Variables, entrada y salida

In shell scripting, variables are like containers for storing data. They help make your scripts flexible and reusable.

Instead of hardcoding values, you can use variables to hold information such as filenames, user inputs, or temporary results.

To create a variable, you simply assign a value to a name. No special keyword is needed!

• Use NAME="Coddy" (no spaces around =).
• Values with spaces usually need quotes (single or double).
• Numbers don't strictly need quotes, but it's often good practice for consistency.

To use the value stored in a variable, you prepend its name with a $ sign.

For example, if you declared GREETING="Hello", you'd use $GREETING to get "Hello". You can also use ${VARIABLE}, which is clearer in complex situations.

Follow these simple rules for variable names:

• Start with a letter or underscore.
• Can contain letters, numbers, and underscores.
• Avoid using special shell keywords.
• Common practice: use uppercase for global/environment variables, lowercase for script-local ones.

Let's see a simple script that declares and uses a variable.

Notice how $MESSAGE retrieves the stored text.

The read command is essential for making your scripts interactive. It waits for the user to type something and press Enter.

• read VARIABLE: Stores input into VARIABLE.
• read -p "Prompt: " VARIABLE: Displays a prompt before waiting for input.

This script uses read -p to ask for your name and then greets you personally.

Try running it and entering your name!

You've seen echo for basic output. It simply prints its arguments followed by a newline.

For more control, printf (like in C) allows formatted output, specifying data types and alignment. It doesn't add a newline automatically, so use \n.

This example combines variables, user input with read, and formatted output using printf.

It calculates a simple sum based on your input.

Consider the following shell script snippet:

#!/bin/bash
NAME="Alice"
read -p "Enter a city: " CITY
echo "Hello, $NAME from $CITY!"

If the user types London and presses Enter, what will be the final output?

Great job! You've learned the fundamentals of shell scripting:

• Variables store data using VAR="value".
• Access variables with $VAR or ${VAR}.
• read gets user input, often with -p for a prompt.
• echo prints simple messages.
• printf offers formatted output.

These are building blocks for dynamic and interactive scripts!