Navigating the Filesystem

pwd Command

pwd

Displays the absolute path of the current working directory.

cd Command

cd [target_directory]

Changes the current shell session's working directory.

Usage:

• cd ..: Move up to the parent directory.
• cd /home/user/docs: Navigate using an absolute path (starts from root /).
• cd ../projects: Navigate using a relative path (based on current location).
• cd ~: Switch to the user's home directory.
• cd -: Return to the previous working directory.

Linux organizes files in an inverted tree structure. The root is denoted by /. Paths act as unique addresses; absolute paths define the full route from the root, while relative paths define the route from your current location.

Listing and Inspecting Files

ls Command

ls [options] [directory]

Lists directory contents. Files starting with . are hidden.

Key Options:

• -a: Show all files, including hidden ones.
• -l: Display detailed metadata (permissions, owner, size, date).
• -d: Show directory information rather than its contents.
• -t: Sort output by modification time.
• -R: Recursively list subdirectories.

Examples:

ls -la
ls -lt /var/log

tree Concept

Every directory (except root) has a parent, and directories can contain multiple children (files or other directories). This structure ensures unique pathing.

File and Directory Manipulation

touch Command

touch [options] filename

Updates the access and modification times of a file. If the file does not exist, it creates an empty file.

mkdir Command

mkdir [options] dirname

Creates new directories.

Key Options:

• -p: Create parent directories as needed. Useful for creating nested paths like mkdir -p project/src/utils.

rm and rmdir Commands

rm [options] target
rmdir [options] dirname

• rmdir: Removes empty directories only.
• rm: Removes files or directories.

Key Options:

• -r: Recursive removal (required for directories).
• -f: Force removal without prompting (bypasses write-protection).
• -i: Interactive mode (prompts before every removal).

Warning: rm -rf / is extremely dangerous and can destroy the operating system.

cp Command

cp [options] source destination

Copies files or directories.

Key Options:

• -r: Copy directories recursivley.
• -i: Prompt before overwriting existing files.
• -f: Force copy without prompts.

mv Command

mv [options] source destination

Moves files (cut/paste) or renames them.

Key Options:

• -i: Prompt before overwriting.
• -f: Do not prompt before overwriting.

Examples:

mv oldname.txt newname.txt  # Rename
mv report.pdf /tmp/         # Move

Viewing Content

cat and tac

cat [options] file

• cat: Prints file content sequentially to standard output.
• tac: Prints file content in reverse order.

Key Options:

• -n: Number all output lines.
• -b: Number non-blank lines only.
• -s: Squeeze multiple blank lines into one.

Output Redirection

The shell handles input (stdin) and output (stdout) as streams.

• >: Output Redirection. Writes output to a file, overwriting existing content.
  • echo "Hello" > file.txt
• >>: Append Redirection. Adds output to the end of a file.
  • echo "World" >> file.txt
• <: Input Redirection. Reads input from a file instead of the keyboard.
  • cat < file.txt
• > empty.txt: Creating an empty file or clearing an existing one.

less and more

less filename

View file content page by page. less is preferred over more because it allows backward navigation.

Controls:

• PageUp / PageDown: Navigate pages.
• /text: Search forward for "text".
• ?text: Search backward for "text".
• n / N: Next/Previous search match.
• q: Quit.

head and tail

head -n 5 filename
tail -n 10 filename

• head: Outputs the first 10 lines (default) of a file.
• tail: Outputs the last 10 lines (default) of a file.
• tail -f logfile: Follows the file output in real-time (useful for logs).

Searching and Filtering

find Command

find [path] [options] [expression]

Searches for files in a directory hierarchy.

Example:

find /home -name "*.conf"

Locates all files ending in .conf starting from /home.

grep Command

grep [options] "pattern" file

Filters text by searching for specific patterns (regex supported).

Key Options:

• -i: Ignore case distinctions.
• -n: Print line numbers with output.
• -v: Invert match (show lines that do NOT match).

Example:

ps aux | grep nginx

Archives and Compression

zip / unzip

zip -r archive.zip folder/
unzip archive.zip -d /target/path

Compresses or extracts .zip files. The -r flag is required to include subdirectories.

tar Command

tar [options] archive_file [files]

Handles tape archives (.tar), often combined with gzip compression.

Templates:

• Compress: tar czvf backup.tar.gz /path/to/backup
  • c: Create
  • z: gzip filter
  • v: Verbose (show progress)
  • f: Filename
• Extract: tar xzvf backup.tar.gz -C /destination
  • x: Extract
  • C: Change to directory

System Information and Utilities

date Command

date +"%Y-%m-%d %H:%M:%S"

Displays or sets the system time.

• Timestamp: date +%s (seconds since Jan 1, 1970).
• Convert Timestamp: date -d @<timestamp>

cal Command

cal [options]

Displays a calendar.

• -3: Show previous, current, and next month.
• -y: Show the whole year.

uname Command

uname -a

Prints system information (Kernel name, hostname, version, architecture).

bc Command

echo "scale=2; 10/3" | bc

A command-line calculator for precision arithmetic.

Documentation and Help

man Command

man [section] command

Accesses the system's manual pages.

• Sections: 1 (User commands), 2 (System calls), 3 (Library functions).
• Search: man -k keyword

nano Editor

A simple terminal-based text editor.

• Ctrl + X: Exit (prompts to save if modified).
• Ctrl + O: Save (Write Out).

Essential Keyboard Shortcuts

• Tab: Auto-completes commands or file paths. Press twice for suggestions.
• Ctrl + C: Interrupt/Send SIGINT to stop the current running process.
• Ctrl + D: Signal EOF (End of File) or logout from the shell.
• Ctrl + R: Search through command history.

Shutdown

shutdown [options] time

• shutdown -h now: Halt the system immeidately.
• shutdown -r +5: Reboot the system in 5 minutes.

Extended Command Reference

Login/Logout: login, exit, last, shutdown, reboot File Ops: file, ln, dd, diff, chmod, chown System: top, df, free, ps, kill, crontab Network: ifconfig, ip, ping, netstat, ssh, scp