Bash Scripting

Comprehensive guide for writing robust, maintainable bash scripts.

Overview

Bash is the standard shell on most Linux and macOS systems. This skill provides templates, patterns, and best practices for creating production-ready shell scripts.

When to Use

•Creating automation scripts
•Writing build/deployment scripts
•Need error handling patterns
•Want script structure templates
•Need to handle arguments and options

Script Template

Minimal Safe Script

bash

#!/bin/bash
set -euo pipefail

main() {
    echo "Hello, World!"
}

main "$@"

Production-Ready Script

bash

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'

# Configuration
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly SCRIPT_NAME="$(basename "$0")"

# Error handling
trap 'echo "Error on line $LINENO" >&2; exit 1' ERR
trap 'echo "Interrupted" >&2; exit 130' INT

# Usage information
usage() {
    cat << EOF
Usage: ${SCRIPT_NAME} [OPTIONS] [ARGUMENTS]

Options:
  -h, --help        Show this help message
  -v, --verbose     Enable verbose output
  -d, --dry-run     Show what would be done without executing

Examples:
  ${SCRIPT_NAME} --verbose
  ${SCRIPT_NAME} --dry-run
EOF
}

# Parse arguments
parse_args() {
    while [[ $# -gt 0 ]]; do
        case $1 in
            -h|--help)
                usage
                exit 0
                ;;
            -v|--verbose)
                VERBOSE=1
                shift
                ;;
            -d|--dry-run)
                DRY_RUN=1
                shift
                ;;
            *)
                echo "Unknown option: $1" >&2
                usage >&2
                exit 1
                ;;
        esac
    done
}

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

log_verbose() {
    [[ ${VERBOSE:-0} -eq 1 ]] && log "$@"
}

log_error() {
    log "ERROR: $*" >&2
}

# Main logic
main() {
    parse_args "$@"
    
    log_verbose "Starting script..."
    
    # Your logic here
    
    log "Script completed successfully"
}

main "$@"

Error Handling

Strict Mode Options

bash

set -e          # Exit on error
set -u          # Error on undefined variables
set -o pipefail # Catch errors in pipelines
set -euo pipefail  # All three combined

Traps

bash

# Catch errors
trap 'echo "Error on line $LINENO" >&2' ERR

# Cleanup on exit
trap 'rm -rf "$TEMP_DIR"' EXIT

# Handle signals
trap 'echo "Interrupted"; exit 130' INT TERM

Variable Best Practices

Quoting

bash

# CORRECT - Always quote variables
file="${1:-default.txt}"
cat "${file}"

# WRONG - Unquoted variables break on spaces
cat ${file}  # Fails if file="my file.txt"

Default Values

bash

name="${1:-default}"      # Use default if unset/null
name="${1:?required}"     # Error if unset/null
name="${1:=default}"      # Assign default if unset
readonly name="${1}"      # Make read-only

Common Patterns

Check if Command Exists

bash

if command -v git >/dev/null 2>&1; then
    echo "Git is installed"
fi

Check if File Exists

bash

if [[ -f "$file" ]]; then
    echo "File exists"
fi

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

Iterate Over Files

bash

for file in *.txt; do
    [[ -f "$file" ]] || continue  # Skip if no match
    process "$file"
done

Read File Line by Line

bash

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

Process Options

bash

while getopts "hvd:" opt; do
    case $opt in
        h) usage; exit 0 ;;
        v) VERBOSE=1 ;;
        d) DIR="$OPTARG" ;;
        ?) usage >&2; exit 1 ;;
    esac
done
shift $((OPTIND-1))

Safety Patterns

Dry Run Mode

bash

run() {
    if [[ ${DRY_RUN:-0} -eq 1 ]]; then
        echo "[DRY RUN] Would execute: $*"
    else
        "$@"
    fi
}

run rm -rf "${dir}"

Confirm Destructive Actions

bash

confirm() {
    read -r -p "Are you sure? [y/N] " response
    [[ "$response" =~ ^[Yy]$ ]]
}

if confirm; then
    rm -rf "$dir"
fi

Safe Temporary Files

bash

TEMP_DIR=$(mktemp -d)
trap 'rm -rf "$TEMP_DIR"' EXIT

Debugging

Verbose Mode

bash

[[ ${VERBOSE:-0} -eq 1 ]] && set -x  # Trace execution

Debug Output

bash

debug() {
    [[ ${DEBUG:-0} -eq 1 ]] && echo "DEBUG: $*" >&2
}

Examples

Backup Script

bash

#!/bin/bash
set -euo pipefail

SOURCE="${1:-.}"
DEST="${2:-backup}"
timestamp=$(date +%Y%m%d_%H%M%S)

mkdir -p "$DEST"
tar -czf "${DEST}/backup_${timestamp}.tar.gz" "$SOURCE"
echo "Backup created: ${DEST}/backup_${timestamp}.tar.gz"

Parallel Processing

bash

#!/bin/bash
set -euo pipefail

process_file() {
    local file="$1"
    # Process logic here
    echo "Processed: $file"
}

export -f process_file

find . -name "*.txt" -print0 | xargs -0 -P4 -I{} bash -c 'process_file "$@"' _ {}