## Description

opflag is a drop-in replacement for Go's flag package, implementing
POSIX/GNU-style --flags.

opflag is compatible with the [GNU extensions to the POSIX recommendations
for command-line options][1]. For a more precise description, see the
"Command-line flag syntax" section below.

[1]: http://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html

opflag is available under the same style of BSD license as the Go language,
which can be found in the LICENSE file.

## Installation

opflag is available using the standard `go get` command.

Install by running:

    go get github.com/xxxserxxx/opflag

Run tests by running:

    go test github.com/xxxserxxx/opflag

## Usage

opflag is a drop-in replacement of Go's native flag package. If you import
opflag under the name "flag" then all code should continue to function
with no changes.

``` go
import flag "github.com/xxxserxxx/opflag"
```

There is one exception to this: if you directly instantiate the Flag struct
there is one more field "Shorthand" that you will need to set.
Most code never instantiates this struct directly, and instead uses
functions such as String(), BoolVar(), and Var(), and is therefore
unaffected.

Define flags using flag.String(), Bool(), Int(), etc.

This declares an integer flag, -flagname, stored in the pointer ip, with type *int.

``` go
var ip *int = flag.Int("flagname", 1234, "help message for flagname")
```

If you like, you can bind the flag to a variable using the Var() functions.

``` go
var flagvar int
func init() {
    flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname")
}
```

Or you can create custom flags that satisfy the Value interface (with
pointer receivers) and couple them to flag parsing by

``` go
flag.Var(&flagVal, "name", "help message for flagname")
```

For such flags, the default value is just the initial value of the variable.

After all flags are defined, call

``` go
flag.Parse()
```

to parse the command line into the defined flags.

Flags may then be used directly. If you're using the flags themselves,
they are all pointers; if you bind to variables, they're values.

``` go
fmt.Println("ip has value ", *ip)
fmt.Println("flagvar has value ", flagvar)
```

After parsing, the arguments after the flag are available as the
slice flag.Args() or individually as flag.Arg(i).
The arguments are indexed from 0 through flag.NArg()-1.

The opflag package also defines some new functions that are not in flag,
that give one-letter shorthands for flags. You can use these by appending
'P' to the name of any function that defines a flag.

``` go
var ip = flag.IntP("flagname", "f", 1234, "help message")
var flagvar bool
func init() {
    flag.BoolVarP("boolname", "b", true, "help message")
}
flag.VarP(&flagVar, "varname", "v", 1234, "help message")
```

Shorthand letters can be used with single dashes on the command line.
Boolean shorthand flags can be combined with other shorthand flags.

The default set of command-line flags is controlled by
top-level functions.  The FlagSet type allows one to define
independent sets of flags, such as to implement subcommands
in a command-line interface. The methods of FlagSet are
analogous to the top-level functions for the command-line
flag set.

## Command line flag syntax

```
--flag    // boolean flags only
--flag=x
```

Unlike the flag package, a single dash before an option means something
different than a double dash. Single dashes signify a series of shorthand
letters for flags. All but the last shorthand letter must be boolean flags.

```
// boolean flags
-f
-abc

// non-boolean flags
-n 1234
-Ifile

// mixed
-abcs "hello"
-abcn1234
```

Flag parsing stops after the terminator "--". Unlike the flag package,
flags can be interspersed with arguments anywhere on the command line
before this terminator.

Integer flags accept 1234, 0664, 0x1234 and may be negative.
Boolean flags (in their long form) accept 1, 0, t, f, true, false,
TRUE, FALSE, True, False.
Duration flags accept any input valid for time.ParseDuration.

## More info

You can see the full reference documentation of the opflag package
[at godoc.org][3], or through go's standard documentation system by
running `godoc -http=:6060` and browsing to
[http://localhost:6060/pkg/github.com/xxxserxxx/opflag][2] after
installation.

[2]: http://localhost:6060/pkg/github.com/xxxserxxx/opflag
[3]: http://godoc.org/github.com/xxxserxxx/opflag
