globalChannelMap not working in template

I have this in my channel preprocessor:

Code:

---

// Modify the message variable below to pre process data



var numoMSH = msg['MSH'].length();

globalChannelMap.put('numofMSH',numoMSH)



return message;

---

No errors when I validate.



In my same channel in the Destination, in the Template for a File Writer:

Code:

---

${message.rawData}



${numofMSH}

---

Which displays the rawData followed by literally "${numofMSH}" as a string.



Any ideas what I'm doing wrong?

globalChannelMap not working in template