Editing crontab (crontab -e) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this crontab entry?

Given the crontab entry below, what will be the output when it runs?

0 5 * * * echo "Backup started" >> /tmp/backup.log

A The message "Backup started" is appended to /tmp/backup.log at 5:00 AM daily.

B The message "Backup started" is printed on the terminal at 5:00 AM daily.

C The message "Backup started" is overwritten in /tmp/backup.log at 5:00 AM daily.

D The message "Backup started" is appended to /tmp/backup.log every minute.

💡 Hint

Remember that '>>' appends output to a file and the schedule '0 5 * * *' means 5:00 AM every day.

The crontab entry runs the echo command at 5:00 AM every day. The '>>' operator appends the output to the file /tmp/backup.log without overwriting it. It does not print to the terminal.

📝 Syntax

intermediate

Which crontab entry is syntactically correct?

Select the crontab entry that will NOT cause a syntax error when saved.

A 15 25 * * * /usr/bin/backup.sh

B 60 12 * * * /home/user/run.sh

C 30 14 * * 1-5 /usr/bin/python3 /home/user/script.py

D 0 0 0 * * /usr/bin/cleanup.sh

💡 Hint

Minutes must be between 0-59 and hours between 0-23 in crontab.

Option C uses valid minute (30) and hour (14) values. Option C has minute 60 which is invalid. Option C has hour 25 which is invalid. Option C has day of month 0 which is invalid.

🔧 Debug

advanced

Why does this crontab entry not run the script?

A user added this line to their crontab:

0 3 * * * /home/user/myscript.sh

But the script does not run at 3 AM. What is the most likely reason?

A The script /home/user/myscript.sh does not have execute permission.

B The crontab entry is missing the username field.

C The time format '0 3 * * *' is invalid.

D Crontab cannot run scripts located in /home directories.

💡 Hint

Check file permissions and execution rights for scripts run by cron.

If the script lacks execute permission, cron cannot run it. The time format is valid, and user crontabs do not require a username field. Cron can run scripts from any accessible directory.

🚀 Application

advanced

Schedule a script to run every 15 minutes using crontab

Which crontab entry correctly runs /usr/local/bin/cleanup.sh every 15 minutes?

A 0,15,30,45 * * * * /usr/local/bin/cleanup.sh

B 15 * * * * /usr/local/bin/cleanup.sh

C 0-15 * * * * /usr/local/bin/cleanup.sh

D */15 * * * * /usr/local/bin/cleanup.sh

💡 Hint

Use the step value syntax to run commands at intervals.

Option D uses '*/15' which means every 15 minutes. Option D runs only at minute 15 each hour. Option D runs at 0,15,30,45 minutes which is also every 15 minutes but option D is the standard concise form. Option D runs every minute from 0 to 15, not every 15 minutes.

🧠 Conceptual

expert

What happens if you edit crontab with 'crontab -e' and save an empty file?

If a user runs 'crontab -e' and deletes all lines, saving an empty file, what is the effect on their scheduled jobs?

A The system restores the previous crontab automatically.

B All scheduled cron jobs for that user are removed and will no longer run.

C Cron jobs continue running as before because empty file is ignored.

D An error is shown and the empty crontab is not saved.

💡 Hint

Think about what an empty crontab means for scheduled tasks.

Saving an empty crontab removes all scheduled jobs for that user. Cron will have no jobs to run for that user until new entries are added.

Practice

1. What does the command crontab -e do in Linux?

easy

A. It deletes all cron jobs for the current user.

B. It lists all scheduled cron jobs for all users.

C. It runs the cron daemon immediately.

D. It opens the current user's cron jobs for editing.

Solution

1. Step 1: Understand the purpose of crontab -e

   This command opens the cron table for the current user in an editor so you can add or change scheduled tasks.

2. Step 2: Compare with other options

   crontab -l lists jobs, not edits. Deleting or running the daemon are different commands.

