Bash Variable Assignment

Overview

This lecture explains how to assign variables in Bash, covering variable scope (local vs global), assignment methods, and useful built-in commands like let and read.

Variable Scope: Local vs Global

• Local variables are defined inside functions or code blocks and only accessible there.
• Global variables are defined outside functions/blocks and accessible everywhere in the script.
• Local variables' values are lost once the function/block ends.
• Echoing a variable outside a function displays the global variable value.

Single Variable Assignment

• Assign variables using var_name=value with no spaces around =.
• Bash does not require specifying data type during assignment.

Integer Assignment

• Assign integers directly, e.g., num=1.

String Assignment

• Assign single words without quotes; multi-word strings use single (') or double (") quotes.
• Single quotes prevent variable expansion; double quotes allow expansion.

Boolean Assignment

• Assign booleans as true_val=true and false_val=false.
• Use these variables in conditionals to control script flow.

Multi-variable Assignment

• Assign multiple variables in one line, e.g., a=1 b=2 c=3.

Default Value Assignment

• Use ${var:-default} to assign a default if var is undefined.
• This prevents errors from uninitialized variables.

Using let Command for Assignments

• let is used for numeric assignments and arithmetic/bitwise/logical operations.
• Example: let "num2 = num1 + 2" performs addition.

Using read Command

• read gets user input and assigns it to variables.
• read animal color splits user input into two variables.
• Use -p for prompt text, -s for silent input (useful for passwords), -a to read into arrays.

Key Terms & Definitions

• Variable — Named storage location for values in a script.
• Local Variable — Variable scoped within a function/block.
• Global Variable — Variable accessible throughout a script.
• let command — Bash built-in for arithmetic and logical operations.
• read command — Bash built-in for reading user input.

Action Items / Next Steps

• Practice assigning and using variables with various data types in Bash.
• Experiment with let and read commands.
• Use default value assignment to avoid uninitialized variables.