Optionparsing

Optionparsing is the process by which a program interprets command-line arguments to identify options (flags and parameters that modify behavior) and positional arguments that convey input data or commands. It supports short options (for example -h) and long options (--help), as well as combinations of short options (-abc can be interpreted as -a -b -c). Values may be supplied as the next token or inline (for example -o value or --output=value). A special token -- is often used to disable further option parsing, after which all tokens are treated as positional arguments.

The result of optionparsing is typically a mapping from option names to values, plus a list of remaining positional arguments. Boolean flags yield true/false, options that require a value produce a string or converted type, and some options may accept multiple values. Defaults are applied when an option is not supplied. Validation and mutual-exclusion rules may be enforced during parsing.

Implementation approaches vary. Some programs implement a custom parser to tailor behavior; many languages provide libraries to simplify parsing and validation. Examples include Python's argparse and optparse, Java's Commons CLI, and various language-specific libraries such as yargs or commander for JavaScript, and GNU getopt/getopt_long for C and C++. Libraries typically offer features like required options, default values, type conversion, help text generation, and support for subcommands or groups.

Optionparsing is central to command-line interfaces; it reduces users' cognitive load by allowing consistent syntax and informative error messages. It is also extended to configuration files and environment variables in more complex tools.