Bash argument parsing with getopts

Parsing Flags with getopts

getopts is the built-in, POSIX-portable way to parse short flags (-v, -o file). For long flags (--verbose) you need getopt (external) or a manual case loop.

#!/usr/bin/env bash
set -euo pipefail

usage() {
  echo "Usage: $0 [-v] [-o output_dir] [-e env] input_file"
  exit 0
}

verbose=false
output_dir="./output"
environment="dev"

# Leading colon = silent error handling (we handle errors)
while getopts ":vo:e:h" opt; do
  case "$opt" in
    v) verbose=true ;;
    o) output_dir="$OPTARG" ;;
    e) environment="$OPTARG" ;;
    h) usage ;;
    :) echo "Option -$OPTARG requires an argument" >&2; exit 1 ;;
    ?) echo "Unknown option: -$OPTARG" >&2; exit 1 ;;
  esac
done

# Shift past parsed options to reach positional args
shift $(( OPTIND - 1 ))
input_file="${1:-}"

[[ -z "$input_file" ]] && { echo "Error: input file required" >&2; exit 1; }

$verbose && echo "Verbose mode ON"
echo "Input: $input_file, Output: $output_dir, Env: $environment"

The colon after a letter in the optstring (o:) means that option requires an argument. A leading colon on the optstring enables silent mode where you handle : and ? cases yourself, giving better error messages.