crontab syntax in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the crontab command in Linux?

The crontab command is used to schedule tasks (called cron jobs) to run automatically at specified times or intervals.

beginner

In crontab syntax, what does the asterisk (*) symbol mean?

The asterisk (*) means "every" possible value for that time field. For example, * in the minute field means every minute.

beginner

Explain the five time fields in a crontab entry.

The five time fields are: minute (0-59), hour (0-23), day of month (1-31), month (1-12), and day of week (0-7, where 0 and 7 both mean Sunday). They define when the command runs.

intermediate

How do you schedule a cron job to run every 5 minutes?

Use */5 in the minute field, like: */5 * * * *. This means every 5 minutes, every hour, every day, every month, every weekday.

intermediate

What does this crontab line do?
0 0 1 * * /home/user/backup.sh

It runs the script /home/user/backup.sh at midnight (00:00) on the first day of every month.

What does the following crontab time field mean?
30 14 * * *

A Run at 14:30 on the first day of every month

B Run every 30 minutes starting at 2 PM

C Run every minute between 2 PM and 3 PM

D Run at 2:30 PM every day

The fields mean minute=30, hour=14 (2 PM), and the rest are wildcards meaning every day, month, and weekday.

Which symbol in crontab means 'every' value for a time field?

A #

B *

C /

D -

The asterisk (*) means every possible value for that time field.

How do you specify a cron job to run only on Sundays?

A Both A and B

B * * * * 7

C * * * * 1

D * * * * 0

In crontab, both 0 and 7 represent Sunday in the day of week field.

What does this crontab entry do?
15 10 * * 1-5 /usr/bin/script.sh

A Runs script.sh at 10:15 AM Monday to Friday

B Runs script.sh at 10:15 AM every day

C Runs script.sh every minute from 10:00 to 10:15 on weekdays

D Runs script.sh at 10:15 AM on weekends

The day of week field 1-5 means Monday to Friday.

Which crontab field controls the month when a job runs?

A First field

B Second field

C Fourth field

D Fifth field

The fourth field in crontab syntax is the month (1-12).

Describe the structure of a crontab entry and what each time field means.

Think about the order and range of each time field.

Explain how to schedule a cron job to run every day at 6:30 AM.

Use exact numbers for hour and minute, wildcards for others.

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