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Flags

Setting and querying flags is simple.

package main

import (
    "fmt"
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

func main() {
    app := &cli.App{
        Flags: []cli.Flag{
            &cli.StringFlag{
                Name:  "lang",
                Value: "english",
                Usage: "language for the greeting",
            },
        },
        Action: func(cCtx *cli.Context) error {
            name := "Nefertiti"
            if cCtx.NArg() > 0 {
                name = cCtx.Args().Get(0)
            }
            if cCtx.String("lang") == "spanish" {
                fmt.Println("Hola", name)
            } else {
                fmt.Println("Hello", name)
            }
            return nil
        },
    }

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

You can also set a destination variable for a flag, to which the content will be scanned. Note that if the Value is set for the flag, it will be shown as default, and destination will be set to this value before parsing flag on the command line.

package main

import (
    "fmt"
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

func main() {
    var language string

    app := &cli.App{
        Flags: []cli.Flag{
            &cli.StringFlag{
                Name:        "lang",
                Value:       "english",
                Usage:       "language for the greeting",
                Destination: &language,
            },
        },
        Action: func(cCtx *cli.Context) error {
            name := "someone"
            if cCtx.NArg() > 0 {
                name = cCtx.Args().Get(0)
            }
            if language == "spanish" {
                fmt.Println("Hola", name)
            } else {
                fmt.Println("Hello", name)
            }
            return nil
        },
    }

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

See full list of flags at https://pkg.go.dev/github.com/urfave/cli/v2

For bool flags you can specify the flag multiple times to get a count(e.g -v -v -v or -vvv)

package main

import (
    "fmt"
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

func main() {
    var count int

    app := &cli.App{
        Flags: []cli.Flag{
            &cli.BoolFlag{
                Name:        "foo",
                Usage:       "foo greeting",
                Count: &count,
            },
        },
        Action: func(cCtx *cli.Context) error {
            fmt.Println("count", count)
            return nil
        },
    }

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

Placeholder Values

Sometimes it's useful to specify a flag's value within the usage string itself. Such placeholders are indicated with back quotes.

For example this:

package main

import (
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

func main() {
    app := &cli.App{
        Flags: []cli.Flag{
            &cli.StringFlag{
                Name:    "config",
                Aliases: []string{"c"},
                Usage:   "Load configuration from `FILE`",
            },
        },
    }

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

Will result in help output like:

--config FILE, -c FILE   Load configuration from FILE

Note that only the first placeholder is used. Subsequent back-quoted words will be left as-is.

Alternate Names

You can set alternate (or short) names for flags by providing a comma-delimited list for the Name. e.g.

package main

import (
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

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

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

That flag can then be set with --lang spanish or -l spanish. Note that giving two different forms of the same flag in the same command invocation is an error.

Ordering

Flags for the application and commands are shown in the order they are defined. However, it's possible to sort them from outside this library by using FlagsByName or CommandsByName with sort.

For example this:

package main

import (
    "log"
    "os"
    "sort"

    "github.com/urfave/cli/v2"
)

func main() {
    app := &cli.App{
        Flags: []cli.Flag{
            &cli.StringFlag{
                Name:    "lang",
                Aliases: []string{"l"},
                Value:   "english",
                Usage:   "Language for the greeting",
            },
            &cli.StringFlag{
                Name:    "config",
                Aliases: []string{"c"},
                Usage:   "Load configuration from `FILE`",
            },
        },
        Commands: []*cli.Command{
            {
                Name:    "complete",
                Aliases: []string{"c"},
                Usage:   "complete a task on the list",
                Action: func(*cli.Context) error {
                    return nil
                },
            },
            {
                Name:    "add",
                Aliases: []string{"a"},
                Usage:   "add a task to the list",
                Action: func(*cli.Context) error {
                    return nil
                },
            },
        },
    }

    sort.Sort(cli.FlagsByName(app.Flags))
    sort.Sort(cli.CommandsByName(app.Commands))

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

Will result in help output like:

--config FILE, -c FILE  Load configuration from FILE
--lang value, -l value  Language for the greeting (default: "english")

Values from the Environment

You can also have the default value set from the environment via EnvVars. e.g.

package main

import (
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

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

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

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

package main

import (
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

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

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

Values from files

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

package main

import (
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

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

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

Note that default values set from file (e.g. FilePath) take precedence over default values set from the environment (e.g. EnvVar).

