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🌟 CMD Challenge Solutions with Detailed Explanations

This repository contains solutions to various command-line tasks from the CMD Challenge. Each challenge is designed to test and improve your shell scripting and command-line skills. Below is the list of challenges and their solutions, along with detailed explanations to help beginners understand the concepts.

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1. Hello World 🚀

Task: Print "hello world".
Solution:

echo "hello world"

Explanation:

• echo is a command used to display text or strings on the terminal.
• By typing echo "hello world", the shell will output the text hello world to the screen.

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2. Current Working Directory 📂

Task: Print the current working directory.
Solution:

pwd

Explanation:

• pwd stands for "print working directory."
• This command displays the full path of the directory you are currently in.

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3. List Files 📜

Task: List all of the files in the current directory, one file per line.
Solution:

ls -1

Explanation:

• ls lists files and directories in the current directory.
• The -1 option ensures that each file is displayed on a separate line.

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4. Last Lines 📖

Task: Print the last 5 lines of access.log.
Solution:

tail -5 access.log

Explanation:

• tail displays the last part of a file.
• The -5 option tells tail to show the last 5 lines of the file access.log.

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5. Find String in a File 🔍

Task: Print all lines in access.log that contain the string "GET".
Solution:

grep GET access.log

Explanation:

• grep searches for a specific pattern in a file.
• Here, it searches for the word GET in the file access.log and prints all matching lines.

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6. Search for Files Containing String 🕵️

Task: Print all files, one per line, that contain the string "500".
Solution:

grep -rl * -e 500

Explanation:

• grep searches files for a given pattern.
• -r means recursive, so it searches in the current directory and all subdirectories.
• -l means it only prints the names of files containing the match, not the matching lines.
• * searches all files in the current directory.
• -e 500 specifies the string 500 to search for.

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7. Search for Files by Extension 📂

Task: Print the relative file paths for all files that start with access.log.
Solution:

find . -name "access.log*"

Explanation:

• find searches for files and directories.
• . specifies the current directory.
• -name tells find to look for files matching a specific name pattern.
• "access.log*" matches any file name starting with access.log.

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8. Search for String in Files Recursively 🔄

Task: Print all matching lines (without the filename or file path) in files starting with access.log and containing the string "500".
Solution:

find . -name "access.log*" | xargs grep -h 500

Explanation:

• find . -name "access.log*" finds all files starting with access.log.
• The | symbol pipes the output of find into the next command.
• xargs passes the file names to grep.
• grep -h searches for 500 but hides file names from the output.

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9. Extract IP Addresses 🌐

Task: Extract all IP addresses from files that start with access.log, printing one per line.
Solution:

find . -name "access.log*" | xargs grep -Eo '^[^ ]+'

Explanation:

• find . -name "access.log*" finds files starting with access.log.
• xargs passes the file names to grep.
• grep -Eo '^[^ ]+' extracts patterns using regular expressions:
  • -E enables extended regular expressions.
  • -o prints only the matching part of the line.
  • '^[^ ]+' matches the beginning of a line until the first space, which is often the IP address in logs.

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10. Delete Files ❌

Task: Delete all files in the current directory, including subdirectories and their contents.
Solution:

find . -delete

Explanation:

• find . searches in the current directory and all subdirectories.
• -delete removes the files and directories found by find. Note: Be very careful when using this command as it will delete everything in the specified directory!

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11. Count Files 🔢

Task: Count the number of files in the current working directory.
Solution:

ls | wc -l

Explanation:

• ls lists files in the current directory.
• The | passes the output of ls to wc (word count).
• wc -l counts the number of lines, which equals the number of files.

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12. Simple Sort 🔀

Task: Print the contents of access.log sorted.
Solution:

sort access.log

Explanation:

• sort arranges the lines of a file in alphabetical or numerical order by default.
• Here, it sorts the lines in access.log.

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13. Count String in Line 🧴

Task: Print the number of lines in access.log containing the string "GET".
Solution:

grep GET access.log | wc -l

Explanation:

• grep GET access.log finds all lines containing GET.
• The | passes these lines to wc -l.
• wc -l counts the number of matching lines.

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14. Split on a Character 🛁

Task: Split the numbers in split-me.txt on the ; character, printing one per line.
Solution:

cat split-me.txt | sed 's/;/
/g'

Explanation:

• cat split-me.txt outputs the content of the file split-me.txt.
• | pipes the output to sed.
• sed 's/;/ /g' replaces each semicolon (;) with a newline character ( ), printing each number on a new line.

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15. Print Number Sequence 📈

Task: Print the numbers 1 to 100 separated by spaces.
Solution:

echo {1..100}

Explanation:

• echo outputs text to the terminal.
• {1..100} is a sequence expression that generates numbers from 1 to 100.
• This prints all numbers in a single line, separated by spaces.

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16. Remove Files with Extension 🧹

Task: Remove all .doc files recursively in the current directory.
Solution:

find . -name "*.doc" -delete

Explanation:

• find . -name "*.doc" searches for files ending with .doc.
• -delete removes these files.

