Cron log monitoring in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Identify the cron job execution from log output

Given the following snippet from a cron log, what command was executed by cron?

Linux CLI

Jun 10 14:05:01 server CRON[12345]: (root) CMD (/usr/bin/backup.sh)

A /usr/bin/update.sh

B /usr/bin/cleanup.sh

C /usr/bin/restart.sh

D /usr/bin/backup.sh

💡 Hint

Look for the CMD part in the log line.

The cron log line shows the command executed after CMD. Here it is /usr/bin/backup.sh.

❓ Troubleshoot

intermediate

Diagnose why a cron job did not run

You see no output or log entries for a cron job scheduled at 3 AM daily. Which of these is the most likely reason?

A The system time is set correctly

B The cron daemon is not running

C The cron job script has execute permission

D The cron job is listed in crontab -l

💡 Hint

If cron daemon is stopped, no jobs run or log.

If the cron daemon is not running, no cron jobs will execute or produce logs.

❓ Configuration

advanced

Configure cron to log all job outputs to a specific file

Which crontab entry correctly redirects both stdout and stderr of /usr/bin/script.sh to /var/log/script.log?

A 0 2 * * * /usr/bin/script.sh > /var/log/script.log 2>&1

B 0 2 * * * /usr/bin/script.sh 2> /var/log/script.log

C 0 2 * * * /usr/bin/script.sh > /var/log/script.log

D 0 2 * * * /usr/bin/script.sh 2>&1 > /var/log/script.log

💡 Hint

Redirect stdout first, then stderr to the same file.

The correct syntax redirects stdout to the file, then redirects stderr (2) to where stdout (1) points.

✅ Best Practice

advanced

Best practice for monitoring cron job failures

Which method is best to get notified immediately when a cron job fails?

A Check cron logs manually every day

B Write output to a local file only

C Configure cron to send email on job failure

D Ignore failures if jobs run mostly fine

💡 Hint

Immediate notification helps quick response.

Configuring cron to send emails on failure alerts you immediately to fix issues.

🧠 Conceptual

expert

Understanding cron log rotation impact

If cron logs are rotated daily and compressed, what is a key consideration for long-term cron job monitoring?

A Archived logs must be decompressed to analyze older cron jobs

B Cron jobs stop running after log rotation

C Cron daemon automatically deletes old logs without compression

D Log rotation disables cron logging temporarily

💡 Hint

Compressed logs need special handling to read.

Rotated logs are compressed files; to review older cron logs, you must decompress them first.

Practice

1. What is the main purpose of monitoring cron logs on a Linux system?

easy

A. To install new cron jobs automatically

B. To check if scheduled tasks ran successfully

C. To delete old cron jobs from the system

D. To speed up the execution of cron jobs

Solution

1. Step 1: Understand cron job scheduling

   Cron jobs are scheduled tasks that run automatically at set times.

2. Step 2: Purpose of cron log monitoring

   Monitoring cron logs helps verify if these tasks ran as planned and if there were any errors.

3. Final Answer:

   To check if scheduled tasks ran successfully -> Option B

4. Quick Check:

   Cron logs track task success = A [OK]

Hint: Cron logs show task run status, not installation or deletion [OK]

Common Mistakes:

• Confusing cron log monitoring with job installation
• Thinking cron logs speed up tasks
• Assuming cron logs delete jobs automatically

2. Which command correctly shows the last 10 lines of the cron log file located at /var/log/cron?

easy

A. tail /var/log/cron

B. head /var/log/cron

C. cat /var/log/cron | grep last 10

D. grep last /var/log/cron

Solution

1. Step 1: Understand the tail command

   The tail command shows the last lines of a file, defaulting to 10 lines.

2. Step 2: Apply tail to cron log

   Using tail /var/log/cron displays the last 10 lines of the cron log file.

3. Final Answer:

   tail /var/log/cron -> Option A

4. Quick Check:

   Last 10 lines = tail [OK]

Hint: Use tail to see recent log entries quickly [OK]

Common Mistakes:

• Using head shows first lines, not last
• Incorrect grep syntax for line count
• Confusing grep with tail for last lines

3. What will be the output of this command if the cron log contains these lines:

Jun 10 10:00:01 server CRON[1234]: (root) CMD (backup.sh)
Jun 10 10:05:01 server CRON[1235]: (root) CMD (cleanup.sh)
Jun 10 10:10:01 server CRON[1236]: (root) CMD (backup.sh)

Command: grep backup.sh /var/log/cron

medium

A. Jun 10 10:00:01 server CRON[1234]: (root) CMD (backup.sh)
Jun 10 10:10:01 server CRON[1236]: (root) CMD (backup.sh)

B. Jun 10 10:05:01 server CRON[1235]: (root) CMD (cleanup.sh)

C. No output

D. Jun 10 10:00:01 server CRON[1234]: (root) CMD (backup.sh)

Solution

1. Step 1: Understand grep filtering

   The grep backup.sh command filters lines containing 'backup.sh'.

2. Step 2: Identify matching lines in the log

   Two lines contain 'backup.sh': the first and third lines.

3. Final Answer:

   Both lines with backup.sh are shown -> Option A

4. Quick Check:

   grep filters matching lines = C [OK]

Hint: grep shows all lines containing the search term [OK]

Common Mistakes:

• Selecting only one matching line
• Expecting output for unrelated terms
• Thinking grep outputs no lines if multiple matches exist

4. You run grep CRON /var/log/cron | tail -5 but get no output, even though you know cron jobs ran recently. What is the most likely reason?

medium

A. Cron jobs do not write to /var/log/cron

B. The grep command syntax is incorrect

C. The tail command does not work with pipes

D. The cron log file is empty or missing

Solution

1. Step 1: Check command correctness

   The command syntax is correct; grep filters lines with 'CRON', tail shows last 5 lines.

2. Step 2: Consider log file state

   If no output appears, the log file may be empty or missing, so no lines match.

3. Final Answer:

   The cron log file is empty or missing -> Option D

4. Quick Check:

   No output usually means empty or missing file [OK]

Hint: No output often means empty or missing log file [OK]

Common Mistakes:

• Blaming grep syntax when it is correct
• Assuming tail can't work with pipes
• Thinking cron never logs to /var/log/cron

5. You want to monitor cron logs in real-time to see new entries as they appear. Which command should you use?

hard

A. grep -r CRON /var/log/cron

B. cat /var/log/cron | head -n 10

C. tail -f /var/log/cron

D. less /var/log/cron

Solution

1. Step 1: Understand real-time log monitoring

   Real-time monitoring means seeing new log lines as they are added.

2. Step 2: Use tail with follow option

   tail -f keeps the file open and shows new lines live.

3. Final Answer:

   tail -f /var/log/cron -> Option C

4. Quick Check:

   tail -f shows live log updates [OK]

Hint: Use tail -f to watch logs live as they update [OK]

Common Mistakes:

• Using grep -r which searches recursively, not live
• Using head only shows start, not live updates
• Using less does not auto-refresh new lines