Go urfave/cli
last modified April 11, 2024
In this article we show how to create command line tools in Golang using the urfave/cli package.
The urfave/cli is a simple and fast package for building command
line applications in Go. It supports commands, subcommands, flags, automatic
help system, dynamic shell completion and generation of markdown documentation.
Simple CLI example
The following example demonstrates how to create a simple skeleton of a CLI application.
package main
import (
"fmt"
"log"
"os"
"github.com/urfave/cli"
)
func main() {
app := cli.NewApp()
app.Name = "my cli application"
app.Action = (func(ctx *cli.Context) error {
fmt.Println("app launched")
return nil
})
err := app.Run(os.Args)
if err != nil {
log.Fatal(err)
}
}
The program shows a short message when run.
import (
"fmt"
"log"
"os"
"github.com/urfave/cli"
)
The package is imported.
app := cli.NewApp()
A new CLI application is created with cli.NewApp. It sets some
defaults to the application.
app.Name = "my cli application"
We set the name for the application through the Name field.
app.Action = (func(ctx *cli.Context) error {
fmt.Println("app launched")
return nil
})
The Action field is set to a function which is called when no
subcommands are specified.
err := app.Run(os.Args)
The Run function parses the arguments slice and routes to the
proper flag/args combination.
$ go build $ simple.exe app launched
We build and run the application.
$ simple.exe -h NAME: my cli application - A new cli application USAGE: simple.exe [global options] command [command options] [arguments...] COMMANDS: help, h Shows a list of commands or help for one command GLOBAL OPTIONS: --help, -h show help
The urfave/cli automatically creates help for our tool.
CLI arguments
Arguments passed to the program can be retrieved via the Args
function.
package main
import (
"fmt"
"log"
"os"
"github.com/urfave/cli/v2"
)
func main() {
app := &cli.App{
Usage: "app a1 a2 ...",
Action: func(ctx *cli.Context) error {
n := ctx.NArg()
fmt.Printf("app received %d arguments\n", n)
if n >= 2 {
first := ctx.Args().First()
rest := ctx.Args().Tail()
fmt.Printf("first argument: %s\n", first)
fmt.Printf("the remaining arguments: %v\n", rest)
} else if n == 1 {
first := ctx.Args().First()
fmt.Printf("first argument: %s\n", first)
}
return nil
},
}
if err := app.Run(os.Args); err != nil {
log.Fatal(err)
}
}
The program retrieves the passed arguments.
n := ctx.NArg()
fmt.Printf("app received %d arguments\n", n)
The NArg returns the number of the command line arguments passed.
first := ctx.Args().First() rest := ctx.Args().Tail()
The First function returns the first argument. The
Tail returns the rest of the arguments (all but the first).
$ nargs.exe 1 2 3 4 app received 4 arguments first argument: 1 the remaining arguments: [2 3 4]
Check arguments
With the Present function, we can check if there are any arguments
passed to the program.
package main
import (
"fmt"
"log"
"os"
"github.com/urfave/cli/v2"
)
func main() {
app := &cli.App{
Action: func(ctx *cli.Context) error {
if ctx.Args().Present() {
fmt.Println(ctx.App.Name)
fmt.Println(ctx.NArg())
fmt.Println(ctx.Args().First())
fmt.Println(ctx.Args().Tail())
} else {
fmt.Println("No arguments specified")
}
return nil
},
}
if err := app.Run(os.Args); err != nil {
log.Fatal(err)
}
}
If there are no arguments, the application prints "No arguments specified" message.
$ checkargs.exe 1 2 3 4 checkargs.exe 4 1 [2 3 4] $ checkargs.exe No arguments specified
The Get function
The Get function returns the nth argument, or else a blank string.
package main
import (
"fmt"
"log"
"os"
"time"
"github.com/urfave/cli/v2"
)
func main() {
app := &cli.App{
Name: "app",
Usage: "app [now] [hello]",
Action: func(ctx *cli.Context) error {
a := ctx.Args().Get(0)
if a == "now" {
now := time.Now()
fmt.Println(now)
} else if a == "hello" {
fmt.Println("hello there!")
} else {
fmt.Printf("Usage: %s\n", ctx.App.Usage)
}
return nil
},
}
if err := app.Run(os.Args); err != nil {
log.Fatal(err)
}
}
In the program, we retrieve the first command line argument and perform an action depending if the argument equals to "now" or "hello".
a := ctx.Args().Get(0)
We retrieve the first argument with Get. In this case we can also
use First.
if a == "now" {
now := time.Now()
fmt.Println(now)
} else if a == "hello" {
fmt.Println("hello there!")
} else {
fmt.Printf("Usage: %s\n", ctx.App.Usage)
}
If the argument equals to "now", we print the current datetime. If it equals to
"hello", we print a message. Otherwise, the usage is printed with
ctx.App.Usage.
