Basic Commands¶

File & Directory Management¶

01. ls - List directory contents¶

• man Page

The ls command lists the contents of a directory. By default, it lists the files in the current directory sorted alphabetically.

Common Flags¶

Examples¶

1. List all files in long format with human-readable sizes

ls -lah

Output Explanation:

drwxr-xr-x  5 user staff  160B Dec 11 10:00 .
drwxr-xr-x 10 user staff  320B Dec 10 14:30 ..
-rw-r--r--  1 user staff  220B Dec 11 09:00 .bashrc
-rw-r--r--  1 user staff  1.2K Dec 11 10:00 file.txt

2. List files sorted by modification time (newest first)

ls -lt

3. List files in reverse alphabetical order

ls -r

Questions¶

• What does the ls command do by default?
• Which flag would you use to see hidden files (starting with .)?
• How can you list files in long format with human-readable sizes?

02. cd - Change directory¶

• man Page

The cd command changes the current working directory of the shell.

Common Usage¶

Examples¶

1. Navigate to a system directory

cd /var/log

2. Move up two levels

cd ../..

3. Toggle between two directories

cd -

4. Change to the home directory

cd ~

Questions¶

• What command changes the current directory?
• How do you go to the parent directory?
• What does cd - do?

03. pwd - Print working directory¶

• man Page

The pwd command prints the full absolute path of the current working directory.

Common Flags¶

Examples¶

1. Check where you are

pwd

Output:

/Users/username/projects/UnixLabs

2. Show the physical path (resolving symlinks)

pwd -P

Questions¶

• What does pwd stand for?
• What is the difference between pwd -L and pwd -P?

04. mkdir - Make directory¶

• man Page

The mkdir command creates new directories.

Common Flags¶

Examples¶

1. Create a directory structure

mkdir -p project/{src,bin,logs}

2. Create a directory with specific permissions

mkdir -m 700 private_folder

3. Create multiple directories at once

mkdir dir1 dir2 dir3

Questions¶

• What does mkdir do?
• Which flag allows creating parent directories if they don’t exist?
• How can you set permissions when creating a directory?

05. touch - Create empty files or update timestamps¶

• man Page

The touch command is used to create an empty file if it doesn’t exist. If the file exists, it updates the access and modification timestamps to the current time.

Common Flags¶

Examples¶

1. Create a new file

touch newfile.txt

2. Update timestamp of an existing file

touch existingfile.txt

3. Create multiple files at once

touch file1.txt file2.txt file3.txt

Questions¶

• What does touch do if the file doesn’t exist?
• What happens if you run touch on an existing file?
• Which flag changes only the access time?

System & Process Management¶

06. cp - Copy files and directories¶

• man Page

The cp command copies files and directories from a source to a destination.

Common Flags¶

Examples¶

1. Copy a file to another directory

cp file.txt /backup/

2. Copy a directory recursively

cp -r src/ src_backup/

3. Safe copy (prompt before overwrite)

cp -i important.txt /data/

4. Copy and preserve attributes

cp -p original.txt copy.txt

Questions¶

• What is the purpose of the cp command?
• Which flag do you use to copy directories recursively?
• What does the -i flag do?

07. mv - Move or rename files¶

• man Page

The mv command moves files or directories from one place to another. It is also used to rename files.

Common Flags¶

Examples¶

1. Rename a file

mv old_name.txt new_name.txt

2. Move files to a directory

mv *.log /var/log/myapp/

3. Move a directory

mv myfolder /new/location/

Questions¶

• How does mv differ from cp?
• What happens if you use mv to move a file to a different name in the same directory?
• Which flag prevents overwriting existing files?

08. rm - Remove files or directories¶

• man Page

The rm command removes (deletes) files and directories. Warning: Deleted files are usually not recoverable.

Common Flags¶

Examples¶

1. Remove a single file

rm file.txt

2. Remove a directory and all its contents (Use with caution)

rm -rf my_dir

3. Interactive removal

rm -i *.txt

4. Remove files forcefully without prompts

rm -f temp*.log

Questions¶

• Why is the rm command dangerous?
• Which flag removes directories recursively?
• What does -f do?

Text Processing¶

09. cat - Concatenate and display files¶

• man Page

The cat command reads data from the file and outputs its content. It is often used to combine files.

Common Flags¶

Examples¶

1. Display file content

cat /etc/hostname

2. Concatenate multiple files

cat part1.txt part2.txt > full_document.txt

3. Create a file from terminal input (Press Ctrl+D to save)

cat > notes.txt

4. Number all lines in a file

cat -n file.txt

Questions¶

• What does cat stand for?
• How can you use cat to create a new file?
• Which flag numbers the output lines?

10. head - Output the first part of files¶

• man Page

The head command outputs the first part of files. By default, it shows the first 10 lines.

