README

Flag

Flag is a drop in replacement for Go's flag package with the addition to parse files and environment variables. If you support the twelve-factor app methodology, Flag complies with the third factor; "Store config in the environment".

An example using a gopher:

$ cat > gopher.go
    package main

    import (
        "fmt"
    	"github.com/namsral/flag"
	)
    
    var age int
    
    flag.IntVar(&age, "age", 0, "age of gopher")
    flag.Parse()
    
    fmt.Print("age:", age)

$ go run gopher.go -age 1
age: 1

Same code but using an environment variable:

$ export AGE=2
$ go run gopher.go
age: 2

Same code but using a configuration file:

$ cat > gopher.conf
age 3

$ go run gopher.go -config gopher.conf
age: 3

The following table shows how flags are translated to environment variables and configuration files:

Type Flag Environment File
int -age 2 AGE=2 age 2
bool -female FEMALE=true female true
float -length 175.5 LENGTH=175.5 length 175.5
string -name Gloria NAME=Gloria name Gloria

This package is a port of Go's flag package from the standard library with the addition of two functions ParseEnv and ParseFile.

Goals

Why?

Why not use one of the many INI, JSON or YAML parsers?

I find it best practice to have simple configuration options to control the behaviour of an applications when it starts up. Use basic types like ints, floats and strings for configuration options and store more complex data structures in the "datastore" layer.

Usage

It's intended for projects which require a simple configuration made available through command-line flags, configuration files and shell environments. It's similar to the original flag package.

Example:

import "github.com/namsral/flag"

flag.String(flag.DefaultConfigFlagname, "", "path to config file")
flag.Int("age", 24, "help message for age")

flag.Parse()

Order of precedence:

  1. Command line options
  2. Environment variables
  3. Configuration file
  4. Default values
Parsing Configuration Files

Create a configuration file:

$ cat > ./gopher.conf
# empty newlines and lines beginning with a "#" character are ignored.
name bob

# keys and values can also be separated by the "=" character
age=20

# booleans can be empty, set with 0, 1, true, false, etc
hacker

Add a "config" flag:

flag.String(flag.DefaultConfigFlagname, "", "path to config file")

Run the command:

$ go run ./gopher.go -config ./gopher.conf

The default flag name for the configuration file is "config" and can be changed by setting flag.DefaultConfigFlagname:

flag.DefaultConfigFlagname = "conf"
flag.Parse()
Parsing Environment Variables

Environment variables are parsed 1-on-1 with defined flags:

$ export AGE=44
$ go run ./gopher.go
age=44

You can also parse prefixed environment variables by setting a prefix name when creating a new empty flag set:

fs := flag.NewFlagSetWithEnvPrefix(os.Args[0], "GO", 0)
fs.Int("age", 24, "help message for age")
fs.Parse(os.Args[1:])
...
$ go export GO_AGE=33
$ go run ./gopher.go
age=33

For more examples see the examples directory in the project repository.

That's it.

License

Copyright (c) 2012 The Go Authors. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  • Neither the name of Google Inc. nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Documentation

Overview

    Package flag implements command-line flag parsing. okdaokddadok Usage:

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

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

    import "flag"
    var ip = flag.Int("flagname", 1234, "help message for flagname")
    

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

    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

    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

    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.

    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.

    Command line flag syntax:

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

    One or two minus signs may be used; they are equivalent. The last form is not permitted for boolean flags because the meaning of the command

    cmd -x *
    

    will change if there is a file called 0, false, etc. You must use the -flag=false form to turn off a boolean flag.

    Flag parsing stops just before the first non-flag argument ("-" is a non-flag argument) or after the terminator "--".

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

    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.

    Example
    Output:
    
    

    Index

    Examples

    Constants

    This section is empty.

    Variables

    View Source
    var CommandLine = NewFlagSet(os.Args[0], ExitOnError)

      CommandLine is the default set of command-line flags, parsed from os.Args. The top-level functions such as BoolVar, Arg, and on are wrappers for the methods of CommandLine.

      View Source
      var DefaultConfigFlagname = "config"

        DefaultConfigFlagname defines the flag name of the optional config file path. Used to lookup and parse the config file when a default is set and available on disk.

