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.