Linux Command-Line Notes

ls: List directory contents
- ls – lists files in the current directory
- ls -l – lists files in long format (permissions, size, owner, etc.)
- ls -a – lists all files, including hidden ones (starting with a dot .)
cd: Change directory
- cd [directory] – change to specified directory
- cd .. – move up one level (parent directory)
- cd / – move to the root directory
- cd ~ – move to the home directory
pwd: Print working directory
- Displays the current directory path.
mkdir: Make directories
- mkdir [directory_name] – creates a new directory
rmdir: Remove directories
- rmdir [directory_name] – deletes an empty directory
rm: Remove files or directories
- rm [file] – removes file
- rm -r [directory] – removes directory recursively
- rm -rf [directory] – forcefully removes a directory without prompting
cp: Copy files or directories
- cp [source][destination] – copies file from source to destination
- cp -r [source_directory][destination] – copies directory recursively
mv: Move or rename files or directories
- mv [source][destination] – moves or renames a file or directory
touch: Create an empty file or update file timestamps
- touch [filename] – creates a new file if it doesn’t exist

cat: Concatenate and display files
- cat [file] – displays the content of a file
- cat [file1][file2] – displays the content of both files in order
less: View file content page by page
- less [file] – allows scrolling through a large file
head: Display the first lines of a file
- head -n [number][file] – displays the first n lines of the file
tail: Display the last lines of a file
- tail -n [number][file] – displays the last n lines of the file
- tail -f [file] – continuously monitors and displays new lines added to a file (useful for log monitoring)
grep: Search for patterns within files
- grep [pattern][file] – searches for a pattern in a file
- grep -r [pattern][directory] – searches recursively in a directory

chmod: Change file permissions
- chmod [permissions][file] – modifies the permission of a file
- chmod u+x [file] – gives the user execution rights for the file
chown: Change file ownership
- chown [owner][file] – changes the ownership of the file
chgrp: Change group ownership of a file
- chgrp [group][file] – changes the group ownership of the file

ps: View currently running processes
- ps – displays processes running in the current session
- ps -aux – shows detailed information about all running processes
top: Real-time view of system processes
- Displays real-time information about running processes, CPU, and memory usage
kill: Terminate processes
- kill [PID] – sends the terminate signal to a process with a specific Process ID (PID)
- kill -9 [PID] – forcefully kills a process
htop: Interactive process viewer (requires installation)
- htop – a more interactive and user-friendly version of top
df: Show disk space usage
- df -h – displays disk usage in human-readable format
du: Show directory space usage
- du -sh [directory] – shows the size of the specified directory
free: Check memory usage
- free -h – displays memory and swap usage in human-readable format
uname: Display system information
- uname -a – displays detailed information about the system (kernel version, architecture, etc.)

ping: Test network connectivity
- ping [hostname/IP] – sends ICMP packets to a host to test connectivity
ifconfig / ip: Network interface configuration
- ifconfig – displays network interface information (Linux pre-2019, now deprecated in favor of ip)
- ip addr – displays IP address and network details
netstat: Display network connections and listening ports
- netstat -tuln – displays TCP/UDP listening ports
curl: Transfer data to or from a server
- curl [URL] – makes HTTP requests to a URL and retrieves the content
wget: Download files from the web
- wget [URL] – downloads a file from the specified URL

tar: Archive and extract files
- tar -cvf [archive.tar][file/directory] – creates an archive
- tar -xvf [archive.tar] – extracts the contents of an archive
gzip / gunzip: Compress and decompress files
- gzip [file] – compresses the file
- gunzip [file.gz] – decompresses the file
zip / unzip: Compress and decompress files in zip format
- zip [archive.zip][file] – compresses file into a zip archive
- unzip [archive.zip] – extracts a zip archive

whoami: Display the current user
- Shows the currently logged-in user.
adduser / useradd: Add a new user
- adduser [username] – adds a new user (simpler, with prompts)
- useradd [username] – adds a new user (requires manual configuration)
passwd: Change user password
- passwd [username] – sets or changes the password for the user
groups: Display group membership
- groups [username] – shows the groups a user belongs to

echo: Print text to the screen
- echo [text] – prints the provided text
sort: Sort lines in a file
- sort [file] – sorts the lines in the file
uniq: Remove duplicate lines from a file
- uniq [file] – removes adjacent duplicate lines
cut: Extract sections from a file
- cut -d [delimiter] -f [fields][file] – extracts specific fields based on delimiter
wc: Word count
- wc -l [file] – counts the number of lines in a file
awk: Pattern scanning and processing language
- Used to manipulate data and generate reports.
sed: Stream editor for modifying files
- sed 's/old/new/g' [file] – replaces all occurrences of old with new in a file

apt / apt-get (Debian-based systems):
- apt-get update – update package lists
- apt-get install [package] – install a package
yum / dnf (Red Hat-based systems):
- yum install [package] – install a package