Variables in Scripts

Variables store data that your scripts can use and manipulate. Understanding variables is essential for writing useful scripts.

Defining Variables

NAME="John"
AGE=25
FILE_PATH="/home/user/data.txt"

Rules:

• No spaces around =
• No $ when setting
• Use $ when accessing

Using Variables

echo $NAME
echo "Hello, $NAME"
echo "Path: ${FILE_PATH}"

Quoting Variables

Always quote variables to handle spaces:

FILE="my file.txt"
cat "$FILE"          # Correct
cat $FILE            # Wrong - tries to cat "my" and "file.txt"

Special Variables

Variable	Meaning
$0	Script name
$1, $2...	Arguments
$#	Number of arguments
$@	All arguments (as separate words)
$*	All arguments (as single word)
$?	Exit status of last command
$$	Current process ID
$!	PID of last background process

Command Line Arguments

#!/bin/bash
# Save as greet.sh

echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "All arguments: $@"
echo "Number of args: $#"

Run it:

./greet.sh Hello World
# Script name: ./greet.sh
# First argument: Hello
# Second argument: World

Exercise: Use Variables

Create and use a variable in the terminal:

Command Substitution

Capture command output in a variable:

# Modern syntax
DATE=$(date)
FILES=$(ls)

# Older syntax
DATE=`date`

Arithmetic

# Using $(( ))
SUM=$((5 + 3))
PRODUCT=$((4 * 7))

# Using let
let "COUNT = COUNT + 1"
let "COUNT++"

# Using expr (older)
RESULT=$(expr 5 + 3)

String Operations

NAME="Linux"

# Length
echo ${#NAME}          # 5

# Substring
echo ${NAME:0:3}       # Lin

# Replace
FILE="data.txt"
echo ${FILE/txt/csv}   # data.csv

Default Values

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

# Set default if unset
echo ${NAME:="Default"}

# Error if unset
echo ${NAME:?"Name required"}

Arrays

# Define array
FRUITS=("apple" "banana" "cherry")

# Access elements
echo ${FRUITS[0]}      # apple
echo ${FRUITS[@]}      # all elements
echo ${#FRUITS[@]}     # count: 3

# Add element
FRUITS+=("date")

Read User Input

#!/bin/bash
echo "Enter your name:"
read NAME
echo "Hello, $NAME!"

# With prompt
read -p "Enter age: " AGE
echo "You are $AGE years old"

# Silent input (for passwords)
read -sp "Password: " PASS

Environment vs Local Variables

LOCAL_VAR="local"      # Only in current shell
export GLOBAL_VAR="global"  # Available to child processes

Key Takeaways

• Set variables with NAME=value (no spaces!)
• Access with $NAME or ${NAME}
• Always quote variables: "$VAR"
• Use $1, $2... for script arguments
• $(command) captures command output
• Use ${#VAR} for string length