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Command Line

The command line (also called the terminal, shell, or CLI — Command Line Interface) is a text-based way to interact with your computer. Instead of clicking icons and menus, you type commands.

It can feel intimidating at first, but learning the basics will make you a much faster and more capable developer. Many tools in software development — Git, package managers, build systems — are primarily used through the command line.

What Is a Shell?​

A shell is the program that reads your commands and runs them. Common shells include:

  • bash — the default on most Linux systems and older macOS
  • zsh — the default on modern macOS (since Catalina)
  • fish — a user-friendly shell with helpful features
  • PowerShell / cmd — Windows shells (though WSL gives you bash on Windows)

On Windows, you can use the Windows Subsystem for Linux (WSL) to get a full bash/zsh environment. This is highly recommended for development.

Opening a Terminal​

  • macOS — Search "Terminal" in Spotlight, or use iTerm2
  • Linux — Usually Ctrl+Alt+T, or find "Terminal" in your apps
  • Windows — Windows Terminal, Git Bash, or WSL
  • VS Code — Built-in terminal with Ctrl+`

Your terminal always has a working directory — the folder you're currently "in." Commands operate relative to this location.

# Print Working Directory — shows where you are
pwd

# List files in the current directory
ls

# List with details (permissions, size, date)
ls -l

# List including hidden files (files starting with .)
ls -a

# Change Directory
cd Documents

# Go up one level (parent directory)
cd ..

# Go to your home directory
cd ~

# Go to the previous directory
cd -

Understanding Paths​

  • Absolute path — starts from the root, always works: /home/username/Documents/project
  • Relative path — relative to where you are now: Documents/project or ../other-project

Working with Files and Directories​

# Create a new directory
mkdir my-project

# Create nested directories
mkdir -p projects/research/data

# Create an empty file
touch notes.txt

# Copy a file
cp notes.txt notes-backup.txt

# Copy a directory and its contents
cp -r my-project my-project-backup

# Move or rename a file
mv notes.txt renamed-notes.txt

# Move a file to a different directory
mv notes.txt Documents/

# Remove a file
rm old-file.txt

# Remove a directory and everything inside it
rm -rf old-directory/
warning

rm -rf permanently deletes files — there's no Trash or Recycle Bin. Double-check what you're deleting before running it.

Reading Files​

# Print the entire contents of a file
cat file.txt

# View a file one screen at a time (press q to quit)
less file.txt

# Print the first 10 lines
head file.txt

# Print the last 10 lines
tail file.txt

# Print the last 10 lines and keep watching for new content
tail -f logfile.txt

Searching​

# Search for a pattern in a file
grep "error" logfile.txt

# Search recursively in all files in a directory
grep -r "TODO" ./src/

# Case-insensitive search
grep -i "error" logfile.txt

# Find files by name
find . -name "*.py"

# Find files modified in the last 7 days
find . -mtime -7

Piping and Redirection​

One of the most powerful features of the command line is piping — connecting the output of one command to the input of another.

# Pipe: send output of one command to another
# Count the number of Python files in the current directory
ls *.py | wc -l

# Find all TODOs in Python files
grep -r "TODO" . | grep ".py"

# View a long output one screen at a time
git log | less

# Redirect output to a file (overwrites)
ls > file-list.txt

# Append output to a file
echo "new line" >> notes.txt

# Discard output (send to /dev/null)
some-noisy-command > /dev/null

Environment Variables​

Environment variables are key-value pairs that configure your shell and the programs running in it.

# Print a variable
echo $HOME
echo $PATH

# Set a variable for the current session
export MY_VAR="hello"

# Use it
echo $MY_VAR

The $PATH variable is a colon-separated list of directories where the shell looks for programs. When you type python, it searches each directory in $PATH in order.

Making Variables Permanent​

Add export MY_VAR="value" to your shell's config file:

  • bash: ~/.bashrc or ~/.bash_profile
  • zsh: ~/.zshrc

Then run source ~/.bashrc (or restart your terminal) to apply changes.

Useful Shortcuts​

ShortcutAction
TabAutocomplete file/command name
Tab TabShow all completions
↑ / ↓Scroll through command history
Ctrl+CStop the running command
Ctrl+DExit the current shell
Ctrl+LClear the screen
Ctrl+AJump to start of line
Ctrl+EJump to end of line
Ctrl+RSearch command history
tip

Tab autocomplete is one of the most useful features. Start typing a filename or command and press Tab — the shell will complete it or show options. Use it constantly.

Running Scripts​

# Run a Python script
python script.py

# Run a shell script
bash myscript.sh

# Make a script executable and run it directly
chmod +x myscript.sh
./myscript.sh

Getting Help​

# Manual page for a command (press q to exit)
man ls

# Brief help flag (works for most commands)
ls --help
git --help
python --help

Command History​

# View your command history
history

# Re-run a specific command by number
!42

# Re-run the last command
!!

Working with Processes​

# List running processes
ps aux

# Search for a process by name
ps aux | grep python

# Kill a process by ID
kill 12345

# Kill a process by name
pkill python

# Run a command in the background
python long-script.py &

# List background jobs
jobs

References​