Variable Basics

Variables are containers for storing data that your scripts can use and manipulate. Understanding variables is fundamental to writing useful bash scripts.

What is a Variable?

A variable is a named storage location that holds a value. In bash:

NAME="Alice"     # Create variable
echo $NAME       # Access variable: prints "Alice"

Creating Variables

The syntax for creating variables is simple but strict:

# Correct - no spaces around =
VARIABLE="value"

# WRONG - spaces cause errors
VARIABLE = "value"    # Error!
VARIABLE= "value"     # Error!
VARIABLE ="value"     # Error!

Variable Naming Rules

Follow these naming conventions:

Rule	Valid	Invalid
Start with letter or underscore	NAME, _count	1name, -var
Use letters, numbers, underscores	file_name2	file-name, my.var
Case sensitive	Name ≠ NAME	-
Convention: UPPERCASE for constants	MAX_SIZE	-
Convention: lowercase for local vars	filename	-

# Good variable names
username="alice"
FILE_PATH="/home/user"
_counter=0
item2="second"

# Bad variable names (will cause errors)
# 2fast="value"      # Can't start with number
# my-var="value"     # Hyphens not allowed
# my.var="value"     # Dots not allowed

Accessing Variables

Use $ to access a variable's value:

NAME="Bob"
echo $NAME         # Bob
echo "$NAME"       # Bob (preferred)
echo "${NAME}"     # Bob (explicit)
echo '$NAME'       # $NAME (literal - single quotes)

Braces Syntax

Use ${VAR} when the variable name could be ambiguous:

FILE="report"
echo "$FILE_2024"      # Looks for variable FILE_2024
echo "${FILE}_2024"    # Correct: report_2024

Exercise: Create and Use Variables

Create a variable and display its value:

Reassigning Variables

Variables can be changed at any time:

COUNT=1
echo $COUNT    # 1

COUNT=2
echo $COUNT    # 2

COUNT=$((COUNT + 1))
echo $COUNT    # 3

Unsetting Variables

Remove a variable with unset:

NAME="Alice"
echo $NAME      # Alice

unset NAME
echo $NAME      # (empty)

Read-Only Variables

Make a variable constant with readonly:

readonly PI=3.14159
PI=3.14         # Error: PI: readonly variable

Or use declare -r:

declare -r MAX_USERS=100
MAX_USERS=200   # Error: MAX_USERS: readonly variable

Command Substitution

Capture command output in a variable:

# Modern syntax (preferred)
CURRENT_DATE=$(date)
FILE_COUNT=$(ls | wc -l)

# Older syntax (backticks)
CURRENT_DATE=`date`

Exercise: Command Substitution

Store the output of a command in a variable:

Arithmetic with Variables

Bash supports integer arithmetic:

# Using $(( ))
A=5
B=3
SUM=$((A + B))        # 8
DIFF=$((A - B))       # 2
PRODUCT=$((A * B))    # 15
QUOTIENT=$((A / B))   # 1 (integer division)
REMAINDER=$((A % B))  # 2

echo "Sum: $SUM"

Increment and Decrement

COUNT=0
((COUNT++))     # Increment: COUNT is now 1
((COUNT--))     # Decrement: COUNT is now 0
((COUNT+=5))    # Add 5: COUNT is now 5

Default Values

Provide default values for unset variables:

# Use default if unset or empty
echo ${NAME:-"Unknown"}    # "Unknown" if NAME is unset

# Set default if unset or empty
echo ${NAME:="Default"}    # Sets NAME to "Default" if unset

# Use alternative if set
echo ${NAME:+"Has value"}  # "Has value" if NAME is set

# Error if unset
echo ${NAME:?"Error: NAME required"}  # Exits with error if unset

Checking if Variable is Set

# Check if variable exists
if [ -z "$NAME" ]; then
    echo "NAME is empty or unset"
fi

if [ -n "$NAME" ]; then
    echo "NAME has a value: $NAME"
fi

Key Takeaways

• Create variables with VAR="value" (no spaces around =)
• Access variables with $VAR or ${VAR}
• Variable names are case-sensitive
• Use $() to capture command output
• Use $(( )) for arithmetic operations
• Use ${VAR:-default} for default values
• Variables are untyped - everything is essentially a string