crontab syntax in Linux CLI - Step-by-Step Execution

Concept Flow - crontab syntax

Start

↓

Read 5 time fields

↓

Match current time with fields?

No→Wait 1 minute

Yes↓

Run scheduled command

↓

Wait 1 minute

↩Back to Read 5 time fields

Crontab reads five time fields, checks if current time matches, runs the command if yes, then waits for the next minute.

Execution Sample

Linux CLI

30 14 * * 1 /home/user/backup.sh

Runs backup.sh at 2:30 PM every Monday.

Execution Table

Step	Minute Field	Hour Field	Day of Month Field	Month Field	Day of Week Field	Current Time	Match?	Action
1	30	14	*	*	1	14:29 Monday	No	Wait
2	30	14	*	*	1	14:30 Monday	Yes	Run /home/user/backup.sh
3	30	14	*	*	1	14:31 Monday	No	Wait
4	30	14	*	*	1	14:30 Tuesday	No	Wait

💡 Crontab waits each minute, runs command only when all time fields match current time.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
Current Time	14:29 Monday	14:29 Monday	14:30 Monday	14:31 Monday	14:30 Tuesday
Match Result	No	No	Yes	No	No
Action	Wait	Wait	Run /home/user/backup.sh	Wait	Wait

Key Moments - 3 Insights

Why does the command run only at 14:30 on Monday and not other times?

Because the minute field is 30, hour is 14, and day of week is 1 (Monday). Only when all match current time (see execution_table step 2) does the command run.

What does the asterisk (*) mean in the day of month and month fields?

An asterisk means 'any value'. So the command runs regardless of day of month or month, as long as other fields match (see execution_table).

If the day of week is set to 1, will the command run on Tuesday?

No, because day of week 1 means Monday only. On Tuesday, the match fails (see execution_table step 4), so the command does not run.

Visual Quiz - 3 Questions

Look at the execution table, what is the action at step 3 when the time is 14:31 Monday?

A Run /home/user/backup.sh

B Run a different command

C Wait

D Exit crontab

💡 Hint

Check the 'Action' column at step 3 in the execution_table.

At which step does the command run according to the execution table?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look for 'Run /home/user/backup.sh' in the Action column.

If the minute field changed from 30 to 31, when would the command run?

A At 14:30 Monday

B At 14:31 Monday

C At 14:29 Monday

D Never

💡 Hint

Match happens when current time matches all fields; see variable_tracker for Current Time values.

Concept Snapshot

Crontab syntax has 5 time fields: minute hour day_of_month month day_of_week
Each field can be a number or * (any).
Crontab runs the command when current time matches all fields.
Example: '30 14 * * 1' runs at 14:30 every Monday.
Use crontab to schedule repeated tasks easily.

Full Transcript

Crontab syntax uses five fields to schedule commands: minute, hour, day of month, month, and day of week. Each field can be a specific number or an asterisk meaning any value. The crontab checks every minute if the current time matches all fields. If yes, it runs the command. For example, '30 14 * * 1 /home/user/backup.sh' runs the backup script at 2:30 PM every Monday. If the time does not match, crontab waits and checks again the next minute. This way, you can automate tasks to run at specific times easily.

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