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Created by Zed A. Shaw Updated 2025-11-14 00:08:54

41: cut

The cut command takes input that's "tabular" and cut it into columns to display. It's a very useful tool when you have data that you only want a slice of, but don't want to write a little program to do the slicing.

The Challenge

To get your version of cut working you'll need to implement these options:

-f -- Sets the list of fields to output.
-d -- Sets what character delimits (separates) the fields. By default this is \t.

The documentation for cut is at:

https://www.gnu.org/software/coreutils/manual/html_node/cut-invocation.html

Requirements

You'll find that cut doesn't use anything new, but it does use almost everything you've seen:

See the list of requirements

Spoilers

As usual, here's my first implementation of cut. Can you do better?

See my first version code

View Source file go-coreutils/cut/main.go Only

package main

import (
  "fmt"
  "flag"
  "strings"
  "log"
  "strconv"
  "bufio"
  "os"
)

type Opts struct {
  FieldOpt string
  Fields []int
  Delim string
}

func ParseOpts() Opts {
  var opts Opts

  opts.Fields = make([]int, 0, 10)

  flag.StringVar(&opts.FieldOpt, "f", "", "fields list")
  flag.StringVar(&opts.Delim, "d", "\t", "delimiter byte")
  flag.Parse()

  if opts.FieldOpt == "" { log.Fatal("-f fields list required") }

  for _, num := range strings.Split(opts.FieldOpt, ",") {
    as_int, err := strconv.Atoi(num)
    if err != nil { log.Fatal("-f can only contain numbers") }

    opts.Fields = append(opts.Fields, as_int)
  }

  return opts
}

func main() {
  opts := ParseOpts()

  scan := bufio.NewScanner(os.Stdin)
  for scan.Scan() {
    line := scan.Text()

    chopped := strings.Split(line, opts.Delim)

    for i, field_num := range opts.Fields {
      if field_num >= len(chopped) {
        fmt.Print("\n")
        continue
      }

      fmt.Print(chopped[field_num])

      if i < len(opts.Fields) - 1 {
        fmt.Print(opts.Delim)
      } else {
        fmt.Print("\n")
      }
    }
  }
}

Testing It

If you want to push your learning further then you can try to implement an automated test for this. I actually need to learn how to test utilities like this with Go, so for now just consider this an extra challenge for later until I learn how to teach it.

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