bash (とsed (とperl……)) があれば動く

slackpost.sh

#!/bin/bash

# Usage: slackpost "<webhook_url>" "<channel>" "<username>" "<message>" ["<attached_message>"]

export LANG=ja_JP.utf8

# ------------
webhook_url=$1
if [[ $webhook_url == "" ]]
then
        echo "No webhook_url specified"
        exit 1
fi

# ------------
shift
channel=$1
if [[ $channel == "" ]]
then
        echo "No channel specified"
        exit 1
fi

# ------------
shift
username=$1
if [[ $username == "" ]]
then
        echo "No username specified"
        exit 1
fi

# ------------
shift

text=$1
escapedText=$(echo $text | sed 's/"/\"/g' | sed "s/'/\'/g" )

if [[ $text == "" ]]
then
        echo "No text specified"
        exit 1
fi

shift

attachedText="$1"
escapedAttachedText=$(echo "$attachedText" | sed 's/"/\"/g' | sed "s/'/\'/g" | perl -pe 's/\n/\\n/g' )

if [[ $attachedText != "" ]]
then
  attachments=", \"attachments\":[{\"color\":\"danger\" , \"text\":\"$escapedAttachedText\"}]"
fi

json="{\"channel\": \"$channel\", \"username\":\"$username\", \"icon_emoji\":\":video_game:\", \"text\":\"$escapedText\" $attachments}"

echo $json

curl -s -d "payload=$json" "$webhook_url"