The Terminal — Going Deeper¶

This builds on 00_COMPUTER_LITERACY_PRIMER.md. Here we cover pipes, redirects, environment variables, and other terminal skills you need for real development.

Learn Your Way¶

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Visualize It¶

See how Python interacts with the operating system via os and sys: Open in Python Tutor

Pipes — connecting commands¶

The | (pipe) sends the output of one command into another:

# Count how many Python files are in the current directory.
ls *.py | wc -l

# Search for "error" in a log file and show only unique lines.
cat app.log | grep "error" | sort | uniq

# Find the 5 largest files.
du -sh * | sort -rh | head -5

Redirects — saving output to files¶

# > writes output to a file (overwrites).
python script.py > output.txt

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

# 2> redirects error messages.
python script.py 2> errors.txt

# &> redirects both output and errors.
python script.py &> all_output.txt

Environment variables¶

Environment variables are values that any program can read. They configure behavior without changing code.

# Set a variable (current session only).
export DATABASE_URL="sqlite:///my.db"
export API_KEY="abc123"

# Read a variable.
echo $DATABASE_URL

# Use in Python.
python -c "import os; print(os.environ.get('DATABASE_URL'))"

.env files¶

For development, store variables in a .env file:

# .env
DATABASE_URL=sqlite:///my.db
API_KEY=abc123
DEBUG=true

Load with python-dotenv:

from dotenv import load_dotenv
load_dotenv()    # Reads .env into environment variables

Never commit .env to git. Add it to .gitignore.

Useful terminal commands¶

Command	What it does	Example
`pwd`	Print current directory	`pwd`
`ls -la`	List all files with details	`ls -la`
`mkdir -p`	Create directory (and parents)	`mkdir -p src/utils`
`rm -r`	Delete directory and contents	`rm -r old_folder`
`cp -r`	Copy directory	`cp -r src/ backup/`
`mv`	Move or rename	`mv old.py new.py`
`which`	Find where a command lives	`which python`
`history`	Show command history	`history \| grep pip`
`cat`	Show file contents	`cat requirements.txt`
`less`	Page through a file	`less long_file.txt`
`grep`	Search in files	`grep "def " *.py`
`wc -l`	Count lines	`wc -l data.csv`

Process management¶

# Run a command in the background.
python server.py &

# See running processes.
ps aux | grep python

# Kill a process by PID.
kill 12345

# Kill by name.
pkill -f "python server.py"

Keyboard shortcuts¶

Shortcut	Action
`Ctrl+C`	Stop the current command
`Ctrl+D`	Exit the shell / end input
`Ctrl+L`	Clear the screen
`Ctrl+R`	Search command history
`Tab`	Auto-complete file/command names
`↑` / `↓`	Navigate command history

Chaining commands¶

# && — run next command only if previous succeeded.
python -m pytest && echo "All tests passed!"

# || — run next command only if previous failed.
python -m pytest || echo "Tests failed!"

# ; — run next command regardless.
echo "Starting..." ; python script.py ; echo "Done."

Practice¶

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