Common Flags¶

Examples¶

1. Show the first 5 lines of a file

head -n 5 /var/log/syslog

2. View the top of a script to check the shebang

head -n 1 script.sh

3. Show the first 10 bytes of a file

head -c 10 file.txt

Questions¶

• What is the default number of lines head displays?
• How do you display the first 20 lines of a file?
• What does -c do?

11. tail - Output the last part of files¶

• man Page

The tail command outputs the last part of files. By default, it shows the last 10 lines. It is extremely useful for monitoring logs.

Common Flags¶

Examples¶

1. Show the last 20 lines of a log file

tail -n 20 error.log

2. Monitor a log file in real-time

tail -f /var/log/nginx/access.log

3. Show the last 50 bytes of a file

tail -c 50 file.txt

Questions¶

• What is tail -f used for?
• How do you display the last 50 lines?
• What is the difference between head and tail?

12. grep - Print lines matching a pattern¶

• man Page

The grep command searches for patterns (strings or regular expressions) in files and prints the matching lines.

Common Flags¶

Examples¶

1. Search for a string in a file

grep "error" application.log

2. Search recursively for a string in the current directory

grep -r "TODO" .

3. Count occurrences of a word

grep -c "localhost" /etc/hosts

4. Search case-insensitively

grep -i "Error" file.txt

Questions¶

• What does grep do?
• Which flag searches recursively?
• How do you invert the match?

13. wc - Print newline, word, and byte counts¶

• man Page

The wc command prints newline, word, and byte counts for each file.

Common Flags¶

Examples¶

1. Count lines in a file

wc -l file.txt

2. Count words

wc -w file.txt

3. Count everything

wc file.txt

Questions¶

• What does wc stand for?
• How do you count only lines?
• What are the three default counts wc displays?

14. sort - Sort lines of text files¶

• man Page

The sort command sorts lines of text files.

Common Flags¶

Examples¶

1. Sort a file alphabetically

sort names.txt

2. Sort numbers

sort -n numbers.txt

3. Sort in reverse order

sort -r names.txt

Questions¶

• How do you sort numerically?
• How do you reverse the sort order?
• Which flag removes duplicates?

15. uniq - Report or omit repeated lines¶

• man Page

The uniq command reports or omits repeated lines. Note: uniq does not detect repeated lines unless they are adjacent.

Common Flags¶

Examples¶

1. Remove adjacent duplicates

sort file.txt | uniq

2. Count occurrences

sort file.txt | uniq -c

3. Show only duplicates

sort file.txt | uniq -d

Questions¶

• Why do you often use sort before uniq?
• How do you count occurrences of lines?
• Which flag shows only duplicate lines?

16. less - Opposite of more¶

• man Page

The less command is a program similar to more, but it allows backward movement in the file as well as forward movement.

Common Flags¶

Examples¶

1. View a file

less file.txt

2. View with line numbers

less -N file.txt

3. Search inside less

/search_term (inside the viewer)

Questions¶

• How do you quit less?
• How do you search forward in less?
• What is the advantage of less over cat?

User & Permissions¶

17. chmod - Change file mode bits¶

• man Page

The chmod command changes the file mode bits (permissions) of a file or directory. Permissions define who can read, write, or execute a file.

Common Flags¶

Permission Modes¶

Symbolic Modes

Octal Modes

Examples¶

1. Make a script executable for the owner

chmod u+x script.sh

2. Set permissions to 755 (rwxr-xr-x)

chmod 755 script.sh

3. Remove write permission for others

chmod o-w file.txt

4. Add execute permission for group

chmod g+x file.sh

Questions¶

• What does chmod do?
• Explain the octal permission 755.
• How do you make a file executable?

18. chown - Change file owner and group¶

• man Page

The chown command changes the user and/or group ownership of a given file.

Common Flags¶

Examples¶

1. Change owner of a file

chown alice file.txt

2. Change owner and group

chown bob:developers project/

3. Change ownership recursively

chown -R www-data:www-data /var/www/html

4. Change only the group

chown :staff file.txt

Questions¶

• What is the difference between chown and chmod?
• Which flag changes ownership recursively?
• How do you change both owner and group?

19. sudo - Execute a command as another user¶

• man Page

The sudo command allows a permitted user to execute a command as the superuser or another user.

Common Flags¶

Examples¶

1. Run command as root

sudo apt update

2. Switch to root user

sudo -i

3. Run command as another user

sudo -u postgres psql

Questions¶

• What does sudo stand for?
• How do you switch to the root user environment?
• How do you run a command as a specific user?

20. whoami - Print effective userid¶

• man Page

The whoami command prints the user name associated with the current effective user ID.

Common Flags¶

Examples¶

1. Check current user

whoami

Questions¶

• What does whoami print?
• Is whoami the same as id -un?
• When is whoami useful?

System Information & Management¶

21. man - An interface to the system reference manuals¶

• man Page

The man command displays the user manual of any command that we can run on the terminal.

