Advanced
Alternate Names¶
You can set alternate (or short) names for flags by providing a list of strings for Aliases
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",
},
},
}
if err := cmd.Run(context.Background(), 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.
Multiple Values per Single Flag¶
As noted in the basics for flag, the simple flags allow only one value per flag and only the last entered value on command line will be returned to user on query.
urfave/cli also supports multi-value flags called slice flags. These flags can take multiple values of same type.
In addition they can be invoked multiple times on the command line and values will be appended to original value
of the flag and returned to the user as a slice
UintSliceFlagIntSliceFlagStringSliceFlagFloatSliceFlag
package main
import (
"fmt"
"log"
"os"
"strings"
"context"
"github.com/urfave/cli/v3"
)
func main() {
cmd := &cli.Command{
Flags: []cli.Flag{
&cli.StringSliceFlag{
Name: "greeting",
Usage: "Pass multiple greetings",
},
},
Action: func(ctx context.Context, cmd *cli.Command) error {
fmt.Println(strings.Join(cmd.StringSlice("greeting"), `, `))
return nil
},
}
if err := cmd.Run(context.Background(), os.Args); err != nil {
log.Fatal(err)
}
}
Multiple values need to be passed as separate, repeating flags, e.g. --greeting Hello --greeting Hola.
Count for bool flag¶
For bool flags you can specify the flag multiple times to get a count(e.g -v -v -v or -vvv)
If you want to support the
-vvvflag, you need to setCommand.UseShortOptionHandling.
package main
import (
"fmt"
"log"
"os"
"context"
"github.com/urfave/cli/v3"
)
func main() {
var count int
cmd := &cli.Command{
UseShortOptionHandling: true,
Flags: []cli.Flag{
&cli.BoolFlag{
Name: "foo",
Usage: "foo greeting",
Aliases: []string{"f"},
Config: cli.BoolConfig{
Count: &count,
},
},
},
Action: func(ctx context.Context, cmd *cli.Command) error {
fmt.Println("count", count)
return nil
},
}
if err := cmd.Run(context.Background(), 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"
"context"
"github.com/urfave/cli/v3"
)
func main() {
cmd := &cli.Command{
Flags: []cli.Flag{
&cli.StringFlag{
Name: "config",
Aliases: []string{"c"},
Usage: "Load configuration from `FILE`",
},
},
}
if err := cmd.Run(context.Background(), 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.
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"
"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",
},
&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(ctx context.Context, cmd *cli.Command) error {
return nil
},
},
{
Name: "add",
Aliases: []string{"a"},
Usage: "add a task to the list",
Action: func(ctx context.Context, cmd *cli.Command) error {
return nil
},
},
},
}
sort.Sort(cli.FlagsByName(cmd.Flags))
if err := cmd.Run(context.Background(), 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")
Required Flags¶
You can mark a flag as 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"
"context"
"github.com/urfave/cli/v3"
)
func main() {
cmd := &cli.Command{
Flags: []cli.Flag{
&cli.StringFlag{
Name: "lang",
Value: "english",
Usage: "language for the greeting",
Required: true,
},
},
Action: func(ctx context.Context, cmd *cli.Command) error {
output := "Hello"
if cmd.String("lang") == "spanish" {
output = "Hola"
}
fmt.Println(output)
return nil
},
}
if err := cmd.Run(context.Background(), os.Args); err != nil {
log.Fatal(err)
}
}
If the command 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"
"context"
"github.com/urfave/cli/v3"
)
func main() {
cmd := &cli.Command{
Flags: []cli.Flag{
&cli.IntFlag{
Name: "port",
Usage: "Use a randomized port",
Value: 0,
DefaultText: "random",
},
},
}
if err := cmd.Run(context.Background(), os.Args); err != nil {
log.Fatal(err)
}
}
Will result in help output like:
--port value Use a randomized port (default: random)
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"
"context"
"github.com/urfave/cli/v3"
)
func main() {
cmd := &cli.Command{
Flags: []cli.Flag{
&cli.IntFlag{
Name: "port",
Usage: "Use a randomized port",
Value: 0,
DefaultText: "random",
Action: func(ctx context.Context, cmd *cli.Command, v int64) error {
if v >= 65536 {
return fmt.Errorf("Flag port value %v out of range[0-65535]", v)
}
return nil
},
},
},
}
if err := cmd.Run(context.Background(), os.Args); err != nil {
log.Fatal(err)
}
}
Will result in help output like:
Flag port value 70000 out of range[0-65535]