Launchd to terminate what it starts

Why not just change your users' behavior on the shutdown process to make this a lot simpler (with minimal effort on scripting/programming the shutdown hook)?

• Tell the users not to use the standard shutdown process (from the menu or through the keyboard shortcut) and instead use your backup AppleScript that's transformed into an app called something like SafeShutdown.app

• Within the SafeShutdown app script, do the backup and then end it with a

  tell application "Finder"
      shut down
  end tell
  

  or with this (will prompt for password)

  do shell script "sudo shutdown -h now" with administrator privileges
  

  or with a

  tell application "System Events"
      shut down
  end tell
  

Run a shell command that starts the other commands:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
  <string>some.label</string>
  <key>ProgramArguments</key>
  <array>
    <string>bash</string>
    <string>-c</string>
    <string>python first_script.py;python second_script.py</string>
  </array>
  <key>RunAtLoad</key>
  <true/>
</dict>
</plist>

Or change the ProgramArguments key to

<key>Program</key>
<string>/path/to/script</string>

and use a script like this:

#!/bin/bash

output=$(python first_script.py 2>&1)
if [[ $? = 0 ]]; then
  python second_script.py --result true
else
  python second_script.py --result --false --reason "$output"
fi