How to use the flag package in Golang
The flag package in Go parses command-line arguments by defining flags like string, int, or bool using flag.StringVar, flag.IntVar, etc., such as flag.StringVar(&host, "host", "localhost", "server address"); after declaring flags, call flag.Parse() to process inputs, enabling access to flag values and positional arguments via flag.Args(); customize help messages by setting flag.Usage for clearer user guidance.
The flag package in Go is used to parse command-line arguments. It allows you to define flags for your program, making it easy to accept user input when running the application from the terminal. Here's how to use it effectively.
Importing and Defining Flags
Start by importing the flag package. You can define flags using functions like flag.String, flag.Int, or flag.Bool. These declare a flag with a name, default value, and usage description.
Example:
flag.StringVar(&host, "host", "localhost", "server address")flag.IntVar(&port, "port", 8080, "server port")
This creates two flags: host (string) and port (int). The variables must be declared beforehand or passed as references directly.
Parsing the Arguments
After defining flags, call flag.Parse() to process the command-line inputs. This should come after all flag declarations and before using the flag values.
Anything that comes after the command name when running the program will be parsed. For example:
go run main.go -host=192.168.1.1 -port=3000After flag.Parse(), the values of host and port will reflect the provided inputs or fall back to defaults.
Handling Positional Arguments
After parsing flags, any remaining arguments are called positional arguments. Use flag.Args() to get them as a slice.
For instance, in:
go run main.go -port=9000 file1.txt file2.txtflag.Args() returns []string{"file1.txt", "file2.txt"}.
If you need the count, use flag.NArg() and flag.Arg(i) for individual access.
Custom Usage and Validation
You can customize the help message by assigning to flag.Usage. This function runs when the user requests help or provides invalid input.
flag.Usage = func() {fmt.Println("Usage: myapp [options]
flag.PrintDefaults()
}
flag.Parse()
This gives clearer instructions than the default output.
Basically, define your flags, parse them, then access their values. Add custom usage if needed. The flag package keeps CLI tools simple and consistent.
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