Time for action – using command substitution

Up to now we have used double-quotes to contain a variable that will be referenced. Unlang also features back-quoted strings, which allows for command substitution. The string inside these back-quotes is evaluated similarly to double-quoted strings where string expansion can take place.

Let's modify the previous exercise to show command substitution in action:

1. Edit the sites-available/default virtual server under the FreeRADIUS configuration directory and add the following inside the post-auth section, at the top of the section:

   if(control:Auth-Type == 'PAP'){ 
       update reply { 
          Reply-Message := `/bin/echo We are using %{control:Auth-Type}`
       } 
   } 

2. Restart FreeRADIUS in debug mode and try to authenticate as alice.

   The feedback of the echo command will now be assigned to the Reply-Message attribute.

What just happened?

We have used back-quotes to perform command substitution. The output of this command has been assigned...