Value Sources - Urfave

Flags can have their default values set from different sources. The following sources are provided by default with urfave/cli

Environment
Text Files

The library also provides a framework for users to plugin their own implementation of value sources to be fetched via other mechanisms(http and so on).

In addition there is a urfave/cli-altsrc repo which hosts some common value sources to read from files or via http/https.

YAML
JSON
TOML

Values from the Environment

To set a value from the environment use cli.EnvVars. e.g.

package main

import (
	"log"
	"os"
	"context"

	"github.com/urfave/cli/v3"
)

func main() {
	cmd := &cli.Command{
		Flags: []cli.Flag{
			&cli.StringFlag{
				Name:    "lang",
				Aliases: []string{"l"},
				Value:   "english",
				Usage:   "language for the greeting",
				Sources: cli.EnvVars("APP_LANG"),
			},
		},
	}

	if err := cmd.Run(context.Background(), os.Args); err != nil {
		log.Fatal(err)
	}
}

If cli.EnvVars contains more than one string, the first environment variable that resolves is used.

package main

import (
	"log"
	"os"
	"context"

	"github.com/urfave/cli/v3"
)

func main() {
	cmd := &cli.Command{
		Flags: []cli.Flag{
			&cli.StringFlag{
				Name:    "lang",
				Aliases: []string{"l"},
				Value:   "english",
				Usage:   "language for the greeting",
				Sources: cli.EnvVars("LEGACY_COMPAT_LANG", "APP_LANG", "LANG"),
			},
		},
	}

	if err := cmd.Run(context.Background(), os.Args); err != nil {
		log.Fatal(err)
	}
}

Values from files

You can also have the default value set from file via cli.File. e.g.

package main

import (
	"log"
	"os"
	"context"

	"github.com/urfave/cli/v3"
)

func main() {
	cmd := &cli.Command{
		Flags: []cli.Flag{
			&cli.StringFlag{
				Name:     "password",
				Aliases:  []string{"p"},
				Usage:    "password for the mysql database",
				Sources: cli.Files("/etc/mysql/password"),
			},
		},
	}

	if err := cmd.Run(context.Background(), os.Args); err != nil {
		log.Fatal(err)
	}
}

Note that default values are set in the same order as they are defined in the Sources param. This allows the user to choose order of priority

Values from alternate input sources (YAML, TOML, and others)

There is a separate package altsrc that adds support for getting flag values from other file input sources.

Currently supported input source formats by that library are:

YAML
JSON
TOML

A simple straight forward usage would be

package main

import (
	"log"
	"os"
	"context"

	"github.com/urfave/cli/v3"
	"github.com/urfave/cli-altsrc/v3"
	yaml "github.com/urfave/cli-altsrc/v3/yaml"
)

func main() {
	cmd := &cli.Command{
		Flags: []cli.Flag{
			&cli.StringFlag{
				Name:    "password",
				Aliases: []string{"p"},
				Usage:   "password for the mysql database",
				Sources: cli.NewValueSourceChain(yaml.YAML("somekey", altsrc.StringSourcer("/path/to/filename"))),
			},
		},
	}

	if err := cmd.Run(context.Background(), os.Args); err != nil {
		log.Fatal(err)
	}
}

Sometime the source name is itself provided by another CLI flag. To allow the library to "lazy-load" the file when needed we use the altsrc.NewStringPtrSourcer function to bind the value of the flag to a pointer that is set as a destination of another flag

package main

import (
	"log"
	"os"
	"context"

	"github.com/urfave/cli/v3"
	"github.com/urfave/cli-altsrc/v3"
	yaml "github.com/urfave/cli-altsrc/v3/yaml"
)

func main() {
	var filename string
	cmd := &cli.Command{
		Flags: []cli.Flag{
			&cli.StringFlag{
				Name:        "file",
				Aliases:     []string{"f"},
				Value:       "/path/to/default",
				Usage:       "filename for mysql database",
				Destination: &filename,
			},
			&cli.StringFlag{
				Name:    "password",
				Aliases: []string{"p"},
				Usage:   "password for the mysql database",
				Sources: cli.NewValueSourceChain(yaml.YAML("somekey", altsrc.NewStringPtrSourcer(&filename))),
			},
		},
	}

	if err := cmd.Run(context.Background(), os.Args); err != nil {
		log.Fatal(err)
	}
}