Common Flags¶

Examples¶

1. Read the manual for ls

man ls

2. Search for commands related to “copy”

man -k copy

3. Get a short description of a command

man -f ls

Questions¶

• What does man do?
• How do you search for manual pages?
• What key do you press to quit the man page viewer?

22. ps - Report a snapshot of the current processes¶

• man Page

The ps command displays information about a selection of the active processes.

Common Flags¶

Examples¶

1. View all running processes

ps -ef

2. View processes for a specific user

ps -u root

3. View processes with CPU/Memory usage (BSD style)

ps aux

Questions¶

• What does ps stand for?
• Which flag selects all processes?
• How do you view processes for a specific user?

23. kill - Terminate a process¶

• man Page

The kill command sends a signal to a process. The default signal is TERM (terminate).

Common Flags¶

Examples¶

1. Terminate a process by PID

kill 1234

2. Force kill a process

kill -9 1234

3. List available signals

kill -l

Questions¶

• What is the default signal sent by kill?
• Which signal forces a process to stop immediately?
• How do you list available signals?

24. top - Display Linux processes¶

• man Page

The top command provides a dynamic real-time view of a running system.

Common Flags¶

Examples¶

1. Start top

top

2. Monitor a specific user

top -u username

3. Refresh every 5 seconds

top -d 5

Questions¶

• What does top display?
• How do you filter processes by user in top?
• How do you change the update interval?

25. df - Report file system disk space usage¶

• man Page

The df command displays the amount of disk space available on the file system containing each file name argument.

Common Flags¶

Examples¶

1. Check disk space usage

df

2. Human readable format

df -h

3. Show file system type

df -Th

Questions¶

• What does df report?
• Which flag shows sizes in human-readable format?
• How do you see the file system type?

26. du - Estimate file space usage¶

• man Page

The du command estimates file space usage.

Common Flags¶

Examples¶

1. Check size of current directory

du -h .

2. Summary of a directory

du -sh /var/log

3. Check size of specific files

du -h file1.txt file2.txt

Questions¶

• What is the difference between df and du?
• How do you get a summary of a directory’s size?
• Which flag gives a grand total?

Advanced Search¶

27. find - Search for files in a directory hierarchy¶

• man Page

The find command searches for files in a directory hierarchy.

Common Flags¶

Examples¶

1. Find files by name

find . -name "*.txt"

2. Find directories only

find /var -type d

3. Find and delete files (Be careful)

find . -name "temp*" -delete

Questions¶

• How do you find files by name?
• How do you find only directories?
• What does -mtime do?

Networking¶

28. ping - Send ICMP ECHO_REQUEST to network hosts¶

• man Page

The ping command uses the ICMP protocol’s mandatory ECHO_REQUEST datagram to elicit an ICMP ECHO_RESPONSE from a host or gateway.

Common Flags¶

Examples¶

1. Ping a host

ping google.com

2. Ping 5 times

ping -c 5 google.com

3. Ping with specific interval

ping -i 0.5 google.com

Questions¶

• What protocol does ping use?
• How do you limit the number of pings?
• What does TTL stand for?

29. curl - Transfer a URL¶

• man Page

The curl command is a tool to transfer data from or to a server.

Common Flags¶

Examples¶

1. Download a file

curl -O https://example.com/file.zip

2. Check headers

curl -I https://google.com

3. Follow redirects

curl -L https://google.com

Questions¶

• What does curl do?
• How do you save the output to a file with the same name?
• Which flag follows redirects?

Archives¶

30. tar - An archiving utility¶

• man Page

The tar command is an archiving utility used to store and extract files from an archive file known as a tarball.

Common Flags¶

Examples¶

1. Create a tar.gz archive

tar -czvf archive.tar.gz /path/to/dir

2. Extract a tar.gz archive

tar -xzvf archive.tar.gz

3. List contents of an archive

tar -tvf archive.tar.gz

Questions¶

• What does tar stand for?
• Which flag creates a new archive?
• How do you compress the archive with gzip?

31. zip - Package and compress (archive) files¶

• man Page

The zip command is a compression and file packaging utility.

Common Flags¶

Examples¶

1. Zip a directory

zip -r archive.zip directory/

2. Zip multiple files

zip archive.zip file1.txt file2.txt

3. Password protect zip

zip -e secure.zip file.txt

Questions¶

• How do you zip a directory recursively?
• How do you password protect a zip file?
• What is the difference between tar and zip?

32. unzip - List, test and extract compressed files in a ZIP archive¶

• man Page

The unzip command will list, test, or extract files from a ZIP archive.

Common Flags¶

Examples¶

1. Unzip a file

unzip archive.zip

2. List contents without extracting

unzip -l archive.zip

3. Extract to specific directory

unzip archive.zip -d /path/to/dest

Questions¶

• How do you list contents of a zip file?
• How do you extract to a specific directory?
• What does unzip -t do?