3. Final Answer:

   It opens the current user's cron jobs for editing. -> Option D

4. Quick Check:

   crontab -e = edit cron jobs [OK]

Hint: Remember: '-e' means edit your cron jobs [OK]

Common Mistakes:

• Confusing '-e' with '-l' which lists jobs
• Thinking it deletes cron jobs
• Assuming it runs cron immediately

2. Which of the following is the correct syntax to schedule a cron job to run every day at 3:30 AM using crontab -e?

easy

A. 30 3 * * * /path/to/script.sh

B. 3 30 * * * /path/to/script.sh

C. * 3 30 * * /path/to/script.sh

D. 30 03 * * /path/to/script.sh

Solution

1. Step 1: Understand cron time fields order

   The order is: minute hour day-of-month month day-of-week. So 30 3 means 3:30 AM.

2. Step 2: Check each option

   30 3 * * * /path/to/script.sh correctly uses '30 3' for 3:30 AM daily. 3 30 * * * /path/to/script.sh swaps minute and hour. * 3 30 * * /path/to/script.sh misplaces day and minute. 30 03 * * /path/to/script.sh misses a field.

3. Final Answer:

   30 3 * * * /path/to/script.sh -> Option A

4. Quick Check:

   Minute Hour Day Month Weekday = 30 3 * * * [OK]

Hint: Minute comes before hour in cron syntax [OK]

Common Mistakes:

• Swapping hour and minute fields
• Missing one of the five time fields
• Using leading zeros incorrectly

3. After adding this line in crontab -e:
0 0 * * 0 /home/user/backup.sh
What will happen?

medium

A. The script runs every day at midnight.

B. The script runs every Sunday at midnight.

C. The script runs every hour on Sundays.

D. The script runs once a month at midnight.

Solution

1. Step 1: Decode the cron schedule

   Fields: minute=0, hour=0, day-of-month=*, month=*, day-of-week=0. Day-of-week 0 means Sunday.

2. Step 2: Interpret the schedule

   The script runs at 00:00 (midnight) on every Sunday.

3. Final Answer:

   The script runs every Sunday at midnight. -> Option B

4. Quick Check:

   0 0 * * 0 = Sunday midnight [OK]

Hint: Day-of-week 0 means Sunday in cron [OK]

Common Mistakes:

• Thinking it runs daily instead of weekly
• Confusing day-of-month with day-of-week
• Assuming 0 means Monday

4. You added this line in crontab -e:
60 12 * * * /home/user/script.sh
Why does this cron job not run?

medium

A. Script path must be absolute, this is relative.

B. Hour 12 is invalid; hours range 0-11.

C. Minute value 60 is invalid; minutes range 0-59.

D. Missing day-of-week field causes error.

Solution

1. Step 1: Check the minute field validity

   Minutes must be between 0 and 59. 60 is invalid and causes cron to ignore the job.

2. Step 2: Verify other fields

   Hour 12 is valid (means noon). Day-of-week is optional. Script path is absolute.

3. Final Answer:

   Minute value 60 is invalid; minutes range 0-59. -> Option C

4. Quick Check:

   Minute must be 0-59, 60 is invalid [OK]

Hint: Minutes max at 59, not 60 [OK]

Common Mistakes:

• Using 60 as a minute value
• Confusing 12-hour clock with 24-hour
• Thinking day-of-week is mandatory

5. You want to run a backup script every weekday at 11:45 PM using crontab -e. Which line correctly schedules this?

hard

A. 45 23 * * 1-5 /home/user/backup.sh

B. 23 45 * * 1-5 /home/user/backup.sh

C. 45 23 * * 0-6 /home/user/backup.sh

D. 45 11 * * 1-5 /home/user/backup.sh

Solution

1. Step 1: Understand the time and days needed

   11:45 PM is hour 23, minute 45. Weekdays are Monday(1) to Friday(5).

2. Step 2: Check each option's correctness

   45 23 * * 1-5 /home/user/backup.sh uses '45 23' for 11:45 PM and '1-5' for weekdays correctly. 23 45 * * 1-5 /home/user/backup.sh swaps minute and hour. 45 23 * * 0-6 /home/user/backup.sh runs every day. 45 11 * * 1-5 /home/user/backup.sh uses hour 11 (11 AM) instead of 23.

3. Final Answer:

   45 23 * * 1-5 /home/user/backup.sh -> Option A

4. Quick Check:

   Minute Hour Days 1-5 = 45 23 Mon-Fri [OK]

Hint: Hour 23 = 11 PM; weekdays 1-5 [OK]

Common Mistakes:

• Swapping hour and minute fields
• Using 11 instead of 23 for PM
• Using 0-6 instead of 1-5 for weekdays