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:

  • YAML
  • JSON
  • TOML

In order to get values for a flag from an alternate input source the following code would be added to wrap an existing cli.Flag like below:

  // --- >8 ---
  altsrc.NewIntFlag(&cli.IntFlag{Name: "test"})

Initialization must also occur for these flags. Below is an example initializing getting data from a yaml file below.

  // --- >8 ---
  command.Before = altsrc.InitInputSourceWithContext(command.Flags, NewYamlSourceFromFlagFunc("load"))

The code above will use the "load" string as a flag name to get the file name of a yaml file from the cli.Context. It will then use that file name to initialize the yaml input source for any flags that are defined on that command. As a note the "load" flag used would also have to be defined on the command flags in order for this code snippet to work.

Currently only YAML, JSON, and TOML files are supported but developers can add support for other input sources by implementing the altsrc.InputSourceContext for their given sources.

Here is a more complete sample of a command using YAML support:

package main

import (
    "fmt"
    "os"

    "github.com/urfave/cli/v2"
    "github.com/urfave/cli/v2/altsrc"
)

func main() {
    flags := []cli.Flag{
        altsrc.NewIntFlag(&cli.IntFlag{Name: "test"}),
        &cli.StringFlag{Name: "load"},
    }

    app := &cli.App{
        Action: func(*cli.Context) error {
            fmt.Println("--test value.*default: 0")
            return nil
        },
        Before: altsrc.InitInputSourceWithContext(flags, altsrc.NewYamlSourceFromFlagFunc("load")),
        Flags:  flags,
    }

    app.Run(os.Args)
}

Required Flags

You can make a flag required by setting the Required field to true. If a user does not provide a required flag, they will be shown an error message.

Take for example this app that requires the lang flag:

package main

import (
    "fmt"
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

func main() {
    app := &cli.App{
        Flags: []cli.Flag{
            &cli.StringFlag{
                Name:     "lang",
                Value:    "english",
                Usage:    "language for the greeting",
                Required: true,
            },
        },
        Action: func(cCtx *cli.Context) error {
            output := "Hello"
            if cCtx.String("lang") == "spanish" {
                output = "Hola"
            }
            fmt.Println(output)
            return nil
        },
    }

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

If the app is run without the lang flag, the user will see the following message

Required flag "lang" not set

Default Values for help output

Sometimes it's useful to specify a flag's default help-text value within the flag declaration. This can be useful if the default value for a flag is a computed value. The default value can be set via the DefaultText struct field.

For example this:

package main

import (
    "log"
    "os"

    "github.com/urfave/cli/v2"
)

func main() {
    app := &cli.App{
        Flags: []cli.Flag{
            &cli.IntFlag{
                Name:        "port",
                Usage:       "Use a randomized port",
                Value:       0,
                DefaultText: "random",
            },
        },
    }

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

Will result in help output like:

--port value  Use a randomized port (default: random)

Precedence

The precedence for flag value sources is as follows (highest to lowest):

  1. Command line flag value from user
  2. Environment variable (if specified)
  3. Configuration file (if specified)
  4. Default defined on the flag

Flag Actions

Handlers can be registered per flag which are triggered after a flag has been processed. This can be used for a variety of purposes, one of which is flag validation

package main

import (
    "log"
    "os"
    "fmt"

    "github.com/urfave/cli/v2"
)

func main() {
    app := &cli.App{
        Flags: []cli.Flag{
            &cli.IntFlag{
                Name:        "port",
                Usage:       "Use a randomized port",
                Value:       0,
                DefaultText: "random",
                Action: func(ctx *cli.Context, v int) error {
                    if v >= 65536 {
                        return fmt.Errorf("Flag port value %v out of range[0-65535]", v)
                    }
                    return nil
                },
            },
        },
    }

    if err := app.Run(os.Args); err != nil {
        log.Fatal(err)
    }
}

Will result in help output like:

Flag port value 70000 out of range[0-65535]

Last update: October 24, 2022