        View Source
        var EnvironmentPrefix = ""

          EnvironmentPrefix defines a string that will be implicitely prefixed to a flag name before looking it up in the environment variables.

          View Source
          var ErrHelp = errors.New("flag: help requested")

            ErrHelp is the error returned if the flag -help is invoked but no such flag is defined.

            View Source
            var Usage = func() {
            	fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0])
            	PrintDefaults()
            }

              Usage prints to standard error a usage message documenting all defined command-line flags. The function is a variable that may be changed to point to a custom function.

              Functions

              func Arg

              func Arg(i int) string

                Arg returns the i'th command-line argument. Arg(0) is the first remaining argument after flags have been processed.

                func Args

                func Args() []string

                  Args returns the non-flag command-line arguments.

                  func Bool

                  func Bool(name string, value bool, usage string) *bool

                    Bool defines a bool flag with specified name, default value, and usage string. The return value is the address of a bool variable that stores the value of the flag.

                    func BoolVar

                    func BoolVar(p *bool, name string, value bool, usage string)

                      BoolVar defines a bool flag with specified name, default value, and usage string. The argument p points to a bool variable in which to store the value of the flag.

                      func Duration

                      func Duration(name string, value time.Duration, usage string) *time.Duration

                        Duration defines a time.Duration flag with specified name, default value, and usage string. The return value is the address of a time.Duration variable that stores the value of the flag.

                        func DurationVar

                        func DurationVar(p *time.Duration, name string, value time.Duration, usage string)

                          DurationVar defines a time.Duration flag with specified name, default value, and usage string. The argument p points to a time.Duration variable in which to store the value of the flag.

                          func Float64

                          func Float64(name string, value float64, usage string) *float64

                            Float64 defines a float64 flag with specified name, default value, and usage string. The return value is the address of a float64 variable that stores the value of the flag.

                            func Float64Var

                            func Float64Var(p *float64, name string, value float64, usage string)

                              Float64Var defines a float64 flag with specified name, default value, and usage string. The argument p points to a float64 variable in which to store the value of the flag.

                              func Int

                              func Int(name string, value int, usage string) *int

                                Int defines an int flag with specified name, default value, and usage string. The return value is the address of an int variable that stores the value of the flag.

                                func Int64

                                func Int64(name string, value int64, usage string) *int64

                                  Int64 defines an int64 flag with specified name, default value, and usage string. The return value is the address of an int64 variable that stores the value of the flag.

                                  func Int64Var

                                  func Int64Var(p *int64, name string, value int64, usage string)

                                    Int64Var defines an int64 flag with specified name, default value, and usage string. The argument p points to an int64 variable in which to store the value of the flag.

                                    func IntVar

                                    func IntVar(p *int, name string, value int, usage string)

                                      IntVar defines an int flag with specified name, default value, and usage string. The argument p points to an int variable in which to store the value of the flag.

                                      func NArg

                                      func NArg() int

                                        NArg is the number of arguments remaining after flags have been processed.

                                        func NFlag

                                        func NFlag() int

                                          NFlag returns the number of command-line flags that have been set.

                                          func Parse

                                          func Parse()

                                            Parse parses the command-line flags from os.Args[1:]. Must be called after all flags are defined and before flags are accessed by the program.

                                            func Parsed

                                            func Parsed() bool

                                              Parsed returns true if the command-line flags have been parsed.

                                              func PrintDefaults

                                              func PrintDefaults()

                                                PrintDefaults prints to standard error the default values of all defined command-line flags.

                                                func Set

                                                func Set(name, value string) error

                                                  Set sets the value of the named command-line flag.

                                                  func String

                                                  func String(name string, value string, usage string) *string

                                                    String defines a string flag with specified name, default value, and usage string. The return value is the address of a string variable that stores the value of the flag.

                                                    func StringVar

                                                    func StringVar(p *string, name string, value string, usage string)

                                                      StringVar defines a string flag with specified name, default value, and usage string. The argument p points to a string variable in which to store the value of the flag.

                                                      func Uint

                                                      func Uint(name string, value uint, usage string) *uint

                                                        Uint defines a uint flag with specified name, default value, and usage string. The return value is the address of a uint variable that stores the value of the flag.