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17. Replace Text in Files ✍️

Task: Delete "challenges are difficult" from all .txt files.
Solution:

find . -name "*.txt" -exec sed -i 's/challenges are difficult//g' {} +

Explanation:

• find . -name "*.txt" locates all .txt files.
• -exec runs a command on each file found.
• sed -i 's/challenges are difficult//g' removes the text "challenges are difficult" from each file in place (-i).

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18. Sum All Numbers ➕

Task: Print the sum of numbers in sum-me.txt.
Solution:

cat sum-me.txt | xargs | sed 's/ /+/g' | bc

Explanation:

• cat sum-me.txt outputs the file's content.
• xargs joins all numbers into a single line.
• sed 's/ /+/g' replaces spaces between numbers with + signs to form a mathematical expression.
• bc evaluates the expression and prints the result.

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19. Just the Files 📁

Task: Print all files in the current directory recursively without leading paths.
Solution:

find . -type f -printf "%f\n"

Explanation:

• find . searches the current directory and subdirectories.
• -type f restricts the search to files (not directories).
• -printf "%f\n" prints only the filenames without their paths.

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20. Remove Extensions from Files 🛠️

Task: Remove file extensions recursively.
Solution:

find . -type f -exec bash -c 'mv "$1" "${1%.*}"' - '{}' \;

Explanation:

• find . -type f finds all files in the current directory and subdirectories.
• -exec bash -c runs a custom bash command for each file.
• mv "$1" "${1%.*}" renames the file by removing everything after the last . (dot), which is the file extension.

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21. Replace Spaces in Filenames 🌌

Task: Replace spaces in filenames with a . character.
Solution:

find . -type f -name "* *" | while read -r file; do mv "$file" "${file// /_}"; done

Explanation:

• find . -type f -name "* *" finds files with spaces in their names.
• The | pipes the list of filenames to a while loop.
• mv "$file" "${file// /_}" renames each file by replacing spaces with underscores (_).

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22. Files Starting with a Number 🔢

Task: Print filenames starting with a number.
Solution:

find . -name '[0-9]*' -type f -printf "%f\n"

Explanation:

• find . -name '[0-9]*' searches for files whose names start with a digit.
• -type f ensures only files are included.
• -printf "%f\n" prints only the filenames.

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23. Print Nth Line 📜

Task: Print the 25th line of faces.txt.
Solution:

sed '25q;d' faces.txt

Explanation:

• sed is used to process and transform text in a file.
• '25q;d' tells sed to quit (q) after reading 25 lines, and delete (d) all but the 25th line.

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24. Remove Duplicate Lines ♻️

Task: Print faces.txt with only the first instance of each duplicate line.
Solution:

awk '!seen[$0]++' faces.txt

Explanation:

• awk processes text line by line.
• !seen[$0]++ ensures each line is printed only the first time it is encountered.

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25. Corrupted Text 🛠️

Task: Remove random ! marks from war_and_peace.txt to restore the original text.
Solution:

< war_and_peace.txt tr -s '!' | sed 's/!//g'

Explanation:

• tr -s '!' squeezes multiple ! into a single one.
• sed 's/!//g' removes all remaining ! characters.

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26. Print Hello World (Alternate) 🚀

Task: Print "Hello World" using escaped space.
Solution:

echo hello\ world

Explanation:

• hello\ world escapes the space with a \, ensuring the two words are treated as part of the same string.

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27. Count Tab Characters 📂

Task: Count lines containing tab characters in file-with-tabs.txt.
Solution:

grep -c $'\t' file-with-tabs.txt

Explanation:

• grep -c counts the number of matching lines.
• $'\t' matches tab characters.

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28. Remove Files Starting with Dash 🗑️

Task: Remove files starting with a dash in the filename.
Solution:

find . -name '-*' -delete

Explanation:

• find . -name '-*' searches for files starting with a dash (-).
• -delete removes those files.

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29. Remove Files without Specific Extensions ❌

Task: Remove all files without .txt and .exe extensions recursively.
Solution:

find . -type f -not \( -name '*.txt' -or -name '*.exe' \) -delete

Explanation:

• find . -type f finds all files.
• -not \( -name '*.txt' -or -name '*.exe' \) excludes .txt and .exe files.
• -delete removes all other files.

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30. Common IPs in Logs 🌐

Task: Print IP addresses common to access.log.1 and access.log.2.
Solution:

comm -12 <(cut -d' ' -f1 access.log.1 | sort) <(cut -d' ' -f1 access.log.2 | sort)

Explanation:

• cut -d' ' -f1 extracts the first column (IP addresses) from each file.
• sort arranges the IPs in order.
• comm -12 finds common lines between the two sorted files.

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31. Reverse Lines 🔄

Task: Print the lines of the file reverse-me.txt in reverse order so that the last line appears first.
Solution:

tac reverse-me.txt

Explanation:

• tac prints a file line by line in reverse order.

