System logs (/var/log) in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the /var/log directory in Linux?

The /var/log directory stores system log files. These logs record events, errors, and messages from the system and applications to help monitor and troubleshoot.

beginner

Which command shows the last 10 lines of a log file?

The tail command shows the last 10 lines by default. For example, tail /var/log/syslog shows the last 10 lines of the syslog file.

beginner

How can you continuously watch new entries added to a log file?

Use tail -f /var/log/filename. This command shows new lines as they are added, like watching a live feed.

intermediate

What is the difference between /var/log/syslog and /var/log/auth.log?

/var/log/syslog records general system messages and events. /var/log/auth.log records authentication-related events like logins and sudo usage.

beginner

Why is it important to check system logs regularly?

Checking logs helps find problems early, understand system behavior, and improve security by spotting unauthorized access or errors.

Which command shows the last 20 lines of /var/log/syslog?

A tail -n 20 /var/log/syslog

B head -n 20 /var/log/syslog

C cat /var/log/syslog | less

D grep 20 /var/log/syslog

tail -n 20 shows the last 20 lines of the file. head shows the first lines, cat | less shows the whole file page by page, and grep 20 searches for the text '20'.

What does the command tail -f /var/log/auth.log do?

A Shows the last lines and updates live as new lines are added

B Deletes the auth.log file

C Shows the first lines of auth.log

D Searches for the word 'auth' in the log

tail -f shows the end of the file and keeps showing new lines as they appear, useful for live monitoring.

Which log file would you check to find failed login attempts?

A /var/log/syslog

B /var/log/kern.log

C /var/log/dmesg

D /var/log/auth.log

/var/log/auth.log records authentication events including failed logins.

What is the main reason to use system logs?

A To store user files

B To monitor system events and troubleshoot problems

C To install software

D To speed up the system

System logs help track what happens on the system and find issues.

Which command would you use to search for the word 'error' in /var/log/syslog?

A cat error /var/log/syslog

B tail error /var/log/syslog

C grep error /var/log/syslog

D head error /var/log/syslog

grep searches for text patterns in files. Here it finds lines containing 'error'.

Explain how you would monitor a system log file in real time to catch new events as they happen.

Think about a command that shows the end of a file and keeps updating.

Describe the difference between /var/log/syslog and /var/log/auth.log and when you would check each.

One is for overall system events, the other for security-related events.

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