Bash — Syntax

Variables

#!/bin/bash

# Assign variable (no spaces!)
name="Alice"
age=30

# Use variable
echo "Name: $name"
echo "Age: ${age}"

# Read input
read -p "Enter name: " username
echo "Hello, $username!"

Command substitution

#!/bin/bash

# Use $(command) to capture output
today=$(date +%Y-%m-%d)
files=$(ls | wc -l)

echo "Today: $today"
echo "Files: $files"

Arithmetic

#!/bin/bash

a=10
b=3

echo "Add: $((a + b))"
echo "Sub: $((a - b))"
echo "Mul: $((a * b))"
echo "Div: $((a / b))"
echo "Mod: $((a % b))"

Special variables

#!/bin/bash

echo "Script name: $0"
echo "First arg: $1"
echo "Second arg: $2"
echo "All args: $@"
echo "Number of args: $#"
echo "Exit status: $?"

String operations

#!/bin/bash

s="Hello, World!"

echo "Length: ${#s}"           # 13
echo "Substring: ${s:0:5}"    # Hello
echo "Replace: ${s/World/Bash}" # Hello, Bash!
echo "Uppercase: ${s^^}"      # HELLO, WORLD!
echo "Lowercase: ${s,,}"      # hello, world!

Mini Practice

Write Bash code that:

1. Declares and uses variables
2. Uses command substitution
3. Performs arithmetic operations
4. Manipulates strings

Up Next

In the next lesson, you'll learn about Comments — documenting your scripts.