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.