bash-basics

Bash Basics Skill

Master the fundamentals of Bash shell scripting with production-ready patterns

Learning Objectives

After completing this skill, you will be able to:

• Write syntactically correct Bash scripts
• Use variables with proper quoting and expansion
• Implement control flow structures (if, case, loops)
• Design reusable functions with error handling
• Apply the strict mode (set -euo pipefail)

Prerequisites

• Basic command line familiarity
• Text editor (vim, nano, or IDE)
• Linux/macOS/WSL environment
• Bash 4.0+ installed

Core Concepts

1. Script Structure

#!/usr/bin/env bash
# Script: example.sh
# Purpose: Demonstrate basic structure
# Usage: ./example.sh [options]

set -euo pipefail    # Strict mode
IFS=$'\n\t'          # Safe IFS

# Constants
readonly VERSION="1.0.0"

# Main logic
main() {
    echo "Hello, World!"
}

main "$@"

2. Variables

# Declaration
name="value"                    # String
declare -i count=0              # Integer
declare -r CONST="immutable"    # Readonly
declare -a array=("a" "b" "c")  # Array
declare -A map=([key]="val")    # Associative array

# Expansion
echo "${name}"                  # Basic
echo "${name:-default}"         # Default if unset
echo "${name:?error msg}"       # Error if unset
echo "${#name}"                 # Length
echo "${name^^}"                # Uppercase
echo "${name,,}"                # Lowercase

3. Control Structures

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

# Case statements
case "$option" in
    start) do_start ;;
    stop)  do_stop ;;
    *)     echo "Unknown" ;;
esac

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

while read -r line; do
    process "$line"
done < file.txt

4. Functions

function greet() {
    local name="${1:?Name required}"
    echo "Hello, $name!"
}

# With return values
function add() {
    local a="${1:-0}"
    local b="${2:-0}"
    echo $((a + b))
}

result=$(add 5 3)

Common Patterns

Error Handling Pattern

die() {
    printf 'ERROR: %s\n' "$1" >&2
    exit "${2:-1}"
}

try_command() {
    if ! "$@"; then
        die "Command failed: $*"
    fi
}

Argument Parsing Pattern

while [[ $# -gt 0 ]]; do
    case "$1" in
        -h|--help)  usage; exit 0 ;;
        -v|--verbose) VERBOSE=true; shift ;;
        --)         shift; break ;;
        -*)         die "Unknown option: $1" ;;
        *)          break ;;
    esac
done

Anti-Patterns

Don't	Do	Why
for f in $(ls)	for f in *	Parsing ls breaks on spaces
result=`cmd`	result=$(cmd)	Backticks don't nest
[ $var = x ]	[[ "$var" = x ]]	Unquoted vars break
cd dir; cmd	(cd dir && cmd)	cd can fail silently

Practice Exercises

1. Hello Script: Write a script that greets by name
2. File Counter: Count files in a directory by extension
3. Backup Script: Create timestamped backups
4. Config Parser: Parse key=value config files

Troubleshooting

Common Errors

Error	Cause	Fix
unbound variable	Using undefined var	Use ${var:-}
syntax error	Missing quotes	Check bracket matching
command not found	PATH issue	Use full path
permission denied	Not executable	chmod +x script

Debug Techniques

# Enable trace
set -x

# Verbose PS4
export PS4='+(${BASH_SOURCE}:${LINENO}): '

# Shellcheck
shellcheck script.sh

Resources

• Bash Manual
• ShellCheck
• Google Shell Style Guide
• Bash Pitfalls