Bash Fundamentals

Master the core concepts of Bash scripting including variables, control structures, functions, and data types.

Getting Started

Hello World

#!/bin/bash
echo "Hello, World!"

Save as hello.sh, then:

chmod +x hello.sh
./hello.sh

Variables

Bash uses dynamic typing. Assign values without declaring types:

name="Alice"
age=30
path="/home/user"

Retrieve variables with $:

echo $name
echo ${name}      # Explicit braces (safer)
echo "${name}"    # With quotes (prevents word splitting)

Variable Naming Rules:

• Start with letter or underscore
• Contain letters, numbers, underscores
• Case-sensitive (Name ≠ name)

Quoting

Quoting prevents word splitting, globbing, and variable expansion in different ways:

Double Quotes — Allow variable and command expansion:

greeting="Hello, $name"
echo "$greeting"  # Expands $name

Single Quotes — Literal strings (no expansion):

path='$HOME/docs'
echo "$path"      # Outputs: $HOME/docs

Escape Character — Backslash escapes special characters:

echo "He said \"Hello\""   # Outputs: He said "Hello"
echo "Price: \$100"        # Outputs: Price: $100

No Quotes — Subject to word splitting and globbing:

files=*.txt       # Expands to all .txt files

Command Substitution

Capture command output into variables:

current_user=$(whoami)
host=$(hostname)
current_date=`date +%Y-%m-%d`   # Backticks (older syntax)

Data Types & Operations

Arrays

Indexed Arrays:

colors=("red" "green" "blue")
colors[3]="yellow"
echo ${colors[0]}          # red
echo ${colors[@]}          # All elements
echo ${#colors[@]}         # Array length

Looping Through Arrays:

for color in "${colors[@]}"; do
  echo "Color: $color"
done

Unset Array Elements:

unset colors[1]           # Remove green
unset colors              # Remove entire array

Associative Arrays

Key-value pairs (Bash 4+):

declare -A person
person[name]="Alice"
person[age]="30"
person[city]="New York"

echo ${person[name]}       # Alice
echo ${!person[@]}         # Keys: name age city
echo ${person[@]}          # Values: Alice 30 New York

String Manipulation

Length:

str="hello"
echo ${#str}               # 5

Substring:

str="Hello, World!"
echo ${str:0:5}            # Hello (offset 0, length 5)
echo ${str:7}              # World! (from position 7)

Parameter Expansion (Remove/Replace):

filename="document.txt"
echo ${filename%.txt}      # document (remove suffix)
echo ${filename#*o}        # document.txt (remove prefix up to 'o')

text="foo bar foo"
echo ${text/foo/FOO}       # FOO bar foo (replace first)
echo ${text//foo/FOO}      # FOO bar FOO (replace all)

Arithmetic

Using $(( )):

result=$((5 + 3))
echo $result               # 8

x=10
y=3
echo $((x * y))            # 30
echo $((x / y))            # 3
echo $((x % y))            # 1
echo $((x ** 2))           # 100 (exponentiation)

Increment/Decrement:

count=0
((count++))
((count+=5))
echo $count                # 6

Using let:

let result=5+3
let x+=10

Control Structures

If/Else

if [ "$age" -gt 18 ]; then
  echo "Adult"
elif [ "$age" -gt 12 ]; then
  echo "Teenager"
else
  echo "Child"
fi

Test Operators:

Operator	Meaning
-eq	Equal
-ne	Not equal
-lt	Less than
-le	Less than or equal
-gt	Greater than
-ge	Greater than or equal
-z	String is empty
-n	String is not empty
=	Strings equal
!=	Strings not equal
-f	File exists and is regular file
-d	Directory exists
-e	File/dir exists
-r	File is readable
-w	File is writable
-x	File is executable

Logical Operators:

if [ "$age" -gt 18 ] && [ "$age" -lt 65 ]; then
  echo "Working age"
fi

if [ "$role" = "admin" ] || [ "$role" = "root" ]; then
  echo "Has privileges"
fi

if [ ! -f "$file" ]; then
  echo "File does not exist"
fi

For Loop

Iterate over list:

for item in apple banana cherry; do
  echo "Fruit: $item"
done

Iterate over range:

for i in {1..5}; do
  echo "Number: $i"
done

C-style for loop:

for ((i=0; i<5; i++)); do
  echo "Count: $i"
done

Iterate over array:

files=("file1.txt" "file2.txt" "file3.txt")
for file in "${files[@]}"; do
  echo "Processing: $file"
done

