Flag package
The flag package is Go’s standard-library package for defining, parsing, retrieving, and documenting command-line flags.
Go in Practice, Second Edition
The Go flag package parses command-line flags and provides APIs for defining flags and generating their help text.
- How it is used
- Defines and parses options for a small Go command-line application.
- How deep it goes
- Works through typed registration, pointer access, parsing, aliases, and help formatting, but leaves richer command interfaces to frameworks.
- Examples
- Uses a bilingual greeting CLI, invalid-flag usage output, and a `VisitAll` help formatter.
- What you can do after
- Apply it via Go's flag package to parse typed options, add explicit aliases, and format help text.
Read how Go in Practice, Second Edition explains flag package →
Learn Go with Pocket-Sized Projects
The flag package is a Go standard-library package for parsing command-line flags and converting their values to supported types.
- How it is used
- Parses named command-line options and separates them from positional arguments in small Go command-line applications.
- How deep it goes
- Explains declaration, defaults, parsing, pointers, and argument retrieval through runnable examples while deferring fuller pointer treatment.
- Examples
- Uses a language-selecting greeting program and a currency converter with source, target, and amount inputs.
- What you can do after
- Apply it via Go's flag package to build CLIs that accept typed options, defaults, positional inputs, and missing-input checks.
Read how Learn Go with Pocket-Sized Projects explains flag package →
Go by Example
The flag package is Go's standard package for parsing command-line flags and defining their names, defaults, and value handling.
- How it is used
- Configures command-line tools by converting optional flags into variables that control program behavior.
- How deep it goes
- Explains FlagSet registration and parsing, then extends to positional arguments, usage customization, and flag.Value custom parsers.
- Examples
- Uses an HTTP request CLI with request-count and concurrency flags, including a million-request invocation.
- What you can do after
- Build it from scratch: create a CLI that registers typed flags, parses them, supplies defaults, and customizes parsing when needed.
Go in Action, Second Edition
The flag package is a Go standard-library package for defining, parsing, and retrieving command-line options and positional arguments.
- How it is used
- Configures flexible CLI tools by accepting options, feature toggles, and positional values at runtime.
- How deep it goes
- Defines typed flags, parsing, and positional-argument retrieval, but stops short of advanced aliases, subcommands, and automatic help.
- Examples
- Demonstrates help and banner flags, JSON/XML conversion, and positional input and output filenames.
- What you can do after
- Apply it via Go’s flag package to build configurable command-line programs with typed options and positional arguments.
Read how Go in Action, Second Edition explains flag package →
Related concepts
- command-line argumentuses · A command-line argument is a value supplied to a program when it is invoked, either as positional input or as an option that configures its behavior.
- flag.Parsehas part · flag.Parse is a Go flag-package function that parses command-line arguments according to defined flags and assigns their values to registered variables.
- value parserused by · A value parser is a function that converts a string flag value to a specified Go type, assigns the result to a variable, and reports parsing errors.
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