Send slack message with preformatted content (e.g. file, command output, etc.) from shell

send-slack-preformatted.sh

#!/usr/bin/env bash

HOOK_URL="https://hooks.slack.com/services/XXX"
FILE="/etc/passwd"
MSG="Here is file content as preformatted text:"$'\n```\n'"$(< "$FILE")"$'\n```\n'
PAYLOAD="$(jq --raw-input --slurp '{text:.}' <<< "$MSG")"

curl \
  -X POST \
  -H 'Content-type: application/json' \
  -d "$PAYLOAD" \
  -Ss \
  "$HOOK_URL" > /dev/null