Linux Basic — getopts Option Parsing

getopts Option Parsing

getopts is the portable way to parse short options (-a -b -f arg) in POSIX-style shells. It is not getopt (which is less portable and complex).

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Basic Pattern

usage() { echo "Usage: $0 [-a] [-b arg]"; exit 1 }

while getopts ":ab:" opt; do
  case $opt in
    a) flag_a=1 ;; 
    b) arg_b=$OPTARG ;;
    :) echo "Option -$OPTARG requires an argument."; usage ;;
    \?) echo "Invalid option: -$OPTARG"; usage ;;
  esac
done

shift $((OPTIND -1))  # remaining args

• :ab: — options string: a no arg, b: requires an arg
• Leading : switches getopts into silent (POSIX) error reporting mode
• $OPTARG holds the option argument

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Boolean Flags

Combine multiple flags easily:

while getopts "abc" opt; do
  case $opt in
    a) a=1;;
    b) b=1;;
    c) c=1;;
  esac
done

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Long Options (Not getopts) — Use a Simple Workaround

getopts doesn’t handle --long natively. For mixed short/long, parse -- manually first:

while [[ "$1" == --* ]]; do
  case "$1" in
    --help) usage;;
    --version) echo "v1"; exit 0;;
  esac
  shift
done

# then fallback to getopts for short options

For full long-option parsing prefer argbash, getopt with caution, or a small parser.

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Idiomatic Usage Message

• Keep usage() short and practical
• Print default values and examples
• Use $0 in help so the message works when the script is renamed

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