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32. Unique Primes 🔢

Task: Print the number of unique prime numbers in random-numbers.txt.
Solution:

cat random-numbers.txt | sort | uniq | factor | awk 'NF==2' | wc -l

Explanation:

• sort | uniq eliminates duplicate numbers.
• factor breaks numbers into their prime factors.
• awk 'NF==2' filters lines with exactly two fields (prime numbers only).
• wc -l counts the number of unique primes.

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33. Find Ports in Netstat 🌐

Task: Print all IPv4 listening ports from netstat.out, sorted in descending order.
Solution:

grep '\bLISTEN\b' netstat.out | grep -oP 'tcp\s+.*:\K\d+' | sort -nr

Explanation:

• grep '\bLISTEN\b' filters lines containing LISTEN.
• grep -oP 'tcp\s+.*:\K\d+' extracts port numbers using Perl-compatible regular expressions.
• sort -nr sorts the port numbers in descending numerical order.

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34. Monitor File Changes 📋

Task: Watch for any changes in the logs directory and display them in real-time.
Solution:

inotifywait -m logs

Explanation:

• inotifywait is a tool for monitoring file system events.
• -m enables monitoring mode to keep the command running.
• logs specifies the directory to monitor. It displays events like file modifications, deletions, and creations.

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35. Detect Failed SSH Login Attempts 🚨

Task: Count the number of failed SSH login attempts in /var/log/auth.log.
Solution:

grep "Failed password" /var/log/auth.log | wc -l

Explanation:

• grep "Failed password" searches for failed login attempts in the log file.
• wc -l counts the number of matching lines.

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36. List Open Network Connections 🌐

Task: Display all open network connections and their statuses.
Solution:

netstat -tuln

Explanation:

• netstat displays network statistics.
• -tuln shows TCP/UDP connections (t/u), listening ports (l), and numeric addresses (n).

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37. Search for SUID Files 🛡️

Task: Find all files with the SUID bit set on the system.
Solution:

find / -perm -4000 2>/dev/null

Explanation:

• find / searches the entire filesystem.
• -perm -4000 matches files with the SUID bit set.
• 2>/dev/null suppresses permission-denied errors.

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38. Analyze Disk Usage 📊

Task: Find the top 5 largest directories in the /var directory.
Solution:

du -h /var | sort -rh | head -n 5

Explanation:

• du -h /var calculates the size of each directory in human-readable format.
• sort -rh sorts by size in reverse order.
• head -n 5 displays the top 5 results.

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39. Check for Active Users 👤

Task: List all currently logged-in users.
Solution:

who

Explanation:

• who displays information about users currently logged into the system.

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40. Identify Open Files by Process 🔍

Task: List all open files by the nginx process.
Solution:

lsof -c nginx

Explanation:

• lsof lists open files.
• -c nginx filters for files opened by processes named nginx.

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41. Verify File Integrity 🔒

Task: Generate a SHA256 checksum for the file backup.tar.gz.
Solution:

sha256sum backup.tar.gz

Explanation:

• sha256sum calculates the SHA256 hash of a file to verify its integrity.

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42. Securely Delete Files 🧹

Task: Permanently delete sensitive.txt without recovery.
Solution:

shred -u sensitive.txt

Explanation:

• shred overwrites the file multiple times before deleting it.
• -u ensures the file is deleted after shredding.

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43. Monitor Memory Usage 📈

Task: Display real-time memory usage.
Solution:

watch -n 1 free -h

Explanation:

• watch -n 1 runs a command every second.
• free -h displays memory usage in human-readable format.

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44. Find Suspicious Processes 🕵️‍♂️

Task: Identify processes using high CPU usage.
Solution:

ps aux --sort=-%cpu | head -n 10

Explanation:

• ps aux lists all running processes.
• --sort=-%cpu sorts them by CPU usage in descending order.
• head -n 10 shows the top 10 results.

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45. Extract Subdomain from URL 🌐

Task: Extract the subdomain from a list of URLs in websites.txt.
Solution:

awk -F[/:] '{print $4}' websites.txt

Explanation:

• awk processes text line by line.
• -F[/:] uses / and : as field separators.
• {print $4} extracts the fourth field, which is the subdomain or domain.

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46. List Recently Modified Files 📆

Task: Find files modified in the last 24 hours.
Solution:

find . -type f -mtime -1

Explanation:

• find . -type f locates all files.
• -mtime -1 matches files modified within the last day.

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47. Check Open Ports 🔓

Task: Display all open ports on the system.
Solution:

ss -tuln

Explanation:

• ss displays socket statistics.
• -tuln shows TCP/UDP connections, listening ports, and numeric addresses.

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48. Kill Processes by Name 🛑

Task: Terminate all processes named apache2.
Solution:

pkill apache2

Explanation:

• pkill sends signals to processes by name.
• apache2 is the name of the processes to terminate.

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49. Detect Hidden Files 📁

Task: Find all hidden files in the current directory.
Solution:

find . -type f -name ".*"

Explanation:

• find . -type f locates all files.
• -name ".*" matches hidden files (names starting with .).

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50. Track System Uptime ⏳

Task: Display how long the system has been running.
Solution:

uptime

Explanation:

• uptime shows the current time, uptime duration, and load averages.

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