Common cron expressions in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the cron expression 0 0 * * * mean?

It means the task runs every day at midnight (00:00). The fields represent: minute=0, hour=0, every day, every month, every weekday.

beginner

Explain the cron expression */15 * * * *.

This runs a task every 15 minutes. The */15 in the minute field means 'every 15 minutes', while the other fields mean every hour, day, month, and weekday.

intermediate

What does 0 9 * * 1-5 schedule?

It schedules a task to run at 9:00 AM every weekday (Monday to Friday). The 1-5 in the weekday field means Monday through Friday.

intermediate

How do you schedule a cron job to run at 5:30 PM on the 1st of every month?

Use 30 17 1 * *. This means minute=30, hour=17 (5 PM), day=1, every month, every weekday.

intermediate

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

It runs a task once a year at midnight on January 1st. The fields mean: minute=0, hour=0, day=1, month=1 (January), every day of the week.

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

A Runs a task every hour at the start of the hour

B Runs a task every minute

C Runs a task once a day at midnight

D Runs a task every day at noon

The minute field is 0, so it runs at minute 0 of every hour, meaning at the start of every hour.

Which cron expression runs a task every Sunday at 3 AM?

A 0 3 * * 7

B Both A and B

C 0 3 * * 1

D 0 3 * * 0

In cron, Sunday can be represented as 0 or 7 in the weekday field.

How often does */10 * * * * run?

A Every 10 hours

B Every 10 seconds

C Every 10 minutes

D Every 10 days

The */10 in the minute field means every 10 minutes.

What does the * symbol mean in a cron expression?

A Every possible value for that field

B No value

C Only one specific value

D A comment

The asterisk means 'every' possible value for that field.

Which cron expression runs a task at 11:45 PM every day?

A 11 45 * * *

B 23 45 * * *

C 45 11 * * *

D 45 23 * * *

The first field is minutes (45), second is hour (23 = 11 PM).

Describe how to read a cron expression and explain what each field means.

Think of the cron expression as a schedule with five parts: minute, hour, day, month, and weekday.

Explain how to schedule a task to run every weekday at 8 AM using a cron expression.

Remember weekdays are numbers 1 to 5 in cron.

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