Common cron expressions in Linux CLI - Mini Project: Build & Apply

Common cron expressions

📖 Scenario: You are managing a Linux server and need to schedule tasks to run automatically at specific times. Cron jobs help you do this by using cron expressions that tell the system when to run your commands.Understanding common cron expressions will help you automate daily, weekly, and monthly tasks easily.

🎯 Goal: Learn to write common cron expressions for scheduling tasks at specific times and intervals.You will create cron expressions for running commands every minute, hourly, daily at midnight, weekly on Sunday at midnight, and monthly on the first day at midnight.

📋 What You'll Learn

Create variables with exact cron expressions as strings

Use the exact variable names: every_minute, hourly, daily_midnight, weekly_sunday_midnight, monthly_first_midnight

Print each variable to show the cron expression

💡 Why This Matters

🌍 Real World

System administrators and developers use cron expressions to automate backups, updates, and maintenance tasks on servers.

💼 Career

Knowing common cron expressions is essential for roles involving Linux server management, DevOps, and automation scripting.

1

Create cron expression for every minute

Create a variable called every_minute and set it to the string "* * * * *" which means run every minute.

Linux CLI

# Create a variable called every_minute with the value "* * * * *"
# Your code here

Hint

This cron expression uses five asterisks separated by spaces to mean every minute of every hour, day, month, and weekday.

2

Add cron expressions for hourly and daily tasks

Add two variables: hourly set to "0 * * * *" to run at the start of every hour, and daily_midnight set to "0 0 * * *" to run daily at midnight.

Linux CLI

every_minute = "* * * * *"
# Add variables hourly and daily_midnight with the exact cron strings
# Your code here

Hint

Use zero for the minute field to run at the start of the hour or day.

3

Add cron expressions for weekly and monthly tasks

Add two variables: weekly_sunday_midnight set to "0 0 * * 0" to run weekly on Sunday at midnight, and monthly_first_midnight set to "0 0 1 * *" to run monthly on the first day at midnight.

Linux CLI

every_minute = "* * * * *"
hourly = "0 * * * *"
daily_midnight = "0 0 * * *"
# Add variables weekly_sunday_midnight and monthly_first_midnight with the exact cron strings
# Your code here

Hint

The last field is the day of the week (0 for Sunday). The third field is the day of the month.

4

Print all cron expressions

Print the variables every_minute, hourly, daily_midnight, weekly_sunday_midnight, and monthly_first_midnight each on its own line.

Linux CLI

every_minute = "* * * * *"
hourly = "0 * * * *"
daily_midnight = "0 0 * * *"
weekly_sunday_midnight = "0 0 * * 0"
monthly_first_midnight = "0 0 1 * *"
# Print all variables each on its own line
# Your code here

Hint

Use separate print statements for each variable.

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