Prepend filenames in a directory with an integer in ascending order of modified time. Integer prefixes are zero-padded based on the number of files in the directory.

sequential_filenames.go

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"math"
	"os"
	"path"
	"sort"
	"strings"
)

func main() {

	if len(os.Args) != 2 {
		log.Fatalf("usage: %s path", os.Args[0])
	}

	dirname := os.Args[1]
	files, err := ioutil.ReadDir(dirname)
	if err != nil {
		log.Fatalln(err)
	}

	// Sort in ascending order of modified time.
	sort.Slice(files, func(i, j int) bool {
		return files[i].ModTime().Unix() < files[j].ModTime().Unix()
	})

	// Figure out how much padding is necessary (based on number of files).
	padding := int(math.Ceil(math.Log10(float64(len(files)))))
	formatter := fmt.Sprintf("%%0%dd", padding)

	for j, file := range files {
		newName := fmt.Sprintf(formatter, j) + fmt.Sprintf("_%s", file.Name())
		newName = strings.ReplaceAll(newName, " ", "_")
		newName = path.Join(dirname, newName)
		fmt.Printf("%s --> %s\n", file.Name(), path.Base(newName))
		err := os.Rename(path.Join(dirname, file.Name()), newName)
		if err != nil {
			log.Fatalln(err)
		}
	}
}