Cron log monitoring in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to view the cron log file.

Linux CLI

tail -f /var/log/[1]

Drag options to blanks, or click blank then click option'

Amessages

Bcron.log

Csyslog

Dcron

Attempts:

3 left

💡 Hint

Common Mistakes

Using syslog or messages file which contain many other logs.

Adding file extensions like .log which may not exist.

The cron log file on many Linux systems is /var/log/cron. Using tail -f lets you watch new cron entries live.

2fill in blank

medium

Complete the command to search cron logs for errors.

Linux CLI

grep '[1]' /var/log/cron

Drag options to blanks, or click blank then click option'

Aerror

Bcron

Cfail

Dwarning

Attempts:

3 left

💡 Hint

Common Mistakes

Searching for 'cron' which matches all lines.

Using 'warning' which may miss errors.

Searching for the word error helps find cron job failures in the log.

3fill in blank

hard

Fix the error in the command to view cron logs with timestamps.

Linux CLI

awk '[1]' /var/log/cron

Drag options to blanks, or click blank then click option'

Aprint

B{print $1, $2, $3, $6}

CBEGIN

D{print}

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting curly braces causing syntax errors.

Using 'print' without braces.

The correct awk syntax requires the action block inside curly braces with print and the fields to display.

4fill in blank

hard

Fill both blanks to filter cron logs for a specific user and command.

Linux CLI

grep '[1]' /var/log/cron | grep '[2]'

Drag options to blanks, or click blank then click option'

Ausername

Broot

Cbackup.sh

Dcron

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'username' literally instead of the actual user.

Filtering by 'cron' twice which is redundant.

First filter by the user root, then by the command backup.sh to find relevant cron entries.

5fill in blank

hard

Fill all three blanks to create a dictionary of commands and their run counts from cron logs.

Linux CLI

awk '{print $6}' /var/log/cron | sort | uniq -c | awk '[1]' | sort -nr > [2] && cat [3]

Drag options to blanks, or click blank then click option'

A{print $2, $1}

Bcron_summary.txt

D{print $1, $2}

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping fields in print causing wrong output order.

Using different file names inconsistently.

This command sequence counts how many times each command runs in cron logs, saves the summary to a file, and then displays it.

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