Parse CLI Arguments with getopts and Long Options

Intermediate10 min

Handle short and long command-line options, required arguments, and positional parameters in bash scripts using getopts and case statements.

Prerequisites

-Bash 4.0+
-Basic bash scripting knowledge

Steps

Parse short options with getopts

Use the built-in getopts for simple short option parsing with flags and arguments.

$ cat > parse_args.sh << 'EOF' #!/usr/bin/env bash set -euo pipefail VERBOSE=false OUTPUT="" while getopts ':hvo:' opt; do case "$opt" in h) echo "Usage: $0 [-v] [-o output] file"; exit 0 ;; v) VERBOSE=true ;; o) OUTPUT="$OPTARG" ;; :) echo "Option -$OPTARG requires an argument" >&2; exit 1 ;; ?) echo "Unknown option: -$OPTARG" >&2; exit 1 ;; esac done shift $((OPTIND - 1)) echo "Verbose: $VERBOSE, Output: $OUTPUT, Remaining: $*" EOF chmod +x parse_args.sh

The colon after an option letter (like o:) means that option requires an argument.

Add long option support with a case statement

Handle --long-option style flags using a while/case loop.

$ cat > parse_long.sh << 'EOF' #!/usr/bin/env bash set -euo pipefail VERBOSE=false OUTPUT="" DRY_RUN=false FILES=() while [[ $# -gt 0 ]]; do case "$1" in -h|--help) echo "Usage: $0 [options] files..."; exit 0 ;; -v|--verbose) VERBOSE=true; shift ;; -o|--output) OUTPUT="${2:?'--output requires a value'}"; shift 2 ;; -d|--dry-run) DRY_RUN=true; shift ;; --) shift; FILES+=("$@"); break ;; -*) echo "Unknown option: $1" >&2; exit 1 ;; *) FILES+=("$1"); shift ;; esac done echo "Verbose=$VERBOSE Output=$OUTPUT DryRun=$DRY_RUN Files=${FILES[*]}" EOF chmod +x parse_long.sh

Validate required arguments

Check that required options and positional arguments are provided.

$ echo '[[ -z "$OUTPUT" ]] && { echo "Error: --output is required" >&2; exit 1; }' && echo '[[ ${#FILES[@]} -eq 0 ]] && { echo "Error: at least one file required" >&2; exit 1; }'

Test the argument parser

Run the script with various option combinations to verify parsing.

$ ./parse_long.sh -v --output result.txt --dry-run file1.txt file2.txt

Full Script

full-script.sh

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

# Argument Parsing Template
VERBOSE=false
OUTPUT=""
DRY_RUN=false
FILES=()

usage() {
  cat << EOF
Usage: $(basename "$0") [OPTIONS] <files...>

Options:
  -h, --help           Show this help
  -v, --verbose        Enable verbose output
  -o, --output FILE    Output file (required)
  -d, --dry-run        Show actions without executing
  --                   End of options

Examples:
  $(basename "$0") -v -o result.txt input.csv
  $(basename "$0") --output=result.txt *.log
EOF
  exit 0
}

while [[ $# -gt 0 ]]; do
  case "$1" in
    -h|--help)     usage ;;
    -v|--verbose)  VERBOSE=true; shift ;;
    -o|--output)   OUTPUT="${2:?'--output requires a value'}"; shift 2 ;;
    --output=*)    OUTPUT="${1#*=}"; shift ;;
    -d|--dry-run)  DRY_RUN=true; shift ;;
    --)            shift; FILES+=("$@"); break ;;
    -*)            echo "Unknown option: $1" >&2; exit 1 ;;
    *)             FILES+=("$1"); shift ;;
  esac
done

[[ -z "$OUTPUT" ]] && { echo "Error: --output is required" >&2; exit 1; }
[[ ${#FILES[@]} -eq 0 ]] && { echo "Error: at least one file required" >&2; exit 1; }

echo "Output: $OUTPUT"
echo "Files: ${FILES[*]}"
$VERBOSE && echo "Verbose mode enabled"
$DRY_RUN && echo "Dry run mode"

FAQ

Discussion

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#!/usr/bin/env bash set -euo pipefail # Argument Parsing Template VERBOSE=false OUTPUT="" DRY_RUN=false FILES=() usage() { cat << EOF Usage: $(basename "$0") [OPTIONS] <files...> Options: -h, --help Show this help -v, --verbose Enable verbose output -o, --output FILE Output file (required) -d, --dry-run Show actions without executing -- End of options Examples: $(basename "$0") -v -o result.txt input.csv $(basename "$0") --output=result.txt *.log EOF exit 0 } while [[ $# -gt 0 ]]; do case "$1" in -h|--help) usage ;; -v|--verbose) VERBOSE=true; shift ;; -o|--output) OUTPUT="${2:?'--output requires a value'}"; shift 2 ;; --output=*) OUTPUT="${1#*=}"; shift ;; -d|--dry-run) DRY_RUN=true; shift ;; --) shift; FILES+=("$@"); break ;; -*) echo "Unknown option: $1" >&2; exit 1 ;; *) FILES+=("$1"); shift ;; esac done [[ -z "$OUTPUT" ]] && { echo "Error: --output is required" >&2; exit 1; } [[ ${#FILES[@]} -eq 0 ]] && { echo "Error: at least one file required" >&2; exit 1; } echo "Output: $OUTPUT" echo "Files: ${FILES[*]}" $VERBOSE && echo "Verbose mode enabled" $DRY_RUN && echo "Dry run mode"