntax 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?

B/

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/' or '-' in the day of week field without proper context.

Leaving the field empty.

The last field in crontab is the day of the week. Using '*' means every day of the week.

2fill in blank

medium

Complete the code to schedule a job to run at 3:30 AM every day.

Linux CLI

[1] 3 * * * /path/to/script.sh

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

A30

B0

C3

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping minutes and hours.

Using '*' instead of specific numbers.

The first field is minutes. To run at 3:30 AM every day, minutes should be 30 and hours 3. The blank is for minutes, so fill it with 30. The full crontab entry is '30 3 * * * /path/to/script.sh'.

3fill in blank

hard

Fix the error in the crontab line to run a job every 5 minutes.

Linux CLI

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

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

A5

B0

C*/5

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Putting '*/5' inside the blank, which duplicates the '*/' prefix.

Using 0 or 1 which do not mean every 5 minutes.

To run a job every 5 minutes, use '*/5' in the minutes field. Since the code already has '*/{{BLANK_1}}', the blank should be '5' to form '*/5'.

4fill in blank

hard

Fill both blanks to schedule a job to run at 2:15 PM on weekdays (Monday to Friday).

Linux CLI

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

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

A14

B13

C1-5

D0-6

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 instead of 14 for hour.

Using 0-6 for weekdays instead of 1-5.

The hour field for 2 PM is 14 (24-hour clock). The day of week field for Monday to Friday is '1-5'.

5fill in blank

hard

Fill all three blanks to schedule a job to run at 11:45 PM on the 1st day of every month in January, March, and May.

Linux CLI

[1] [2] [3] 1,3,5 * /path/to/script.sh

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

A45

B23

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 11 instead of 23 for hour.

Using 0 for day of month instead of 1.

Minutes is 45, hour is 23 (11 PM), day of month is 1.

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