shell-bash

Shell & Bash Scripting

Overview

Shell scripting patterns for automation, system administration, and CLI tools.

---

Script Fundamentals

Script Structure

#!/usr/bin/env bash
#
# Script: backup.sh
# Description: Backup files to remote server
# Usage: ./backup.sh [options] <source> <destination>
#

set -euo pipefail  # Exit on error, undefined vars, pipe failures
IFS=$'\n\t'        # Safer word splitting

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

# Default values
VERBOSE=false
DRY_RUN=false
COMPRESS=true

# Cleanup on exit
cleanup() {
    local exit_code=$?
    # Cleanup temporary files
    rm -f "${TEMP_FILE:-}"
    exit "$exit_code"
}
trap cleanup EXIT

# Logging functions
log() {
    local level="$1"
    shift
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] [$level] $*" | tee -a "$LOG_FILE"
}

info() { log "INFO" "$@"; }
warn() { log "WARN" "$@" >&2; }
error() { log "ERROR" "$@" >&2; }
debug() { [[ "$VERBOSE" == true ]] && log "DEBUG" "$@" || true; }

die() {
    error "$@"
    exit 1
}

# Usage
usage() {
    cat <<EOF
Usage: $SCRIPT_NAME [options] <source> <destination>

Options:
    -v, --verbose     Enable verbose output
    -n, --dry-run     Show what would be done
    -h, --help        Show this help message

Examples:
    $SCRIPT_NAME /data /backup
    $SCRIPT_NAME -v --dry-run /home/user /mnt/backup
EOF
}

# Main function
main() {
    parse_args "$@"
    validate_inputs
    perform_backup
}

# Run main if script is executed directly
if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
    main "$@"
fi

Argument Parsing

# Using getopts (POSIX)
parse_args_getopts() {
    while getopts ":vnh" opt; do
        case $opt in
            v) VERBOSE=true ;;
            n) DRY_RUN=true ;;
            h) usage; exit 0 ;;
            \?) die "Invalid option: -$OPTARG" ;;
            :) die "Option -$OPTARG requires an argument" ;;
        esac
    done
    shift $((OPTIND - 1))

    SOURCE="${1:-}"
    DESTINATION="${2:-}"
}

# Using manual parsing (supports long options)
parse_args() {
    while [[ $# -gt 0 ]]; do
        case "$1" in
            -v|--verbose)
                VERBOSE=true
                shift
                ;;
            -n|--dry-run)
                DRY_RUN=true
                shift
                ;;
            -c|--compress)
                COMPRESS=true
                shift
                ;;
            --no-compress)
                COMPRESS=false
                shift
                ;;
            -h|--help)
                usage
                exit 0
                ;;
            --)
                shift
                break
                ;;
            -*)
                die "Unknown option: $1"
                ;;
            *)
                break
                ;;
        esac
    done

    # Positional arguments
    SOURCE="${1:-}"
    DESTINATION="${2:-}"
}

# Validation
validate_inputs() {
    [[ -z "$SOURCE" ]] && die "Source path is required"
    [[ -z "$DESTINATION" ]] && die "Destination path is required"
    [[ -e "$SOURCE" ]] || die "Source does not exist: $SOURCE"
}