                                                        func Uint64

                                                        func Uint64(name string, value uint64, usage string) *uint64

                                                          Uint64 defines a uint64 flag with specified name, default value, and usage string. The return value is the address of a uint64 variable that stores the value of the flag.

                                                          func Uint64Var

                                                          func Uint64Var(p *uint64, name string, value uint64, usage string)

                                                            Uint64Var defines a uint64 flag with specified name, default value, and usage string. The argument p points to a uint64 variable in which to store the value of the flag.

                                                            func UintVar

                                                            func UintVar(p *uint, name string, value uint, usage string)

                                                              UintVar defines a uint flag with specified name, default value, and usage string. The argument p points to a uint variable in which to store the value of the flag.

                                                              func Var

                                                              func Var(value Value, name string, usage string)

                                                                Var defines a flag with the specified name and usage string. The type and value of the flag are represented by the first argument, of type Value, which typically holds a user-defined implementation of Value. For instance, the caller could create a flag that turns a comma-separated string into a slice of strings by giving the slice the methods of Value; in particular, Set would decompose the comma-separated string into the slice.

                                                                func Visit

                                                                func Visit(fn func(*Flag))

                                                                  Visit visits the command-line flags in lexicographical order, calling fn for each. It visits only those flags that have been set.

                                                                  func VisitAll

                                                                  func VisitAll(fn func(*Flag))

                                                                    VisitAll visits the command-line flags in lexicographical order, calling fn for each. It visits all flags, even those not set.

                                                                    Types

                                                                    type ErrorHandling

                                                                    type ErrorHandling int

                                                                      ErrorHandling defines how to handle flag parsing errors.

                                                                      const (
                                                                      	ContinueOnError ErrorHandling = iota
                                                                      	ExitOnError
                                                                      	PanicOnError
                                                                      )

                                                                      type Flag

                                                                      type Flag struct {
                                                                      	Name     string // name as it appears on command line
                                                                      	Usage    string // help message
                                                                      	Value    Value  // value as set
                                                                      	DefValue string // default value (as text); for usage message
                                                                      }

                                                                        A Flag represents the state of a flag.

                                                                        func Lookup

                                                                        func Lookup(name string) *Flag

                                                                          Lookup returns the Flag structure of the named command-line flag, returning nil if none exists.

                                                                          type FlagSet

                                                                          type FlagSet struct {
                                                                          	// Usage is the function called when an error occurs while parsing flags.
                                                                          	// The field is a function (not a method) that may be changed to point to
                                                                          	// a custom error handler.
                                                                          	Usage func()
                                                                          	// contains filtered or unexported fields
                                                                          }

                                                                            A FlagSet represents a set of defined flags. The zero value of a FlagSet has no name and has ContinueOnError error handling.

                                                                            func NewFlagSet

                                                                            func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet

                                                                              NewFlagSet returns a new, empty flag set with the specified name and error handling property.

                                                                              func NewFlagSetWithEnvPrefix

                                                                              func NewFlagSetWithEnvPrefix(name string, prefix string, errorHandling ErrorHandling) *FlagSet

                                                                                NewFlagSetWithEnvPrefix returns a new empty flag set with the specified name, environment variable prefix, and error handling property.

                                                                                func (*FlagSet) Arg

                                                                                func (f *FlagSet) Arg(i int) string

                                                                                  Arg returns the i'th argument. Arg(0) is the first remaining argument after flags have been processed.

                                                                                  func (*FlagSet) Args

                                                                                  func (f *FlagSet) Args() []string

                                                                                    Args returns the non-flag arguments.

                                                                                    func (*FlagSet) Bool

                                                                                    func (f *FlagSet) Bool(name string, value bool, usage string) *bool

                                                                                      Bool defines a bool flag with specified name, default value, and usage string. The return value is the address of a bool variable that stores the value of the flag.

                                                                                      func (*FlagSet) BoolVar

                                                                                      func (f *FlagSet) BoolVar(p *bool, name string, value bool, usage string)

                                                                                        BoolVar defines a bool flag with specified name, default value, and usage string. The argument p points to a bool variable in which to store the value of the flag.

