go-cli

A library to define command line interfaces in Go.

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Package go-cli provides a declarative way to define full featured command-line interfaces. It follows the POSIX/GNU-style guidelines and supports custom bash completion. The current version support only simple commands with flags. Future versions will support composite commands with sub-commands, like go and git.

The options supported by a command are defined by the fields of a struct with specific struct-tags, and the command itself is defined by a Run() method attached to that struct.

Features

The implementation of go-clo follows the conventions outlined in The Open Group Base Specifications Issue 7, 2018 edition, Chapter 12. Utility Conventions

Short flags support

If a command defines -c, -v and -z as boolean flags and -f as a string option, then:

• -cvz is equivalent to -c -v -z
• -cvzf foo is equivalent to -c -v -z -f foo
• -cvzffoo is equivalent to -c -v -z -f foo
• -cffoovz is equivalent to -c -f foovz

Long flags support

Long flags always start with a -- and can accept value either inline after an = sign, or as the next argument.

• --filename foo
• --filename=foo
• --filename= specifies an empty filename

Boolean long flags do not accept a value unless attached with an = sign; --bool-flag=false is the only way to set a boolean flag with a default value of true back to false.

Positional and addition arguments

All non-option command-line arguments must be captured by a field in the command options struct, otherwise an error is generated. Non-option arguments can be captures as either positional arguments or additional arguments; additional arguments must be be backed by a slice type.

A special delimiter -- marks the end of option flags and capture the remaining arguments as non-option, assigned to positional and additional arguments fields. Note that after parsing, it is not possible to determine if arguments were specified before or after a -- delimiter.

Limitation

• Each option flag can appear only once unless it is backed by an slice type.
• The order in which arguments are specified on the command line cannot be retrieved after parsing (except for arguments backed by a slice type in which values are stored in order).
• All option and non-option arguments are handled independently of their position on the command-line, so you cannot have different flag values attached to different arguments. For example, the following hypothetical compiler command cannot be handled by go-cli: cc -O2 foo.cpp -O0 bar.ccp.
• -vvvv to e.g. increase verbosity to level 4 is not supported.

Installation

go get github.com/maargenton/go-cli

Usage

type cmd struct {
    Port      *string  `opts:"arg:1, name:port" desc:"..."`
    Format    *string  `opts:"-f, --format"     desc:"..."`
    Verbose   bool     `opts:"-v, --verbose"    desc:"..."`
    List      []string `opts:"-l, --list, sep:\\, , env:LIST"`
}

func (options *cmd) Run() error {
    // This function is invoked once the options struct has been
    // initialized with default values, values from the environment
    // and values from the command-line.
    return nil;
}

func main() {
    cli.Run(&cli.Command{
        Handler:     &cmd{},
        Description: "...",
    })
}

The cmd type is a struct that will capture all the options and arguments passed on the command line. The main function defines a command referring to the cmd type, and through cli.Run() parses the command-line arguments and environment variable to configure the command before calling cmd.Run().

Struct tags

The details of the command-line interface are defined on the struct with opts struct tags. Each tag contains a comma-separated list of a short name and/or long name and additional options in the form key:value. Within the value, reserved characters (comma and colon) can be escaped with a double-backslash \\. For example, to accept a comma-separated list of values, a option should include sep:\\,.

The opts tag can consist of:

• -b,--baudrate: either or both of a short and long flag name for the option
• arg:<n> : captures a positional argument
• args : captures all remaining arguments
• default: : a default value for the field if not specified on the command-line
• env: : the name of an environment variable that can override the default
• sep:: a list separator characters for fields that can accept multiple values. To use a comma or colon as separator, those characters must be escaped with a double-backslash (\\, or \\:). To use spaces and newlines as separator, the special value \\s can be used. Unless a separator is specified with this option, additional values must be specified by repeating the option flag multiple times.
• keep-spaces : when specified with no value, this option preserves the spaces around the argument values, that would otherwise be trimmed by default.
• keep-empty : for fields thar accept multiple values using a separator, this option preserve empty values after splitting and trimming, that would otherwise be dropped by default.
• name:: the display name for the value, used when printing out description of the field.

