How do you parse command-line flags in Go?
Aug 02, 2025 pm 04:24 PM
Go's flag package can easily parse command line parameters. 1. Use flag.Type() to define type flags such as strings, integers, and booleans; 2. You can parse flags to variables through flag.TypeVar() to avoid pointer operations; 3. After calling flag.Parse(), use flag.Args() to obtain subsequent positional parameters; 4. Implementing the flag.Value interface can support custom types to meet most simple CLI requirements. Complex scenarios can be replaced by spf13/cobra library.
Parsing command-line flags in Go is straightforward using the built-in flag package. It allows you to define and parse flags of various types (string, int, bool, etc.) with minimal code.
Here's how it works:
1. Define Flags with flag.Type()
You declare flags using functions like flag.String() , flag.Int() , flag.Bool() , etc. Each takes a name, default value, and usage description.
package main
import (
"flag"
"fmt"
)
func main() {
// Define flags
name := flag.String("name", "World", "your name")
age := flag.Int("age", 0, "your age")
verbose := flag.Bool("verbose", false, "enable verbose mode")
// Parse the command-line flags
flag.Parse()
// Use the values
fmt.Printf("Hello, %s!\n", *name)
fmt.Printf("Age: %d\n", *age)
fmt.Printf("Verbose: %t\n", *verbose)
}Run it like:
go run main.go -name Alice -age 30 -verbose
Output:
Hello, Alice! Age: 30 Verbose: true
2. Using Struct Tags with flag (via Pointers)
The flag.Type() functions return a pointer to the value. So you must dereference them with * when using.
Alternatively, you can declare a variable first and use flag.TypeVar() :
var name string var age int var verbose bool flag.StringVar(&name, "name", "World", "your name") flag.IntVar(&age, "age", 0, "your age") flag.BoolVar(&verbose, "verbose", false, "enable verbose mode") flag.Parse()
This avoids points in the variable usage later.
3. Positional Arguments After Flags
After flag.Parse() , any remaining arguments are accessible via flag.Args() , and you can get the count with flag.NArg() .
fmt.Printf("Extra arguments: %v\n", flag.Args())For:
go run main.go -name Bob file1.txt file2.txt
flag.Args() returns ["file1.txt", "file2.txt"] .
4. Custom Types and flag.Value Interface
You can define custom flag types by implementing the flag.Value interface:
type Greets []string
func (g *Greets) String() string {
return fmt.Sprint(*g)
}
func (g *Greets) Set(value string) error {
*g = append(*g, value)
return nil
}Use it:
var greets Greets flag.Var(&greets, "greet", "add a greeting") flag.Parse()
Now you can pass multiple flags:
go run main.go -greet Hello -greet Hi -greet "Howdy"
And greets will contain all three strings.
Basically, flag covers most use cases out of the box. For more advanced needs (like subcommands or config files), people often use libraries like spf13/cobra , but for simple CLI tools, flag is clean and sufficient.
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