Bash Scripting: A Complete Guide

#!/usr/bin/env bash
#
# Script: backup.sh
# Description: Creates timestamped backups with rotation
# Author: Your Name
# Usage: ./backup.sh <source_dir> [destination_dir]

set -euo pipefail

# --- Configuration ---
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly TIMESTAMP="$(date +%Y%m%d_%H%M%S)"

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

# --- Main ---
main() {
    log "Starting backup..."
    # actual logic here
    log "Backup complete"
---

main "$@"

set -euo pipefail

# Assignment (no spaces around =)
name="Alice"
count=5
result=$(ls -la)       # command substitution
today=$(date +%Y-%m-%d)

# Usage
echo "$name"            # always quote variables!
echo "Hello, ${name}!"  # braces for clarity/expansion

# Default values
echo "${name:-default}"   # use default if unset or empty
echo "${name:=default}"   # assign default if unset
echo "${name:?error msg}" # error if unset
echo "${name:+alt}"       # use alt if set, else nothing

# Readonly and local
readonly API_KEY="abc123"     # cannot be changed
local temp_var="temporary"   # function-local scope (inside functions)

# Arrays
files=("file1.txt" "file2.txt" "file3.txt")
echo "${files[0]}"            # 'file1.txt'
echo "${files[@]}"            # all elements
echo "${#files[@]}"           # array length

# Associative arrays (Bash 4+)
declare -A config
config[host]="localhost"
config[port]=8080
echo "${config[host]}"

string="hello-world.txt"

# Length
echo "${#string}"        # 16

# Substring
echo "${string:0:5}"     # 'hello'
echo "${string:6}"       # 'world.txt'

# Pattern removal (prefix)
echo "${string#*.}"      # 'txt' — remove shortest prefix matching *.  
echo "${string##*.}"     # 'txt' — remove longest prefix

# Pattern removal (suffix)
echo "${string%.*}"      # 'hello-world' — remove shortest suffix
echo "${string%%.*}"     # 'hello' — remove longest suffix

# Replace
echo "${string/hello/hi}"     # 'hi-world.txt' (first match)
echo "${string//o/_}"         # 'hell_-w_rld.txt' (all matches)
echo "${string/#hello/hi}"    # replace at start
echo "${string/%txt/csv}"     # replace at end

# Case modification (Bash 4+)
echo "${string^^}"            # uppercase: 'HELLO-WORLD.TXT'
echo "${string,,}"            # lowercase: 'hello-world.txt'
echo "${string^}"             # capitalize first character

# if/then/elif/else/fi
if [[ -f "$file" ]]; then
    echo "File exists"
elif [[ -d "$file" ]]; then
    echo "Directory exists"
else
    echo "Not found"
fi

# File tests
[[ -f "$file" ]]   # regular file exists
[[ -d "$dir" ]]    # directory exists
[[ -e "$path" ]]   # any file system entry exists
[[ -x "$bin" ]]    # is executable
[[ -z "$var" ]]    # string is empty
[[ -n "$var" ]]    # string is not empty

# String comparison
[[ "$a" == "$b" ]]   # equal (use == not =)
[[ "$a" != "$b" ]]   # not equal
[[ "$a" < "$b" ]]    # lexicographic less than
[[ -z "$var" ]]      # empty

# Numeric comparison
(( count > 5 ))      # arithmetic context (preferred)
[[ $count -gt 5 ]]   # old test style

# Pattern matching
[[ "$filename" == *.txt ]]    # wildcard match
[[ "$email" =~ ^[a-z]+@ ]]   # regex match

# Logical operators
[[ -n "$var" && -f "$file" ]]  # and
[[ -n "$var" || -f "$file" ]]  # or
[[ ! -f "$file" ]]             # not

# For loop over items
for item in one two three; do
    echo "$item"
done

# For loop with range (Bash 3+)
for i in {1..5}; do
    echo "$i"
done

# For loop with C-style syntax
for ((i = 0; i < 5; i++)); do
    echo "$i"
done

# While loop
while read -r line; do
    echo "Line: $line"
done < input.txt

# While with counter
count=0
while (( count < 5 )); do
    echo "$count"
    ((count++))
done

# Loop over command output
for file in *.txt; do
    [[ -f "$file" ]] || continue  # skip if no matches
    echo "Processing: $file"
done

# Loop over array
for item in "${files[@]}"; do
    echo "$item"
done

# Define function
greet() {
    local name="$1"            # first argument
    local greeting="${2:-Hello}" # second arg with default

    if [[ -z "$name" ]]; then
        echo "Usage: greet <name> [greeting]" >&2
        return 1
    fi

    echo "${greeting}, ${name}!"
---

# Call function
greet "Alice"               # "Hello, Alice!"
greet "Bob" "Hi"            # "Hi, Bob!"
greet                       # prints usage and returns 1

# Return values
get_os() {
    case "$(uname)" in
        Linux)  echo "linux" ;;
        Darwin) echo "macos" ;;
        *)      echo "unknown" ;;
    esac
---

os=$(get_os)
echo "Running on $os"

# Functions returning numeric status (0-255)
is_valid_port() {
    (( $1 > 0 && $1 <= 65535 ))
---

if is_valid_port 8080; then
    echo "Port is valid"
fi

#!/usr/bin/env bash

usage() {
    echo "Usage: $0 [options] <input>"
    echo "  -o, --output FILE   Output file"
    echo "  -v, --verbose       Verbose output"
    echo "  -h, --help          Show this help"
    exit 1
---

# Parse with getopt
args=$(getopt -o o:vh --long output:,verbose,help -n "$0" -- "$@")
if [[ $? -ne 0 ]]; then usage; fi
eval set -- "$args"

verbose=false
output=""

while true; do
    case "$1" in
        -o|--output)
            output="$2"
            shift 2
            ;;
        -v|--verbose)
            verbose=true
            shift
            ;;
        -h|--help)
            usage
            ;;
        --)
            shift
            break
            ;;
    esac
done

input="${1:?Missing input file}"  # required positional arg

echo "Input: $input"
echo "Output: ${output:-stdout}"
echo "Verbose: $verbose"

# Custom error handler
error_handler() {
    local line=$1
    local code=$2
    echo "Error on line $line: exit code $code" >&2
---

trap 'error_handler $LINENO $?' ERR

# Ensure cleanup on exit
cleanup() {
    rm -f /tmp/tempfile.$$
    echo "Cleanup done"
---
trap cleanup EXIT

# Check command existence
command -v curl >/dev/null 2>&1 || {
    echo "curl is required but not installed." >&2
    exit 1
---

# Fail fast with &&/|| chains
mkdir -p "$backup_dir" || {
    echo "Failed to create backup directory" >&2
    exit 1
---