main.go

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"net/http"
	"os"
)

const (
	EnvSlackWebhook         = "SLACK_WEBHOOK"
	EnvSlackIcon            = "SLACK_ICON"
	EnvSlackChannel         = "SLACK_CHANNEL"
	EnvSlackTitle           = "SLACK_TITLE"
	EnvSlackMessage         = "SLACK_MESSAGE"
	EnvSlackColor           = "SLACK_COLOR"
	EnvSlackUserName        = "SLACK_USERNAME"
	EnvSlackMarkdownSupport = "SLACK_MARKDOWN"
)

type Webhook struct {
	Text            string       `json:"text,omitempty"`
	UserName        string       `json:"username,omitempty"`
	IconURL         string       `json:"icon_url,omitempty"`
	IconEmoji       string       `json:"icon_emoji,omitempty"`
	Channel         string       `json:"channel,omitempty"`
	UnfurlLinks     bool         `json:"unfurl_links"`
	Attachments     []Attachment `json:"attachments,omitempty"`
	MarkdownSupport bool         `json:"mrkdwn,omitempty"`
}

type Attachment struct {
	Fallback string  `json:"fallback"`
	Pretext  string  `json:"pretext,omitempty"`
	Color    string  `json:"color,omitempty"`
	Fields   []Field `json:"fields,omitempty"`
}

type Field struct {
	Title string `json:"title"`
	Value string `json:"value,omitempty"`
	Short bool   `json:short"`
}

func main() {
	endpoint := os.Getenv(EnvSlackWebhook)
	if endpoint == "" {
		fmt.Fprintln(os.Stderr, "URL is required")
		os.Exit(1)
	}
	text := os.Getenv(EnvSlackMessage)
	if text == "" {
		fmt.Fprintln(os.Stderr, "Message is required")
		os.Exit(1)
	}

	_, isMarkdownSupportEnabled := os.LookupEnv(EnvSlackMarkdownSupport)

	msg := Webhook{
		UserName:        os.Getenv(EnvSlackUserName),
		IconURL:         os.Getenv(EnvSlackIcon),
		Channel:         os.Getenv(EnvSlackChannel),
		MarkdownSupport: isMarkdownSupportEnabled,
		Attachments: []Attachment{
			{
				Fallback: envOr(EnvSlackMessage, "This space intentionally left blank"),
				Color:    os.Getenv(EnvSlackColor),
				Fields: []Field{
					{
						Title: os.Getenv(EnvSlackTitle),
						Value: envOr(EnvSlackMessage, "EOM"),
					},
				},
			},
		},
	}

	if err := send(endpoint, msg); err != nil {
		fmt.Fprintf(os.Stderr, "Error sending message: %s\n", err)
		os.Exit(2)
	}
}

func envOr(name, def string) string {
	if d, ok := os.LookupEnv(name); ok {
		return d
	}
	return def
}

func send(endpoint string, msg Webhook) error {
	enc, err := json.Marshal(msg)
	if err != nil {
		return err
	}
	b := bytes.NewBuffer(enc)
	res, err := http.Post(endpoint, "application/json", b)
	if err != nil {
		return err
	}

	if res.StatusCode >= 299 {
		return fmt.Errorf("Error on message: %s\n", res.Status)
	}
	fmt.Println(res.Status)
	return nil
}