$ app.exe now 2023-10-03 12:34:08.5846632 +0200 CEST m=+0.003355301 $ app.exe hello hello there!
CLI commands
A command is a specific action that the tool performs. The previous
example had two commands: now and hello. Now we
rewrite the program to use commands.
package main
import (
"fmt"
"log"
"os"
"time"
"github.com/urfave/cli/v2"
)
func main() {
app := &cli.App{
Name: "app",
Commands: []*cli.Command{
{
Name: "now",
Usage: "Show current local datetime",
Action: func(c *cli.Context) error {
now := time.Now()
fmt.Println(now)
return nil
},
},
{
Name: "hello",
Usage: "Show hello message",
Action: func(c *cli.Context) error {
fmt.Println("Hello there!")
return nil
},
},
},
}
if err := app.Run(os.Args); err != nil {
log.Fatal(err)
}
}
The commands are specified in the Commands field.
{
Name: "now",
Usage: "Show current local datetime",
Action: func(c *cli.Context) error {
now := time.Now()
fmt.Println(now)
return nil
},
},
We specify the Name, the Usage and the
Action for the command.
CLI flags
Flags are used to pass values to the command line applications.
-count=x -count x --count=x --count x
There are several ways to specify flags. Also note that boolean flags do not require a value.
package main
import (
"fmt"
"log"
"os"
"github.com/urfave/cli/v2"
)
func main() {
app := &cli.App{
Flags: []cli.Flag{
&cli.StringFlag{Name: "name"},
},
Action: func(ctx *cli.Context) error {
name := ctx.Value("name")
fmt.Println(name)
return nil
},
}
if err := app.Run(os.Args); err != nil {
log.Fatal(err)
}
}
The program accepts a name flag.
Flags: []cli.Flag{
&cli.StringFlag{Name: "name"},
},
Flags are specified via the Flags field. The name flag
is a StringFlag.
Action: func(ctx *cli.Context) error {
name := ctx.Value("name")
fmt.Println(name)
return nil
},
In the Action function we get the value of the flag using
Value.
$ flag.exe --name "John Doe" John Doe
Aliases
We can give aliases to our flags. An alias is a different name for the flag.
package main
import (
"fmt"
"log"
"os"
"github.com/urfave/cli/v2"
)
func main() {
app := &cli.App{
Flags: []cli.Flag{
&cli.StringFlag{Name: "config", Aliases: []string{"cfg", "conf"}},
},
Action: func(ctx *cli.Context) error {
fmt.Println(ctx.String("config"))
fmt.Println(ctx.String("cfg"))
fmt.Println(ctx.String("conf"))
return nil
},
}
if err := app.Run(os.Args); err != nil {
log.Fatal(err)
}
}
The config flag has cfg and conf aliases.
Flags: []cli.Flag{
&cli.StringFlag{Name: "config", Aliases: []string{"cfg", "conf"}},
},
The aliases are specified with the Aliases option.
fmt.Println(ctx.String("config"))
fmt.Println(ctx.String("cfg"))
fmt.Println(ctx.String("conf"))
We get retrieve the value via all three options.
$ aliases.exe -cfg file.txt file.txt file.txt file.txt
Destination
The Destination option can be used to specify the variable to which
the passed value will be copied.
package main
import (
"fmt"
"log"
"os"
"github.com/urfave/cli/v2"
)
func main() {
var name string
app := &cli.App{
Flags: []cli.Flag{
&cli.StringFlag{Name: "name", Destination: &name},
},
Action: func(ctx *cli.Context) error {
msg := fmt.Sprintf("Hello %s!", name)
fmt.Println(msg)
return nil
},
}
if err := app.Run(os.Args); err != nil {
log.Fatal(err)
}
}
In the program, we copy the value from the passed name argument
to the name variable.
Application version
Application version can be set via Version and
VersionFlag.
package main
import (
"os"
"github.com/urfave/cli/v2"
)
func main() {
cli.VersionFlag = &cli.BoolFlag{
Name: "version",
Aliases: []string{"V"},
Usage: "shows the app version",
}
app := &cli.App{
Name: "app",
Usage: "app demonstrating version",
Version: "v1.0",
}
app.Run(os.Args)
}
In the example we set the application version.
cli.VersionFlag = &cli.BoolFlag{
Name: "version",
Aliases: []string{"V"},
Usage: "shows the app version",
}
We specify the flag name, alias, and usage. The BoolFlag is used.
app := &cli.App{
Name: "app",
Usage: "app demonstrating version",
Version: "v1.0",
}
We specify the version string.
$ app.exe --version app version v1.0
Source
In this article we have shown how to create Go command line tools using the urfave/cli package.
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