crontab syntax in Linux CLI - Commands & Configuration

Introduction

Crontab syntax lets you schedule tasks to run automatically at specific times on your Linux system. It helps you automate repetitive jobs like backups or updates without needing to run them manually.

When you want to run a script every day at midnight to clean temporary files.

When you need to send a report email every Monday morning automatically.

When you want to check system health every hour without logging in.

When you want to update software packages weekly without manual intervention.

When you want to restart a service every day to keep it fresh.

Commands

This command lists all the scheduled cron jobs for the current user so you can see what tasks are set to run.

Terminal

crontab -l

Expected Output

30 2 * * * /home/user/backup.sh

0 9 * * 1 /home/user/send_report.sh

→

-l - List current user's cron jobs

This command schedules a new cron job to run the cleanup script every day at midnight by writing the job directly into crontab.

Terminal

echo "0 0 * * * /home/user/cleanup.sh" | crontab -

Expected Output

No output (command runs silently)

This opens the crontab file in an editor so you can add, edit, or remove scheduled tasks manually.

Terminal

crontab -e

Expected Output

No output (command runs silently)

→

-e - Edit the current user's crontab

This command removes all cron jobs for the current user, clearing the schedule completely.

Terminal

crontab -r

Expected Output

No output (command runs silently)

→

-r - Remove all cron jobs for the current user

Key Concept

If you remember nothing else from crontab syntax, remember: the five time fields (minute, hour, day of month, month, day of week) control when your task runs.

Common Mistakes

Using wrong numbers or ranges in the time fields, like 60 for minutes or 25 for hours.

Crontab expects minutes 0-59 and hours 0-23; invalid numbers cause the job to not run.

Always use valid ranges: minutes 0-59, hours 0-23, day of month 1-31, month 1-12, day of week 0-7 (0 and 7 both mean Sunday).

Forgetting to specify the full path to scripts or commands in the cron job.

Cron runs with a limited environment and may not find commands without full paths, causing failures.

Always use absolute paths like /home/user/script.sh or /usr/bin/python in your cron jobs.

Editing crontab file directly without using 'crontab -e'.

Direct edits to crontab files may not be recognized by the cron daemon, so changes won't apply.

Always use 'crontab -e' to edit cron jobs safely and apply changes immediately.

Summary

Use 'crontab -l' to list your current scheduled tasks.

Add new jobs by editing with 'crontab -e' or piping commands into crontab.

Remember the five time fields control when your task runs: minute, hour, day of month, month, day of week.

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