                                                                                        func (*FlagSet) Duration

                                                                                        func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration

                                                                                          Duration defines a time.Duration flag with specified name, default value, and usage string. The return value is the address of a time.Duration variable that stores the value of the flag.

                                                                                          func (*FlagSet) DurationVar

                                                                                          func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string)

                                                                                            DurationVar defines a time.Duration flag with specified name, default value, and usage string. The argument p points to a time.Duration variable in which to store the value of the flag.

                                                                                            func (*FlagSet) Float64

                                                                                            func (f *FlagSet) Float64(name string, value float64, usage string) *float64

                                                                                              Float64 defines a float64 flag with specified name, default value, and usage string. The return value is the address of a float64 variable that stores the value of the flag.

                                                                                              func (*FlagSet) Float64Var

                                                                                              func (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string)

                                                                                                Float64Var defines a float64 flag with specified name, default value, and usage string. The argument p points to a float64 variable in which to store the value of the flag.

                                                                                                func (*FlagSet) Init

                                                                                                func (f *FlagSet) Init(name string, errorHandling ErrorHandling)

                                                                                                  Init sets the name, environment name prefix, and error handling property for a flag set. By default, the zero FlagSet uses an empty name, EnvironmentPrefix, and the ContinueOnError error handling policy.

                                                                                                  func (*FlagSet) Int

                                                                                                  func (f *FlagSet) Int(name string, value int, usage string) *int

                                                                                                    Int defines an int flag with specified name, default value, and usage string. The return value is the address of an int variable that stores the value of the flag.

                                                                                                    func (*FlagSet) Int64

                                                                                                    func (f *FlagSet) Int64(name string, value int64, usage string) *int64

                                                                                                      Int64 defines an int64 flag with specified name, default value, and usage string. The return value is the address of an int64 variable that stores the value of the flag.

                                                                                                      func (*FlagSet) Int64Var

                                                                                                      func (f *FlagSet) Int64Var(p *int64, name string, value int64, usage string)

                                                                                                        Int64Var defines an int64 flag with specified name, default value, and usage string. The argument p points to an int64 variable in which to store the value of the flag.

                                                                                                        func (*FlagSet) IntVar

                                                                                                        func (f *FlagSet) IntVar(p *int, name string, value int, usage string)

                                                                                                          IntVar defines an int flag with specified name, default value, and usage string. The argument p points to an int variable in which to store the value of the flag.

                                                                                                          func (*FlagSet) Lookup

                                                                                                          func (f *FlagSet) Lookup(name string) *Flag

                                                                                                            Lookup returns the Flag structure of the named flag, returning nil if none exists.

                                                                                                            func (*FlagSet) NArg

                                                                                                            func (f *FlagSet) NArg() int

                                                                                                              NArg is the number of arguments remaining after flags have been processed.

                                                                                                              func (*FlagSet) NFlag

                                                                                                              func (f *FlagSet) NFlag() int

                                                                                                                NFlag returns the number of flags that have been set.

                                                                                                                func (*FlagSet) Parse

                                                                                                                func (f *FlagSet) Parse(arguments []string) error

                                                                                                                  Parse parses flag definitions from the argument list, which should not include the command name. Must be called after all flags in the FlagSet are defined and before flags are accessed by the program. The return value will be ErrHelp if -help was set but not defined.

                                                                                                                  func (*FlagSet) ParseEnv

                                                                                                                  func (f *FlagSet) ParseEnv(environ []string) error

                                                                                                                    ParseEnv parses flags from environment variables. Flags already set will be ignored.

                                                                                                                    func (*FlagSet) ParseFile

                                                                                                                    func (f *FlagSet) ParseFile(path string) error

                                                                                                                      ParseFile parses flags from the file in path. Same format as commandline argumens, newlines and lines beginning with a "#" charater are ignored. Flags already set will be ignored.

                                                                                                                      func (*FlagSet) Parsed

                                                                                                                      func (f *FlagSet) Parsed() bool

                                                                                                                        Parsed reports whether f.Parse has been called.

                                                                                                                        func (*FlagSet) PrintDefaults

                                                                                                                        func (f *FlagSet) PrintDefaults()

                                                                                                                          PrintDefaults prints, to standard error unless configured otherwise, the default values of all defined flags in the set.

