System logs (/var/log) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this command?

You run the command tail -n 3 /var/log/syslog on a Linux system. What does this command do?

Linux CLI

tail -n 3 /var/log/syslog

A Shows the first 3 lines of the syslog file

B Shows the last 3 lines of the syslog file

C Deletes the last 3 lines of the syslog file

D Shows all lines except the last 3 lines of the syslog file

💡 Hint

Think about what the 'tail' command does with the '-n' option.

The tail command shows the end of a file. The -n 3 option tells it to show the last 3 lines.

💻 Command Output

intermediate

What does this command output?

You run grep -i error /var/log/auth.log | wc -l. What does this command do?

Linux CLI

grep -i error /var/log/auth.log | wc -l

A Shows all lines with the word 'error' in auth.log

B Deletes lines containing 'error' from auth.log

C Counts all lines in auth.log

D Counts how many lines contain the word 'error' (case insensitive) in auth.log

💡 Hint

Remember what grep and wc -l do.

grep -i error finds lines with 'error' ignoring case. wc -l counts those lines.

🔧 Debug

advanced

Why does this command fail?

You try to run cat /var/log/secure | grep 'Failed password' but get a permission denied error. Why?

Linux CLI

cat /var/log/secure | grep 'Failed password'

A The grep command syntax is wrong

B The file /var/log/secure does not exist

C You need root permissions to read /var/log/secure

D The pipe symbol '|' is not supported in this shell

💡 Hint

Check file permissions for /var/log/secure.

The /var/log/secure file is usually readable only by root. Without root, you get permission denied.

🚀 Application

advanced

Create a command to monitor new log entries

Which command will continuously show new lines added to /var/log/messages in real time?

A tail -f /var/log/messages

B head -n 10 /var/log/messages

C cat /var/log/messages

D grep 'new' /var/log/messages

💡 Hint

Think about commands that follow file changes live.

tail -f shows the end of a file and updates as new lines are added.

🧠 Conceptual

expert

What is the purpose of log rotation in /var/log?

Why do Linux systems use log rotation for files in /var/log?

A To prevent log files from growing too large and consuming disk space

B To delete all logs automatically after one day

C To merge all logs into one big file

D To encrypt log files for security

💡 Hint

Think about disk space and file management.

Log rotation moves or compresses old logs to keep file sizes manageable and save disk space.

Practice

1. What is the primary purpose of files located in the /var/log/ directory on a Linux system?

easy

A. To hold user personal documents and files

B. To contain system configuration files

C. To save temporary files created during program execution

D. To store system and application logs recording events and errors

Solution

1. Step 1: Understand the role of /var/log/

   The /var/log/ directory is designed to keep logs about system and application activities.

2. Step 2: Differentiate from other directories

   Other directories like /home store user files, /etc holds configs, and /tmp stores temporary files.

3. Final Answer:

   To store system and application logs recording events and errors -> Option D

4. Quick Check:

   /var/log/ = system logs [OK]

Hint: Remember: /var/log holds logs, not configs or user files [OK]

Common Mistakes:

• Confusing /var/log with /etc for configs
• Thinking /var/log stores user documents
• Mixing up temporary files with logs

2. Which command correctly displays the last 10 lines of the /var/log/syslog file?

easy

A. tail /var/log/syslog

B. head /var/log/syslog

C. cat /var/log/syslog | head

D. less /var/log/syslog

Solution

1. Step 1: Recall command functions

   The tail command shows the last lines of a file; by default, it shows the last 10 lines.

2. Step 2: Compare options

   head shows the first lines, cat | head also shows first lines, and less opens a pager for browsing but does not directly show last lines.

3. Final Answer:

   tail /var/log/syslog -> Option A

4. Quick Check:

   tail = last 10 lines [OK]

Hint: Use tail to see last lines quickly [OK]

Common Mistakes:

• Using head instead of tail for last lines
• Confusing less with tail output
• Piping cat to head for last lines

3. What will be the output of the command grep 'error' /var/log/syslog | wc -l?

medium

A. It displays all lines from /var/log/syslog except those with 'error'

B. It shows the number of lines containing the word 'error' in /var/log/syslog

C. It counts all words in /var/log/syslog

D. It shows the first line containing 'error' in /var/log/syslog

Solution

1. Step 1: Understand grep and wc -l

   grep 'error' /var/log/syslog filters lines containing 'error'. wc -l counts the number of lines from that output.

2. Step 2: Combine command effects

   The pipeline counts how many lines in syslog have the word 'error'.

3. Final Answer:

   It shows the number of lines containing the word 'error' in /var/log/syslog -> Option B

4. Quick Check:

   grep + wc -l = count matching lines [OK]

Hint: grep filters lines; wc -l counts them [OK]

Common Mistakes:

• Thinking grep excludes matching lines
• Confusing wc -l with word count
• Assuming output shows lines, not count

4. You run tail -n 20 /var/log/auth.log but get an error: "tail: cannot open '/var/log/auth.log' for reading: Permission denied". What is the best way to fix this?

medium

A. Run the command with sudo: sudo tail -n 20 /var/log/auth.log

B. Change the file permissions with chmod 777 /var/log/auth.log

C. Copy the file to your home directory and run tail there

D. Use head instead of tail

Solution

1. Step 1: Identify permission issue

   The error says permission denied, meaning the user lacks rights to read the file.

2. Step 2: Use elevated privileges safely

   Using sudo runs the command as root, allowing access without changing file permissions.

3. Final Answer:

   Run the command with sudo: sudo tail -n 20 /var/log/auth.log -> Option A

4. Quick Check:

   Permission denied? Use sudo [OK]

Hint: Use sudo for restricted log files [OK]

Common Mistakes:

• Changing permissions insecurely with chmod 777
• Copying files without permissions
• Using head instead of fixing permissions

5. You want to monitor new entries added to /var/log/syslog in real-time and filter only lines containing the word 'failed'. Which command achieves this?

hard

A. cat /var/log/syslog | tail -f

B. grep failed /var/log/syslog

C. tail -f /var/log/syslog | grep failed

D. less /var/log/syslog | grep failed

Solution

1. Step 1: Understand real-time monitoring

   tail -f shows new lines added to a file live as they appear.

2. Step 2: Filter output for 'failed'

   Piping tail -f output to grep failed shows only lines containing 'failed' in real-time.

3. Final Answer:

   tail -f /var/log/syslog | grep failed -> Option C

4. Quick Check:

   tail -f + grep = live filtered log [OK]

Hint: Use tail -f piped to grep for live filtered logs [OK]

Common Mistakes:

• Using grep alone shows only current matches, no live updates
• Using cat with tail -f incorrectly
• Trying to grep inside less which is not live