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💻 Common Useful Linux Commands 🐧

Common Linux Commands and Descriptions

Linux FileSystem Structure

/
├── bin/              # Essential user binaries (e.g., ls, cat), accessible by all users.
├── boot/             # Files needed to boot the system, like the kernel and bootloader configs.
├── dev/              # Device files representing hardware (e.g., /dev/sda for hard drives).
├── etc/              # System-wide configuration files (e.g., fstab, hosts).
│   ├── init.d/       # Service scripts (start, stop, restart)
│   ├── nginx/        # Configuration for the nginx web server
│   └── ssh/          # SSH configuration files
├── home/             # User home directories for personal data.
│   └── user/         # Directory for user 'user',
├── lib/              # Shared libraries essential for the system and kernel modules.
├── media/            # Mount points for removable media, external devices like USBs or DVDs.
├── mnt/              # Temporary mount directory
├── opt/              # Optional software packages or third-party apps.
├── proc/             # Process and kernel information
├── root/             # Home directory for root user
├── sbin/             # System binaries for administrative tasks (e.g., reboot, iptables).
├── tmp/              # Temporary files, often cleared on reboot.
├── usr/              # User programs and libraries, typically for installed packages.
│   ├── bin/          # User commands
│   ├── lib/          # User libraries
│   └── share/        # Shared files
└── var/              # Variable files (logs, databases, etc.)
    ├── log/          # Log files
    ├── cache/        # Application cache data
    └── tmp/          # Temporary files created by applications

File and Directory Operations

ls – Lists the contents of a directory.

ls
ls -l        # Long listing format
ls -a        # List all files, including hidden ones

cd – Changes the current directory.

cd /path/to/directory
cd ..        # Go up one directory
cd ~         # Go to the home directory

mkdir – Creates a new directory.
```
mkdir new_directory
```
rmdir – Removes an empty directory.
```
rmdir directory_name
```

cp – Copies files or directories.

cp source_file destination
cp -r source_directory destination_directory   # Copy directories recursively

mv – Moves or renames files and directories.

mv old_name new_name
mv file_name /path/to/destination/

rm – Removes files or directories.

rm file_name
rm -r directory_name   # Remove directories recursively

touch – Creates an empty file or updates the timestamp of an existing file.
```
touch file_name
```

File Viewing & Manipulation

cat – Displays the contents of a file.
```
cat file_name
```
less – Allows you to view file contents page by page.
```
less file_name
```

head – Shows the first 10 lines of a file (default).

head file_name
head -n 5 file_name    # Show the first 5 lines

tail – Shows the last 10 lines of a file (default).

tail file_name
tail -n 5 file_name    # Show the last 5 lines

grep – Searches for patterns within files.

grep 'search_term' file_name
grep -r 'search_term' /path/to/directory    # Search recursively in directories

Permissions & Ownership

chmod – Changes file permissions.

chmod 755 file_name    # Gives read, write, execute permissions to the owner and read, execute to others
chmod +x script.sh     # Make file executable

chown – Changes the file owner and group.
```
chown user:group file_name
```

umask – Sets default file creation permissions.

umask 022   # Sets default permissions to 755 for directories and 644 for files

Process Management

ps – Displays the currently running processes.
```
ps
ps aux      # Show all processes
```
top – Displays real-time system processes and resource usage.
```
top
```

kill – Terminates a process by its PID.

kill process_id
kill -9 process_id    # Forcefully kill a process

htop – Interactive process viewer (requires installation).
```
htop
```

System Information

df – Shows disk space usage.
```
df -h      # Human-readable format
```
du – Shows disk usage for files and directories.
```
du -h /path/to/directory
```
free – Displays memory usage.
```
free -h    # Human-readable format
```
uname – Shows system information.
```
uname -a   # Display all system info
```
uptime – Shows how long the system has been running.
```
uptime
```
whoami – Displays the current logged-in user.
```
whoami
```
hostname – Displays or sets the system's hostname.
```
hostname
```
lscpu – Displays CPU architecture information.
```
lscpu
```

Network Commands

ping – Tests connectivity to a host.
```
ping google.com
```
ifconfig – Displays network interface information (may require net-tools installation on some systems).
```
ifconfig
```

ip – Configures network interfaces and routing.

ip addr show      # Show IP addresses of network interfaces
ip route show     # Show routing table

curl – Fetches data from a URL.
```
curl https://example.com
```
wget – Downloads files from the web.
```
wget https://example.com/file.zip
```

Package Management

apt-get (for Debian/Ubuntu-based distributions) – Installs, updates, or removes software packages.

sudo apt-get update           # Update package list
sudo apt-get install package  # Install a package
sudo apt-get remove package   # Remove a package

yum (for RedHat/CentOS-based distributions) – Installs, updates, or removes software packages.

sudo yum update               # Update package list
sudo yum install package      # Install a package
sudo yum remove package       # Remove a package

File Compression

tar – Archives or extracts files.

tar -czvf archive_name.tar.gz /path/to/directory   # Create a compressed archive
tar -xzvf archive_name.tar.gz                      # Extract a compressed archive

zip – Compresses files into a zip archive.
```
zip archive_name.zip file1 file2
```
unzip – Extracts a zip archive.
```
unzip archive_name.zip
```

Miscellaneous

echo – Prints a message or variables to the terminal.
```
echo "Hello, World!"
```
date – Displays or sets the system date and time.
```
date
```

alias – Creates an alias for a command.

alias ll='ls -la'    # Create a shortcut for 'ls -la'

history – Shows the command history.
```
history
```
clear – Clears the terminal screen.
```
clear
```

These are just a few of the many powerful commands in Linux, but they cover most of the common operations you'll perform daily.

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