                                                                                                                          func (*FlagSet) Set

                                                                                                                          func (f *FlagSet) Set(name, value string) error

                                                                                                                            Set sets the value of the named flag.

                                                                                                                            func (*FlagSet) SetOutput

                                                                                                                            func (f *FlagSet) SetOutput(output io.Writer)

                                                                                                                              SetOutput sets the destination for usage and error messages. If output is nil, os.Stderr is used.

                                                                                                                              func (*FlagSet) String

                                                                                                                              func (f *FlagSet) String(name string, value string, usage string) *string

                                                                                                                                String defines a string flag with specified name, default value, and usage string. The return value is the address of a string variable that stores the value of the flag.

                                                                                                                                func (*FlagSet) StringVar

                                                                                                                                func (f *FlagSet) StringVar(p *string, name string, value string, usage string)

                                                                                                                                  StringVar defines a string flag with specified name, default value, and usage string. The argument p points to a string variable in which to store the value of the flag.

                                                                                                                                  func (*FlagSet) Uint

                                                                                                                                  func (f *FlagSet) Uint(name string, value uint, usage string) *uint

                                                                                                                                    Uint defines a uint flag with specified name, default value, and usage string. The return value is the address of a uint variable that stores the value of the flag.

                                                                                                                                    func (*FlagSet) Uint64

                                                                                                                                    func (f *FlagSet) Uint64(name string, value uint64, usage string) *uint64

                                                                                                                                      Uint64 defines a uint64 flag with specified name, default value, and usage string. The return value is the address of a uint64 variable that stores the value of the flag.

                                                                                                                                      func (*FlagSet) Uint64Var

                                                                                                                                      func (f *FlagSet) Uint64Var(p *uint64, name string, value uint64, usage string)

                                                                                                                                        Uint64Var defines a uint64 flag with specified name, default value, and usage string. The argument p points to a uint64 variable in which to store the value of the flag.

                                                                                                                                        func (*FlagSet) UintVar

                                                                                                                                        func (f *FlagSet) UintVar(p *uint, name string, value uint, usage string)

                                                                                                                                          UintVar defines a uint flag with specified name, default value, and usage string. The argument p points to a uint variable in which to store the value of the flag.

                                                                                                                                          func (*FlagSet) Var

                                                                                                                                          func (f *FlagSet) Var(value Value, name string, usage string)

                                                                                                                                            Var defines a flag with the specified name and usage string. The type and value of the flag are represented by the first argument, of type Value, which typically holds a user-defined implementation of Value. For instance, the caller could create a flag that turns a comma-separated string into a slice of strings by giving the slice the methods of Value; in particular, Set would decompose the comma-separated string into the slice.

                                                                                                                                            func (*FlagSet) Visit

                                                                                                                                            func (f *FlagSet) Visit(fn func(*Flag))

                                                                                                                                              Visit visits the flags in lexicographical order, calling fn for each. It visits only those flags that have been set.

                                                                                                                                              func (*FlagSet) VisitAll

                                                                                                                                              func (f *FlagSet) VisitAll(fn func(*Flag))

                                                                                                                                                VisitAll visits the flags in lexicographical order, calling fn for each. It visits all flags, even those not set.

                                                                                                                                                type Getter

                                                                                                                                                type Getter interface {
                                                                                                                                                	Value
                                                                                                                                                	Get() interface{}
                                                                                                                                                }

                                                                                                                                                  Getter is an interface that allows the contents of a Value to be retrieved. It wraps the Value interface, rather than being part of it, because it appeared after Go 1 and its compatibility rules. All Value types provided by this package satisfy the Getter interface.

                                                                                                                                                  type Value

                                                                                                                                                  type Value interface {
                                                                                                                                                  	String() string
                                                                                                                                                  	Set(string) error
                                                                                                                                                  }

                                                                                                                                                    Value is the interface to the dynamic value stored in a flag. (The default value is represented as a string.)

                                                                                                                                                    If a Value has an IsBoolFlag() bool method returning true, the command-line parser makes -name equivalent to -name=true rather than using the next command-line argument.

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