Check tailscale expiry on Linux and mac

check-tailscale-expiry.sh

#!/bin/bash

message=""
# what OS is this?
cur_os=$(uname -s)

# do we have jq?
if ! which jq >/dev/null 2>&1 ; then
  echo "please install jq" >&2
  exit 1
fi

case $cur_os in 
  Linux)
    status=$(tailscale status --json |\
      jq --arg tendays "$(date -d '10 day' +%s)" \
      '.Self.KeyExpiry | fromdateiso8601 as $expiry | $expiry < ($tendays | tonumber)')
    if [[ "$status" == "true" ]]; then
      message="Tailscale key expiry is less than 10 days. Renew with 'tailscale up'"
    fi
    ;;
  Darwin)
    # MacOS doesn't put the tailscale binary in PATH
    tailscale='/Applications/Tailscale.app/Contents/MacOS/Tailscale'
    # MacOS doesn't have GNU date, but 86400 * 10 = 864000. I don't care if we are
    # a little off a couple of times a year due to Daylight Savings.
    tendays=$(($(date +%s) + 864000))
    status=$($tailscale status --json |\
      jq --arg tendays "$tendays" \
      '.Self.KeyExpiry | fromdateiso8601 as $expiry | $expiry < ($tendays | tonumber)')
    if [[ "$status" == "true" ]]; then
      message="Tailscale key expiry is less than 10 days. Renew soon!"
    fi
    ;;
   *)
     message="Unknown OS; can't check Tailscale expiry"
     ;;
esac

if [[ -n "$message" ]] ; then
  echo "$message" >&2
fi