Command line arguments

Context

When you run a program from the terminal, you can pass arguments. In Go, these are available via os.Args – a slice of strings where os.Args[0] is the program name, and os.Args[1:] are the arguments.

For more complex flag parsing (e.g., -port 8080), the flag package provides a standard solution.

Using `os.Args`

package main

import (
    "fmt"
    "os"
)

func main() {
    // Print all arguments including program name
    for i, arg := range os.Args {
        fmt.Printf("args[%d] = %s\n", i, arg)
    }

    // Get positional arguments (skip program name)
    if len(os.Args) < 2 {
        fmt.Println("Please provide at least one argument")
        return
    }
    firstArg := os.Args[1]
    fmt.Println("First argument:", firstArg)
}

Using the `flag` package

The flag package parses Unix‑style flags (e.g., -name=value, --name value).

package main

import (
    "flag"
    "fmt"
)

func main() {
    var name string
    var age int
    flag.StringVar(&name, "name", "World", "name to greet")
    flag.IntVar(&age, "age", 0, "age of the person")
    flag.Parse()

    fmt.Printf("Hello, %s (age %d)\n", name, age)
    fmt.Println("Remaining args:", flag.Args())
}

Run with:

go run main.go -name Alice -age 30 extra1 extra2

Output

Hello, Alice (age 30)
Remaining args: [extra1 extra2]

Example

Print all received arguments.

Code example

package main

import (
    "fmt"
    "os"
)

func main() {
    fmt.Println("Arguments:", os.Args)
}

Output (if run as `go run main.go hello world`)

Arguments: [/tmp/go-build123/b001/exe/main hello world]

Command line arguments

Context

Using os.Args

Using the flag package