Crate gflags

source ·
Expand description


Command line flag library in the style of gflags (formerly Google Commandline Flags).

Quoting directly from the C++ gflags documentation, because the concept is the same here:

Commandline flags are flags that users specify on the command line when they run an executable. In the command

fgrep -l -f /var/tmp/foo johannes brahms

-l and -f /var/tmp/foo are the two commandline flags. (johannes and brahms, which don’t start with a dash, are commandline arguments.)

Typically, an application lists what flags the user is allowed to pass in, and what arguments they take – in this example, -l takes no argument, and -f takes a string (in particular, a filename) as an argument. Users can use a library to help parse the commandline and store the flags in some data structure.

Gflags, the commandline flags library used within Google, differs from other libraries, such as getopt(), in that flag definitions can be scattered around the source code, and not just listed in one place such as main(). In practice, this means that a single source-code file will define and use flags that are meaningful to that file. Any application that links in that file will get the flags, and the gflags library will automatically handle that flag appropriately.

There’s significant gain in flexibility, and ease of code reuse, due to this technique.

This style of flag registration is better suited for large scale development than maintaining a single central list of flags, as the central list would become an endless source of merge conflicts in an application developed simultaneously by hundreds of developers.

Defining flags

Flags may be defined from any source file through the gflags::define! macro. There is no central list of all the flags of the application. (That’s the point and advantage of gflags for large-scale development compared to other flags libraries.)

gflags::define! {
    /// Include 'advanced' options in the menu listing.
    --big_menu = true

gflags::define! {
    /// Comma-separated list of languages to offer in the 'lang' menu.
    -l, --language <LANG> = "english,french,german"

Flags are required to have a long name (like --verbose) and may optionally have a short name (like -v). Flags must have exactly one long name and at most one short name; multiple different aliases for the same flag is not supported.

Flags of a type other than bool may have an optional value-placeholder like <LANG>. This is optional and purely cosmetic. It appears in help text.

Accessing flags

Somewhere early in your application, call gflags::parse() to parse the command line. This call returns a Vec<&str> containing everything on the command line which is not a flag (these are sometimes known as positional arguments) in a vector.

After gflags::parse() has been called, the value of each flag is available in the .flag field of the flag’s long name.

gflags::define! {
    --print-args = false

fn main() {
    let args = gflags::parse();

    if PRINT_ARGS.flag {
        println!("args = {:?}", args);

As shown in this snippet, flag names may contain hyphens, in which case the variable through which the flag’s value can be accessed has underscores in place of the hyphens.

Additionally every flag provides a method .is_present() to query whether that flag was provided on the command line. When using flags for which a default value is not provided, be sure to check .is_present() because accessing .flag when not present will cause a panic. Note also that flags without a default value must specify their data type, as below.

use std::path::Path;

gflags::define! {
    /// Search for patterns from the given file, with one pattern per line.
    -f, --file: &Path

fn main() {
    let patterns = gflags::parse();

    if FILE.is_present() {
        let path = FILE.flag;
        println!("searching for patterns from file: {}", path.display());
    } else {
        println!("searching for patterns given on command line: {:?}", patterns);

Printing help

There is no built-in -h flag for help, but you can define your own and call gflags::print_help_and_exit() to render the documentation of all flags.

gflags::define! {
    -h, --help = false

fn main() {
    if HELP.flag {

    /* ... */

For some of the flag definitions shown in this documentation, the help text would be rendered as follows.

            Include 'advanced' options in the menu listing.

    -f, --file
            Search for patterns from the given file, with one pattern per line.

    -l, --language <LANG>
            Comma-separated list of languages to offer in the 'lang' menu.

The flags are listed in alphabetical order by long name.

You will likely want to print your own content above this including the application name, version, author, introductory explanation, and usage strings.

Custom data types

The gflags::define! macro is extensible to custom data types by providing an impl of gflags::custom::Value for your type.

use gflags::custom::{Arg, Error, Result, Value};

gflags::define! {
    --color <WHEN>: Color = Color::Auto

enum Color {

impl Value for Color {
    fn parse(arg: Arg) -> Result<Self> {
        match arg.get_str() {
            "never" => Ok(Color::Never),
            "always" => Ok(Color::Always),
            "auto" => Ok(Color::Auto),
            _ => Err(Error::new("invalid color")),


Interface for defining your own flag data type.


Entry point for registering a flag from any source file.


The state associated with a single flag.


Initialize the value of all flags based on arguments from the command line at runtime.
Initialize the value of all flags, accepting non-UTF8 positional arguments.
Print the names and descriptions of all the flags.