A separate desc struct tag contains the description for the option.

Supported field types

Fields in the command options struct can be:

• any parsable value type
• a pointer to a parsable type
• a slice of parsable value type.

A parsable types is a type whose value that can be set from a string using some standard interface, and includes:

• bool type
• All built-in integer and float types,
• string type
• time.Duration
• Any type that conforms to flag.Value.
• Any type that conforms to encoding.TextUnmarshaler
• Any other type that registers a custom parser function through value.RegisterParser()

Unless otherwise initialized, all pointer fields are initialized to nil, all slice fields are initialized to an empty slice, and all scalar fields are initialized to their built-in zero value. If the command needs to differentiate between the built-in zero value of a scalar field and a specified zero value, a pointer type should be used and checked against nil.

By default, slice field values must be provided by repeating the option flag multiple times, once of each value. If a separator is defined (sep:), multiple or all values can be provided with one command-line argument. To provide multiple values through an environment variable, a separated must be defined.

New in v0.5.0: A breaking change has been introduced to better handle lists of values and spaces around values. Prior behavior can be restores with the keep-spaces option for all fields and keep-empty for lists. With this new behavior, spaces around the values are automatically trimmed, and empty values after trimming are dropped from value lists. In effect, this allows for trailing separators to be ignored, and for list values to be specified with additional spaces around the separators, including newlines. Note that, even with keep-empty, if the last character is a separator, the last empty values is always drop, as was the case in prior versions.

In a command-line interface, all option flags are optional. Required options should use positional arguments. Positional arguments are required unless defined with a pointer type, and only if all subsequent positional arguments are also optional.

Optional command behavior

Every command struct must define a Run() error function to comply with the cli.Handler interface. The struct can also define additional methods to support specific behaviors:

• Version() string, if defined, adds a -v, --version option that print the command version returned by this function
• Usage(name string, width int) string, if defined, let the command completely redefine the usage printout triggered by -h, --help option
• Complete(opt *option.T, partial string) []option.Description, if defined, let the command override the list of suggestions offered during completion of an option or an argument. By default, the completion mechanism emulates the default behavior of bash completion and suggests matching local files.

Completion support

Completion integration with bash is supported by running:

eval $(<command> --bash-completion-script)

Once setup, bash will invoke the command to get completion suggestions, with two special environment variables set, COMP_WORD and COMP_INDEX.

When using zsh, you can still leverage bash completion scripts by adding the following to you ~/.zshrc, before th eval command:

autoload -U +X compinit && compinit
autoload -U +X bashcompinit && bashcompinit

If for some reason a command should not support the built-in completion, the completion machinery can be disabled by setting cmd.DisableCompletion on the root command.

Enum support

To help with command-line handling of enumerated types, the go-cli package includes a code generator command for use with go:generate that adds the necessary flag.Value interface methods to enumerated types.

The command is primarily intended to be run in the context of go:generate, from a file that contains definitions for one or more enumerated types. In that context, it is executed from the folder of that file and processes the current package. It generates one file with _enumer.go suffix for every file in the package that contains an enumerated type.

For example, the file pkg/strcase/format.go contains the following line:

//go:generate go run github.com/maargenton/go-cli/cmd/enumer format.go

which generate format_enumer.go for the Format type. The command is restricted to process only the definitions found in format.go.

The string representation of enumerated values is flexible and configurable, and defaults to filtered-hyphen-case. The generated Parse...() function and Set() method accept all supported string representations; the String() method prints the value in the specified representation. The representation format can be specified on the command-line with -f or --format option and applies to all the enum types generated by one invocation of the command.

For additional convenience, the generated code also defines methods to support both encoding.TextMarshaler and encoding.TextUnmarshaler, making the enum values represented as a strings in both json and yaml serialization.