crontab syntax in Linux CLI - Mini Project: Build & Apply

Understanding Crontab Syntax

📖 Scenario: You are managing a Linux server and want to schedule tasks to run automatically at specific times. To do this, you will learn how to write crontab entries that tell the system when to run your commands.

🎯 Goal: Build a simple crontab entry that runs a backup script every day at 2:30 AM.

📋 What You'll Learn

Create a crontab line with the correct time fields

Use the exact time 2:30 AM for scheduling

Specify the command to run the backup script located at /usr/local/bin/backup.sh

Print the final crontab line to verify

💡 Why This Matters

🌍 Real World

System administrators use crontab to schedule backups, updates, and maintenance tasks automatically without manual intervention.

💼 Career

Knowing crontab syntax is essential for Linux system administrators, DevOps engineers, and anyone managing servers to automate routine tasks efficiently.

1

Set up the time fields for the crontab entry

Create five variables named minute, hour, day_of_month, month, and day_of_week with these exact values: minute = '30', hour = '2', day_of_month = '*', month = '*', and day_of_week = '*'.

Linux CLI

# Your code here

Hint

Remember, crontab time fields are minute, hour, day of month, month, and day of week.

2

Set the command to run in the crontab entry

Create a variable called command and set it to the string '/usr/local/bin/backup.sh' which is the path to the backup script.

Linux CLI

minute = '30'
hour = '2'
day_of_month = '*'
month = '*'
day_of_week = '*'
# Your code here

Hint

The command is the exact path to the script you want to run.

3

Combine the time fields and command into one crontab line

Create a variable called crontab_line that combines minute, hour, day_of_month, month, day_of_week, and command separated by spaces in this exact order: minute, hour, day_of_month, month, day_of_week, command.

Linux CLI

minute = '30'
hour = '2'
day_of_month = '*'
month = '*'
day_of_week = '*'
command = '/usr/local/bin/backup.sh'
# Your code here

Hint

Use an f-string to join all parts with spaces.

4

Print the final crontab line

Write a print statement to display the value of crontab_line.

Linux CLI

minute = '30'
hour = '2'
day_of_month = '*'
month = '*'
day_of_week = '*'
command = '/usr/local/bin/backup.sh'
crontab_line = f"{minute} {hour} {day_of_month} {month} {day_of_week} {command}"
# Your code here

Hint

The output should show the full crontab line exactly.

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