Homemade CLI Tools

main.go

// This is an example for:
// https://www.tegh.net/2021/12/12/homemade-cli-tools/
package main

import (
	"errors"
	"fmt"
	"io"
	"log"
	"os"

	"github.com/spf13/cobra"
)

func main() {
	var name string
	c := &cobra.Command{
		Use: "tool",
		Run: func(cmd *cobra.Command, args []string) {
			if name != "" {
				fmt.Println(name + ": ")
			}
			for _, a := range args {
				fmt.Print(a, " ")
			}
			if len(args) > 0 {
				fmt.Println()
			}

			file, fileClose, err := inputFileOrStdin("")
			if err != nil {
				log.Fatalln(err)
			}
			defer fileClose()
			b, err := io.ReadAll(file)
			if err != nil {
				log.Fatalln(err)
			}
			fmt.Println(string(b))
		},
	}
	c.Flags().StringVarP(
		&name,    // a pointer to the variable to be set
		"name",   // the name of the flag (use it with `double dash`)
		"n",      // a short name of the flag (to be used with a single `dash`)
		"",       // the default value
		"a name", // a short usage description
	)
	if err := c.Execute(); err != nil {
		os.Exit(1)
	}
}

func inputFileOrStdin(inputFilePath string) (*os.File, func() error, error) {
	if inputFilePath != "" {
		file, err := os.Open(inputFilePath)
		if err != nil {
			return nil, nil, err
		}
		return file, file.Close, nil
	}
	fi, err := os.Stdin.Stat()
	if err != nil {
		return nil, nil, err
	}
	if fi.Size() == 0 && fi.Mode()&os.ModeNamedPipe == 0 {
		return nil, nil, errors.New("no input file provided and stdin is empty")
	}
	log.Println("os.Stdin size:", fi.Size())
	return os.Stdin, func() error { return nil }, nil
}