csvknife: a utility to cut a column from CSV data

csvknife.go

package main

// csvknife cuts a specified column from a CSV file.
//
// To build:
//
//   go build .
//
// If you're reading this online, you can clone your own copy with a
// command like this:
//
//   git clone https://gist.github.com/3f54f1362b3521e0c0cf9288f33d3355.git csvknife
//
//
// Copyright 2024 Eric Wollesen
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import (
	"encoding/csv"
	"flag"
	"fmt"
	"log"
	"os"
	"strconv"
)

var (
	usageMsg = `
The first argument is the 1-indexed column number of the CSV data to print.

Suggested shell example to cut column 3 (1-indexed) without a header (i.e. from Sumologic) into
jq for filtering:

  $ %s --skip-header 3 < data.csv | jq --slurp
`

	usageFn = func() {
		var out = flag.CommandLine.Output()
		var arg0 = os.Args[0]
		fmt.Fprintf(out, "Usage of %s:\n", arg0)
		flag.PrintDefaults()
		fmt.Fprintf(out, usageMsg, arg0)
	}
)

func main() {
	var skipHeader = false
	flag.BoolVar(&skipHeader, "skip-header", false, "when true, skip the first line")
	flag.Usage = usageFn
	flag.Parse()

	colNum := func(arg string) int {
		i, err := strconv.Atoi(arg)
		if err != nil {
			log.Fatal(fmt.Errorf("parsing column number: %w", err))
		}
		return i
	}(flag.Arg(0))

	records, err := csv.NewReader(os.Stdin).ReadAll()
	if err != nil {
		log.Fatal(fmt.Errorf("reading CSV data: %w", err))
	}

	for idx, record := range records {
		if idx == 0 && skipHeader {
			continue
		}
		fmt.Println(record[colNum-1])
	}
}

go.mod

module csvknife

go 1.21.5