While Loop

counter=0
while [ $counter -lt 5 ]; do
  echo "Counter: $counter"
  ((counter++))
done

Read input line-by-line:

while IFS= read -r line; do
  echo "Line: $line"
done < file.txt

Case Statement

case "$1" in
  start)
    echo "Starting service..."
    ;;
  stop)
    echo "Stopping service..."
    ;;
  restart)
    echo "Restarting service..."
    ;;
  *)
    echo "Usage: $0 {start|stop|restart}"
    exit 1
    ;;
esac

Functions

Basic Function

greet() {
  echo "Hello, $1!"
}

greet "Alice"              # Hello, Alice!

Function with Local Variables

calculate() {
  local x=$1
  local y=$2
  local result=$((x + y))
  echo $result
}

sum=$(calculate 10 20)
echo "Sum: $sum"           # Sum: 30

Function Return Values

is_positive() {
  if [ $1 -gt 0 ]; then
    return 0              # Success
  else
    return 1              # Failure
  fi
}

if is_positive 5; then
  echo "5 is positive"
fi

Capture output:

get_user_info() {
  echo "Alice:30:NYC"
}

info=$(get_user_info)
echo "$info"              # Alice:30:NYC

Exit Codes & Error Handling

Exit Codes

Every command returns an exit code (0 = success, non-zero = failure):

ls /nonexistent 2>/dev/null
echo $?                   # Non-zero (command failed)

Check exit code immediately:

if command; then
  echo "Success"
else
  echo "Failed"
fi

Error Prevention

Set error flags:

#!/bin/bash
set -e    # Exit on any error
set -u    # Exit if undefined variable used
set -o pipefail  # Return error if pipe command fails

Handle errors:

command || {
  echo "Error occurred"
  exit 1
}

Built-in Commands

Command	Purpose
echo	Print text
printf	Formatted output
read	Read user input
test / [	Evaluate condition
pwd	Print working directory
cd	Change directory
export	Set environment variable
source / .	Execute commands from file
type	Show command type
command	Execute command

Exercises

Exercise 1: Temperature Converter

Create a script that converts Celsius to Fahrenheit:

#!/bin/bash

if [ $# -ne 1 ]; then
  echo "Usage: $0 <celsius>"
  exit 1
fi

celsius=$1
fahrenheit=$(echo "scale=2; $celsius * 9/5 + 32" | bc)
echo "$celsius°C = $fahrenheit°F"

Exercise 2: File Info Script

Create a script that displays file information:

#!/bin/bash

if [ ! -f "$1" ]; then
  echo "File not found: $1"
  exit 1
fi

echo "File: $1"
echo "Size: $(wc -c < "$1") bytes"
echo "Lines: $(wc -l < "$1")"
echo "Modified: $(stat -c %y "$1")"

Exercise 3: Array Processing

Create a script that processes an array:

#!/bin/bash

numbers=(5 12 3 8 15 2)
sum=0
count=0

for num in "${numbers[@]}"; do
  sum=$((sum + num))
  ((count++))
done

average=$((sum / count))
echo "Sum: $sum"
echo "Count: $count"
echo "Average: $average"

Exercise 4: Simple Menu

Create an interactive menu system:

#!/bin/bash

while true; do
  echo "=== Menu ==="
  echo "1) List files"
  echo "2) Show current directory"
  echo "3) Exit"
  read -p "Choose option: " choice

  case $choice in
    1) ls -l ;;
    2) pwd ;;
    3) exit 0 ;;
    *) echo "Invalid choice" ;;
  esac
done

Key Takeaways

• Variables are dynamically typed; use $ or ${} to expand
• Quotes control expansion: "" (expand) vs '' (literal)
• Arrays are 0-indexed; associative arrays use keys
• Control structures (if/for/while/case) power script logic
• Functions promote reusability and code organization
• Exit codes (0 = success) drive error handling
• Use set -e and set -u to catch common errors early

Next Steps

Explore Advanced Bash topics like process substitution, here documents, signal handling, and text processing with sed/awk.