Common cron expressions in Linux CLI - Step-by-Step Execution

Concept Flow - Common cron expressions

Start

↓

Read cron expression

↓

Parse fields: min, hour, day, month, weekday

↓

Match current time with fields

↓

If all match?

No→Wait for next minute

Yes↓

Run scheduled command

↓

Repeat every minute

Cron reads the expression fields, checks if current time matches, and runs the command if all fields match.

Execution Sample

Linux CLI

*/5 * * * * echo "Hello"
0 0 * * 0 /backup.sh
30 14 1 * * /monthly_report.sh

These cron lines run commands every 5 minutes, weekly on Sunday midnight, and monthly on the 1st at 14:30.

Execution Table

Step	Cron Field	Value	Current Time	Match?	Action
1	Minute	*/5	00	Yes	Check next field
2	Hour	*	14	Yes	Check next field
3	Day of Month	*	1	Yes	Check next field
4	Month	*	6	Yes	Check next field
5	Day of Week	*	3 (Wednesday)	Yes	Run command: echo "Hello"
6	Minute	0	00	Yes	Check next field
7	Hour	0	00	Yes	Check next field
8	Day of Month	*	1	Yes	Check next field
9	Month	*	6	Yes	Check next field
10	Day of Week	0 (Sunday)	3 (Wednesday)	No	Do not run /backup.sh
11	Minute	30	30	Yes	Check next field
12	Hour	14	14	Yes	Check next field
13	Day of Month	1	1	Yes	Check next field
14	Month	*	6	Yes	Check next field
15	Day of Week	*	3 (Wednesday)	Yes	Run command: /monthly_report.sh
16	End				Wait for next minute

💡 Execution stops after checking all fields and running commands if matched, then waits for next minute.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
Minute	Current time	00	00	30	30
Hour	Current time	14	14	14	14
Day of Month	Current time	1	1	1	1
Month	Current time	6	6	6	6
Day of Week	Current time	3	3	3	3
Command Run	None	echo "Hello"	None	/monthly_report.sh	None

Key Moments - 3 Insights

Why does the /backup.sh command not run even though the minute and hour match?

Because the day of week field is 0 (Sunday) but current day is 3 (Wednesday), so the condition fails at step 10 in the execution table.

What does the '*/5' in the minute field mean?

It means every 5 minutes, so at minutes 0, 5, 10, 15, etc. This is shown in step 1 where minute 00 matches '*/5'.

Can a cron job run if only some fields match?

No, all fields must match the current time for the command to run, as shown by the checks in the execution table steps.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the /backup.sh command fail to run?

A Step 5

B Step 15

C Step 10

D Step 1

💡 Hint

Check the 'Match?' column for /backup.sh in the execution table rows.

According to the variable tracker, what is the value of 'Minute' after the third check?

A 30

B 00

C 14

D 3

💡 Hint

Look at the 'Minute' row in variable_tracker under 'After 3'.

If the day of week for /backup.sh was changed to 3, what would happen?

A The command would run every minute

B The command would run at midnight on Wednesday

C The command would never run

D The command would run only on Sundays

💡 Hint

Refer to the execution table where day of week mismatch caused no run at step 10.

Concept Snapshot

Cron expressions have 5 fields: minute, hour, day of month, month, day of week.
Each field can be a number, '*', or step values like '*/5'.
Cron runs the command only if all fields match the current time.
Common patterns: '*/5 * * * *' runs every 5 minutes.
'0 0 * * 0' runs weekly on Sunday at midnight.
'30 14 1 * *' runs monthly on the 1st at 14:30.

Full Transcript

Cron expressions consist of five fields: minute, hour, day of month, month, and day of week. The cron daemon checks each field against the current time every minute. If all fields match, it runs the scheduled command. For example, '*/5 * * * *' runs a command every 5 minutes. '0 0 * * 0' runs a command weekly on Sunday at midnight. '30 14 1 * *' runs a command monthly on the first day at 14:30. If any field does not match, the command does not run. This is shown in the execution table where the /backup.sh command does not run because the day of week does not match. Understanding each field and how they match current time is key to using cron effectively.