Shell Scripting (Bash) Master Class #

Engineering-Grade Reference Manual for Bash Scripting
A comprehensive guide to shell scripting automation and best practices

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Table of Contents #

1. Bash Fundamentals
2. Variables & Data Types
3. Control Structures
4. Functions
5. Arrays
6. String Manipulation
7. File Operations
8. Process Management
9. Error Handling
10. Best Practices

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Bash Fundamentals #

🔹 Shebang and Script Basics #

#!/bin/bash
# This is a comment

# Make script executable
chmod +x script.sh

# Run script
./script.sh

# Run with bash explicitly
bash script.sh

🔹 Variables #

# Variable assignment (no spaces around =)
NAME="John"
AGE=30

# Using variables
echo "Name: $NAME"
echo "Age: ${AGE}"

# Command substitution
CURRENT_DATE=$(date +%Y-%m-%d)
FILES=$(ls -1)

# Read user input
read -p "Enter your name: " USERNAME
echo "Hello, $USERNAME"

# Environment variables
export PATH="/usr/local/bin:$PATH"
echo $HOME
echo $USER

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Variables & Data Types #

🔹 Special Variables #

$0  # Script name
$1, $2, $3  # Positional parameters
$#  # Number of arguments
$@  # All arguments as separate words
$*  # All arguments as single word
$?  # Exit status of last command
$$  # Process ID of script
$!  # Process ID of last background command

🔹 Variable Operations #

# Default values
echo "${VAR:-default}"  # Use default if VAR is unset
echo "${VAR:=default}"  # Set and use default if VAR is unset
echo "${VAR:?error}"    # Error if VAR is unset

# String length
NAME="John"
echo "${#NAME}"  # 4

# Arithmetic
NUM=10
((NUM++))
echo $((NUM + 5))

# Let command
let "result = 5 + 3"

---

Control Structures #

🔹 If Statements #

# Basic if
if [ "$NAME" = "John" ]; then
    echo "Hello John"
fi

# If-else
if [ $AGE -gt 18 ]; then
    echo "Adult"
else
    echo "Minor"
fi

# If-elif-else
if [ $SCORE -ge 90 ]; then
    echo "A"
elif [ $SCORE -ge 80 ]; then
    echo "B"
else
    echo "C"
fi

# Multiple conditions
if [ $AGE -gt 18 ] && [ "$NAME" = "John" ]; then
    echo "Adult named John"
fi

# File tests
if [ -f "file.txt" ]; then
    echo "File exists"
fi

if [ -d "directory" ]; then
    echo "Directory exists"
fi

if [ -r "file.txt" ]; then
    echo "File is readable"
fi

🔹 Case Statements #

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

🔹 Loops #

# For loop
for i in 1 2 3 4 5; do
    echo "Number: $i"
done

# For loop with range
for i in {1..10}; do
    echo $i
done

# For loop with files
for file in *.txt; do
    echo "Processing $file"
done

# While loop
COUNT=0
while [ $COUNT -lt 5 ]; do
    echo "Count: $COUNT"
    ((COUNT++))
done

# Until loop
COUNT=0
until [ $COUNT -ge 5 ]; do
    echo "Count: $COUNT"
    ((COUNT++))
done

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

---

Functions #

🔹 Function Basics #

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

# Call function
greet "John"

# Function with return value
function add() {
    local result=$(($1 + $2))
    echo $result
}

SUM=$(add 5 3)
echo "Sum: $SUM"

# Function with return code
function check_file() {
    if [ -f "$1" ]; then
        return 0
    else
        return 1
    fi
}

if check_file "test.txt"; then
    echo "File exists"
fi

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Arrays #

🔹 Array Operations #

# Declare array
FRUITS=("apple" "banana" "orange")

# Access elements
echo "${FRUITS[0]}"  # apple
echo "${FRUITS[@]}"  # all elements
echo "${#FRUITS[@]}" # array length

# Add element
FRUITS+=("grape")

# Loop through array
for fruit in "${FRUITS[@]}"; do
    echo "Fruit: $fruit"
done

# Associative arrays (Bash 4+)
declare -A COLORS
COLORS[red]="#FF0000"
COLORS[green]="#00FF00"

echo "${COLORS[red]}"

# Loop through associative array
for key in "${!COLORS[@]}"; do
    echo "$key: ${COLORS[$key]}"
done

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String Manipulation #

🔹 String Operations #

STRING="Hello World"

# Length
echo "${#STRING}"

# Substring
echo "${STRING:0:5}"  # Hello
echo "${STRING:6}"    # World

# Replace
echo "${STRING/World/Universe}"  # Replace first
echo "${STRING//o/0}"            # Replace all

# Uppercase/Lowercase
echo "${STRING^^}"  # HELLO WORLD
echo "${STRING,,}"  # hello world

# Remove prefix/suffix
FILE="document.txt"
echo "${FILE%.txt}"    # document
echo "${FILE#*.}"      # txt

---

File Operations #

🔹 File Handling #

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

# Write to file
echo "Hello" > file.txt   # Overwrite
echo "World" >> file.txt  # Append

# Check if file exists
if [ -f "file.txt" ]; then
    echo "File exists"
fi

# Create directory
mkdir -p /path/to/directory

# Copy files
cp source.txt destination.txt

# Move files
mv old.txt new.txt

# Delete files
rm file.txt
rm -rf directory/

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Process Management #

🔹 Process Control #

# Run in background
long_running_command &

# Get process ID
PID=$!

# Wait for process
wait $PID

# Kill process
kill $PID
kill -9 $PID  # Force kill

# Check if process is running
if ps -p $PID > /dev/null; then
    echo "Process is running"
fi

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Error Handling #

🔹 Exit Codes and Traps #

#!/bin/bash
set -e  # Exit on error
set -u  # Exit on undefined variable
set -o pipefail  # Exit on pipe failure

# Trap errors
trap 'echo "Error on line $LINENO"' ERR

# Cleanup on exit
cleanup() {
    echo "Cleaning up..."
    rm -f /tmp/tempfile
}
trap cleanup EXIT

# Check command success
if ! command -v docker &> /dev/null; then
    echo "Docker not found"
    exit 1
fi

# Custom error handling
function error_exit() {
    echo "Error: $1" >&2
    exit 1
}

[ -f "required.txt" ] || error_exit "required.txt not found"

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Best Practices #

✅ Script Template #

#!/bin/bash

# Script: backup.sh
# Description: Backup files to remote server
# Author: Your Name
# Date: 2024-01-01

set -euo pipefail  # Exit on error, undefined vars, pipe failures

# Constants
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly BACKUP_DIR="/backup"
readonly LOG_FILE="/var/log/backup.log"

# Functions
log() {
    echo "[$(date +'%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG_FILE"
}

error() {
    log "ERROR: $*" >&2
    exit 1
}

cleanup() {
    log "Cleaning up..."
    # Cleanup code here
}

main() {
    trap cleanup EXIT
    
    log "Starting backup..."
    
    # Main logic here
    
    log "Backup completed successfully"
}

# Run main function
main "$@"

✅ Best Practices #

1. Use shellcheck for linting
2. Quote variables to prevent word splitting
3. Use set -euo pipefail for safety
4. Add error handling and logging
5. Make scripts idempotent
6. Use functions for organization
7. Add usage/help messages
8. Use meaningful variable names

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🎓 Shell Scripting Master Class Complete